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28 The Reproductive System: Embryology and Human Development PowerPoint® Lecture Presentations prepared by Steven Bassett Southeast Community College Lincoln, Nebraska © 2012 Pearson Education, Inc. Introduction • Development involves: • Differentiation of cells • Reorganization of cells • Development can be characterized by different periods of time • Prenatal development • Embryology • Postnatal development © 2012 Pearson Education, Inc. An Overview of Development • Development can be characterized by different periods of time • Prenatal development • Conception to delivery • Involves embryology (development during the prenatal period) • Postnatal development • Development from birth to maturity © 2012 Pearson Education, Inc. Fertilization • Fertilization is the joining of two haploid cells to create a diploid cell • Function of the haploid cells • Spermatozoon • Delivers the paternal chromosomes to the ovum • Ovum • Provides the maternal chromosomes • Provides nourishment for embryonic development © 2012 Pearson Education, Inc. Fertilization • Fertilization occurs in the ampulla of the uterine tube • 200 million sperm cells enter the vaginal canal • Only about 10,000 make it to the uterine tubes • Less than 100 actually contact the egg • Only one will fertilize the egg © 2012 Pearson Education, Inc. Fertilization • Fertilization details • When the egg is ovulated, it is surrounded by the corona radiata, which protects the egg as it is being ovulated • Numerous sperm cells release hyaluronidase, from their acrosomal cap, in an effort to decompose the corona radiata ANIMATION Fertilization and the Preparations for Cleavage © 2012 Pearson Education, Inc. Figure 28.1a Fertilization and Preparation for Cleavage A secondary oocyte surrounded by spermatozoa © 2012 Pearson Education, Inc. Figure 28.1b Fertilization and Preparation for Cleavage Oocyte at Ovulation Ovulation releases a secondary oocyte and the first polar body; both are surrounded by the corona radiata. The oocyte is suspended in metaphase of meiosis II. Corona radiata First polar body Fertilization and Oocyte Activation Acrosomal enzymes from multiple sperm create gaps in the corona radiata. A single sperm then makes contact with the oocyte membrane, and membrane fusion occurs, triggering oocyte activation and completion of meiosis. Fertilizing spermatozoon Second polar body Pronucleus Formation Begins The sperm is absorbed into the cytoplasm, and the female pronucleus develops. Nucleus of fertilizing spermatozoon Female pronucleus Zona pellucida Cytokinesis Begins Amphimixis Occurs and Cleavage Begins The first cleavage division nears completion roughly 30 hours after fertilization. Further events are diagrammed in Figure 28.2 Metaphase of first cleavage division Spindle Formation and Cleavage Preparation The male pronucleus develops, and spindle fibers appear in preparation for the first cleavage division. Male pronucleus Blastomeres Events at fertilization and immediately thereafter © 2012 Pearson Education, Inc. Female pronucleus Figure 28.1b Fertilization and Preparation for Cleavage (Part 1 of 2) Oocyte at Ovulation Ovulation releases a secondary oocyte and the first polar body; both are surrounded by the corona radiata. The oocyte is suspended in metaphase of meiosis II. Corona radiata First polar body Fertilization and Oocyte Activation Acrosomal enzymes from multiple sperm create gaps in the corona radiata. A single sperm then makes contact with the oocyte membrane, and membrane fusion occurs, triggering oocyte activation and completion of meiosis. Fertilizing spermatozoon Zona pellucida Events at fertilization and immediately thereafter © 2012 Pearson Education, Inc. Second polar body Pronucleus Formation Begins The sperm is absorbed into the cytoplasm, and the female pronucleus develops. Nucleus of fertilizing spermatozoon Female pronucleus Figure 28.1b Fertilization and Preparation for Cleavage (Part 2 of 2) Cytokinesis Begins Amphimixis Occurs and Cleavage Begins The first cleavage division nears completion roughly 30 hours after fertilization. Further events are diagrammed in Figure 28.2 Metaphase of first cleavage division Spindle Formation and Cleavage Preparation The male pronucleus develops, and spindle fibers appear in preparation for the first cleavage division. Male pronucleus Blastomeres Events at fertilization and immediately thereafter © 2012 Pearson Education, Inc. Female pronucleus Prenatal Development • Prenatal development is gestation period (9 months) • Prenatal development is divided into trimesters • First trimester (rudiments of all organs appear) • Second trimester (fetus looks like a human) • Third trimester (organs become functional, rapid growth) © 2012 Pearson Education, Inc. Prenatal Development • The First Trimester • 1 to 12 weeks • Four events within the first trimester • • • • Cleavage (sequence of cell reproduction) Implantation (implantation into endometrial lining) Placentation (formation of the placenta) Embryogenesis (development of the embryo) © 2012 Pearson Education, Inc. Prenatal Development • Cleavage and Blastocyst Formation • Cell division results in the formation of blastomeres • A solid ball of cells eventually develops – this is a morula • Some cells migrate to one “edge” of the morula creating a mass of cells and a hollow cavity called the blastocoele • The ball of cells is now called the blastocyst • The outer layer of the blastocyst consists of cells called the trophoblast © 2012 Pearson Education, Inc. Figure 28.2 Cleavage and Blastocyst Formation Blastomeres Polar bodies 4-cell stage 2-cell stage Early morula DAY 2 DAY 1 DAY 3 Advanced morula DAY 4 First cleavage division Inner cell mass DAY 0: Fertilization DAY 6 Ovulation Blastocoele DAYS 7–10: Implantation in uterine wall (See Figure 28.3) © 2012 Pearson Education, Inc. Trophoblast Blastocyst Prenatal Development • Cleavage and Blastocyst Formation (continued) • Trophoblast cells provide nutrients to the developing embryo • The inner cell mass consists of stem cells that will develop into all the cells of the body © 2012 Pearson Education, Inc. Prenatal Development • Implantation • Upon contact with the endometrial lining, the trophoblast cells divide rapidly • The trophoblast cells “fuse” with the endometrial lining forming a syncytial trophoblast • This layer of cells releases hyaluronidase to erode away more of the endometrial lining so the mass can implant © 2012 Pearson Education, Inc. Prenatal Development • Implantation (continued) • Upon implantation, the inner cell mass separates from the trophoblast area • When the inner cell mass separates from the trophoblast, two cavities form: • Amnionic cavity • Blastocoele cavity ANIMATION Implantation © 2012 Pearson Education, Inc. Figure 28.3 Stages in the Implantation Process DAY 6 FUNCTIONAL LAYER OF ENDOMETRIUM UTERINE CAVITY Uterine glands Blastocyst DAY 7 Trophoblast Blastocoele Inner cell mass DAY 8 Syncytial trophoblast DAY 9 Cellular trophoblast Hypoblast Epiblast Developing primary villi Blastodisc Amniotic cavity Lacuna © 2012 Pearson Education, Inc. Prenatal Development • Formation of the Blastodisc • A layer of cells forms between the amnionic cavity and the blastocoele cavity • The layers are called • Epiblast • Hypoblast © 2012 Pearson Education, Inc. Figure 28.4a Blastodisc Organization and Gastrulation Syncytial trophoblast ENDOMETRIUM Amniotic cavity Cellular trophoblast Epiblast of blastodisc Hypoblast of blastodisc Lacunae Yolk sac Blastocoele Uterine lining DAY 10 The blastodisc begins as two layers: the epiblast, facing the amniotic cavity, and the hypoblast, exposed to the blastocoele. Migration of epiblast cells around the amniotic cavity is the first step in the formation of the amnion. Migration of hypoblast cells creates a sac that hangs below the blastodisc. This is the first step in yolk sac formation. © 2012 Pearson Education, Inc. Prenatal Development • Gastrulation and Germ Layer Formation • Eventually some cells of the epiblast move toward the center of the blastodisc creating a primitive streak • This movement is called gastrulation • As the cells move toward the primitive streak area, they begin to migrate between the epiblast and hypoblast layers • This creates three distinct layers of cells © 2012 Pearson Education, Inc. Prenatal Development • Gastrulation and Germ Layer Formation • The three layers of cells are: • Ectoderm (derived from the epiblast layer) • Mesoderm (new layer between the epiblast and hypoblast) • Endoderm (derived from the hypoblast layer) © 2012 Pearson Education, Inc. Figure 28.4 Blastodisc Organization and Gastrulation Syncytial trophoblast ENDOMETRIUM Amniotic cavity Cellular trophoblast Epiblast of blastodisc Ectoderm Blastodisc Hypoblast of blastodisc Lacunae Primitive streak Yolk sac Blastocoele Uterine lining DAY 10 The blastodisc begins as two layers: the epiblast, facing the amniotic cavity, and the hypoblast, exposed to the blastocoele. Migration of epiblast cells around the amniotic cavity is the first step in the formation of the amnion. Migration of hypoblast cells creates a sac that hangs below the blastodisc. This is the first step in yolk sac formation. © 2012 Pearson Education, Inc. Mesoderm Endoderm DAY 12 Migration of epiblast cells into the region between epiblast and hypoblast gives the blastodisc a third layer. From the time this process (gastrulation) begins, the epiblast is called ectoderm, the hypoblast endoderm, and the migrating cells mesoderm. Prenatal Development • The ectoderm, mesoderm, and endoderm are collectively known as the germ layers • Each layer will form specific tissues and organs of the body • Germ layers will also form structures involved in embryonic survival called extraembryonic membranes © 2012 Pearson Education, Inc. Table 28.1 The Fates of the Primary Germ Layers © 2012 Pearson Education, Inc. Prenatal Development • Formation of Extraembryonic Membranes • There are four major extraembryonic membranes • • • • Yolk sac Amnion Allantois Chorion © 2012 Pearson Education, Inc. Prenatal Development • Formation of Extraembryonic Membranes • Yolk sac • This is a pouch that extends from the hypoblast cells into the blastocoele • It is the early site for blood cell formation • Amnion • Amnionic fluid fills the amnion cavity, which surrounds and cushions the embryo and fetus © 2012 Pearson Education, Inc. Prenatal Development • Formation of Extraembryonic Membranes (continued) • Allantois • Eventually gives rise to the urinary bladder • Chorion • The mesoderm and trophoblast layers form together to form the chorion • The chorion will eventually develop extensions into the endometrium © 2012 Pearson Education, Inc. Figure 28.5 The Embryonic Membranes and Placenta Formation Migration of mesoderm around the inner surface of the trophoblast creates the chorion. Mesodermal migration around the outside of the amniotic cavity, between the ectodermal cells and the trophoblast forms the amnion. Mesodermal migration around the endodermal pouch creates the yolk sac. The embryonic disc bulges into the amniotic cavity at the head fold. The allantois, an endodermal extension surrounded by mesoderm, extends toward the trophoblast. Amniotic cavity (containing amniotic fluid) Amnion Allantois Syncytial trophoblast Cellular trophoblast Yolk sac Chorion Mesoderm Yolk sac Head fold of embryo Chorion Syncytial trophoblast Chorionic villi of placenta Blastocoele The developing embryo and extraembryonic membranes bulge into the uterine cavity. The trophoblast pushing out into the uterine lumen remains covered by endometrium but no longer participates in nutrient absorption and embryo support. The embryo moves away from the placenta, and the body stalk and yolk stalk fuse to form an umbilical stalk. The embryo now has a head fold and a tail fold. Constriction of the connections between the embryo and the surrounding trophoblast narrows the yolk stalk and body stalk. Tail fold Uterus Body stalk Myometrium Yolk stalk Decidua basalis Yolk sac Umbilical stalk Embryonic gut Placenta Yolk sac Embryonic head fold Chorionic villi of placenta Decidua capsularis Decidua parietalis Uterine lumen Decidua parietalis Decidua basalis Umbilical cord Placenta Amniotic cavity The amnion has expanded greatly, filling the uterine cavity. The fetus is connected to the placenta by an elongated umbilical cord that contains a portion of the allantois, blood vessels, and the remnants of the yolk stalk. © 2012 Pearson Education, Inc. Amnion Chorion Decidua capsularis Prenatal Development • Placentation • The placenta begins to form when the chorion produces villi (chorionic villi) that extend into the endometrial lining • The body stalk connects the embryo to the chorion • As the fetus develops and moves farther into the uterus, it obtains its nutrients via the umbilical cord © 2012 Pearson Education, Inc. Figure 28.5 The Embryonic Membranes and Placenta Formation (Part 6 of 8) The developing embryo and extraembryonic membranes bulge into the uterine cavity. The trophoblast pushing out into the uterine lumen remains covered by endometrium but no longer participates in nutrient absorption and embryo support. The embryo moves away from the placenta, and the body stalk and yolk stalk fuse to form an umbilical stalk. Uterus Myometrium Decidua basalis Umbilical stalk Placenta Yolk sac Chorionic villi of placenta Decidua capsularis Decidua parietalis Uterine lumen Decidua parietalis Decidua basalis Umbilical cord Placenta Amniotic cavity The amnion has expanded greatly, filling the uterine cavity. The fetus is connected to the placenta by an elongated umbilical cord that contains a portion of the allantois, blood vessels, and the remnants of the yolk stalk. © 2012 Pearson Education, Inc. Amnion Chorion Decidua capsularis Figure 28.6a A Three-Dimensional View of Placental Structure Decidua capsularis Amnion Umbilical cord (cut) Placenta Yolk sac Decidua basalis Chorion Umbilical vein Umbilical arteries Chorionic villi Area filled with maternal blood Decidua parietalis Myometrium Uterine cavity Cervical (mucous) plug in cervical canal External os Maternal blood vessels Cervix Vagina Amnion Trophoblast (cellular and syncytial layers) For clarity, the uterus is shown after the embryo has been removed and the umbilical cord cut. Blood flows into the placenta through ruptured maternal arteries. It then flows around chorionic villi, which contain fetal blood vessels. Fetal blood arrives through paired umbilical arteries and leaves through a single umbilical vein. Maternal blood reenters the venous system of the mother through the broken walls of small uterine veins. Maternal blood flow is shown by arrows; note that no actual mixing of maternal and fetal blood occurs. © 2012 Pearson Education, Inc. Prenatal Development • Embryogenesis • Shortly after gastrulation, embryogenesis begins • By week 4, a head fold and tail fold develop • The embryo is separated from the blastodisc area © 2012 Pearson Education, Inc. Figure 28.5 The Embryonic Membranes and Placenta Formation (Part 3 of 8) The embryo now has a head fold and a tail fold. Constriction of the connections between the embryo and the surrounding trophoblast narrows the yolk stalk and body stalk. Tail fold Body stalk Yolk stalk Yolk sac Embryonic gut Embryonic head fold © 2012 Pearson Education, Inc. Prenatal Development • Embryogenesis • At about 12 weeks, organs begin to form • This is called organogenesis © 2012 Pearson Education, Inc. Figure 28.7b The First Trimester Medulla Ear Pharyngeal arches Forebrain Eye Heart Somites Body stalk Arm bud Tail Leg bud Fiber-optic view of human development at week 4 © 2012 Pearson Education, Inc. Figure 28.7c The First Trimester Chorionic villi Amnion Umbilical cord Placenta Fiber-optic view of human development at week 8 © 2012 Pearson Education, Inc. Figure 28.7d The First Trimester Amnion Umbilical cord Fiber-optic view of human development at week 12 © 2012 Pearson Education, Inc. Prenatal Development • Second Trimester • Fetus is covered by the amnion sac • Fetus grows faster than the placenta • Third Trimester • All fetal organs become functional © 2012 Pearson Education, Inc. Prenatal Development • Changes in the uterus during gestation • Uterus will increase in length from 7.5 cm to 30 cm • Contains almost 5 L of fluid • The uterus and contents weigh about 22 pounds • Abdominal organs are pushed out of their normal positions © 2012 Pearson Education, Inc. Figure 28.9a The Growth of the Uterus and Fetus Placenta Uterus Umbilical cord Fetus at 16 weeks Amniotic fluid Cervix Vagina Pregnancy at four months (16 weeks) showing the positions of the uterus, fetus, and placenta © 2012 Pearson Education, Inc. Figure 28.9b The Growth of the Uterus and Fetus 9 months 8 months 7 months 6 months 5 months After dropping, in preparation to delivery 4 months 3 months Changes in the size of the uterus during the second and third trimesters © 2012 Pearson Education, Inc. Figure 28.9c The Growth of the Uterus and Fetus Liver Small intestine Stomach Pancreas Transverse colon Aorta Fundus of uterus Common iliac vein Umbilical cord Cervical (mucus) plug in cervical canal Placenta Urinary bladder Pubic symphysis Vagina External os Rectum Urethra Pregnancy at full term. Note the position of the uterus and fetus and the displacement of abdominal organs relative to part (d). © 2012 Pearson Education, Inc. Labor and Delivery • The goal of labor is parturition (expulsion of the fetus) • There are three stages of labor • Dilation stage • Expulsion stage • Placental stage © 2012 Pearson Education, Inc. Labor and Delivery • The dilation stage • The fetus begins to move down the cervical canal • The cervix dilates • The amnion sac breaks • The expulsion stage • The birth of the child • The placental stage • Ejection of the placenta © 2012 Pearson Education, Inc. Figure 28.10 The Stages of Labor Placenta Umbilical cord The Dilation Stage Sacral promontory Pubic Cervix Vagina symphysis Cervical canal Fully developed fetus before labor begins The Expulsion Stage The Placental Stage Uterus © 2012 Pearson Education, Inc. Ejection of the placenta Figure 28.10 The Stages of Labor (Part 1 of 3) Placenta Umbilical cord The Dilation Stage Sacral promontory Pubic Cervix Vagina symphysis Cervical canal Fully developed fetus before labor begins © 2012 Pearson Education, Inc. Figure 28.10 The Stages of Labor (Part 2 of 3) The Expulsion Stage © 2012 Pearson Education, Inc. Figure 28.10 The Stages of Labor (Part 3 of 3) The Placental Stage Uterus © 2012 Pearson Education, Inc. Ejection of the placenta The Neonatal Period • This is the period from birth to one month • Events that occur from birth to one month are: • Lungs fill with air • Blood circulation changes with the closing of the ductus arteriosus and the foramen ovale of the heart • Heart rate drops from 120–140 beats per minute to about 70 • Breathing rate drops from 30 per minute to the normal rate © 2012 Pearson Education, Inc. The Neonatal Period • Events that occur from birth to one month are (continued): • Kidneys filter the infant’s own blood • Digestive system becomes active • Metabolic rate is increased to maintain warmth for a few days after birth © 2012 Pearson Education, Inc. Embryology Summary • Overview of Embryology • The development of the: • • • • • • • • • Integumentary System Skull Vertebral Column Appendicular Skeleton Muscles Nervous System Spinal Cord Brain Special Sense Organs © 2012 Pearson Education, Inc. • Endocrine System • Heart • Cardiovascular System • Lymphoid System • Respiratory System • Digestive System • Urinary System • Reproductive System Embryology Summary • The Development of the Integumentary System • 1 month: epithelium overlies mesenchyme • 3 months: epithelium multiplies to form layers; mesenchyme develops into connective tissue thus forming the dermis • 4 months: epidermis begins to grow into the dermis thus forming columns of cells; hair follicles, sweat glands, and sebaceous glands develop from these columns © 2012 Pearson Education, Inc. Embryology Summary 28.1 The Development of the Integumentary System (Part 1 of 6) The Development of the Integumentary System Ectoderm Mesoderm 1 MONTH At the start of the second month, the superficial ectoderm is a simple epithelium overlying loosely organized mesenchyme. © 2012 Pearson Education, Inc. Embryology Summary 28.1 The Development of the Integumentary System (Part 2 of 6) The Development of the Integumentary System Germinative cells Connective tissue Epithelial column Blood vessel 3 MONTHS Over the following weeks, the epithelium becomes stratified through repeated divisions of the basal or germinative cells. The underlying mesenchyme differentiates into embryonic connective tissue containing blood vessels that bring nutrients to the region. © 2012 Pearson Education, Inc. Mesenchyme 4 MONTHS During the third and fourth months, small areas of epidermis undergo extensive divisions and form cords of cells that grow into the dermis. These are epithelial columns. Mesenchymal cells surround the columns as they extend deeper and deeper into the dermis. Hair follicles, sebaceous glands, and sweat glands develop from these columns. Embryology Summary • The Development of the Integumentary System (continued) • Skin: at 3 months, the epithelium continues to thicken • Melanocytes migrate into the stratum basale layer • Nails: develop from the thickening of the epidermis © 2012 Pearson Education, Inc. Embryology Summary 28.1 The Development of the Integumentary System (Part 3 of 6) The Development of the Integumentary System NAILS Ectoderm SKIN Nail field Fingertip Loose connective tissue As basal cell divisions continue, the epithelial layer thickens and the basal lamina is thrown into irregular folds. Pigment cells called melanocytes migrate into the area and squeeze between the germinative cells. The epithelium now resembles the epidermis of the adult. Melanocyte Germinative cell Dermis Dense connective tissue Subcutaneous layer 4 MONTHS © 2012 Pearson Education, Inc. The embryonic connective tissue differentiates into the dermis. Fibroblasts and other connective tissue cells form from mesenchymal cells or migrate into the area. The density of fibers increases. Loose connective tissue extends into the ridges, but a deeper, less vascular region is dominated by a dense, irregular collagen fiber network. Below the dermis the embryonic connective tissue develops into the subcutaneous layer, a layer of loose connective tissue. 4 MONTHS Nails begin as thickenings of the epidermis near the tips of the fingers and toes. These thickenings settle into the dermis, and the borderline with the general epidermis becomes distinct. Initially, nail production involves all of the germinative cells of the nail field. Nail root Eponychium Nail bed Nail BIRTH By the time of birth, nail production is restricted to the nail root. Embryology Summary 28.1 The Development of the Integumentary System (Part 4 of 6) The Development of the Integumentary System HAIR FOLLICLES Hair Sebaceous gland Hair column Sebaceous gland Papilla 5 MONTHS A hair follicle develops as a deep column surrounds a papilla, a small mass of connective tissue. Hair growth will occur in the epithelium covering the papilla. An outgrowth from the epithelial column forms a sebaceous gland. © 2012 Pearson Education, Inc. BIRTH At birth a hair projects from the follicle, and the secretions of the sebaceous gland lubricate the hair shaft. Embryology Summary 28.1 The Development of the Integumentary System (Part 5 of 6) The Development of the Integumentary System EXOCRINE GLANDS Epithelial column Duct of sweat gland Mesenchyme 5 MONTHS A sweat gland develops as an epithelial column elongates, coils, and becomes hollow. © 2012 Pearson Education, Inc. BIRTH At birth, sweat gland ducts carry the secretions of the gland cells to the skin surface. Embryology Summary 28.1 The Development of the Integumentary System (Part 6 of 6) The Development of the Integumentary System MAMMARY GLANDS Hollowing nipple Epidermis Epidermal thickening 5 MONTHS Developing duct Mammary glands develop in a comparable fashion, but the epidermal thickenings are much broader and extensive branching occurs. © 2012 Pearson Education, Inc. Branching duct Fat BIRTH At birth, the mammary glands have not completed their development. In females, further elaboration of the duct and gland system occurs at puberty, but functional maturity does not occur until late in pregnancy. Embryology Summary • The Development of the Skull • 5 weeks • Cartilage begins to appear • Cartilage within the brain area enlarges to form the chondrocranium • Skull walls and floor are beginning but no roof • 9 weeks • Endochondral ossification appears; frontal and parietal structures appear © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 1 of 7) The Development of the Skull Pharyngeal cartilages First pharyngeal arch (mandibular) Second arch (hyoid) Arches 3, 4, 6 Brain Eye Nose 5 WEEKS 5-WEEK EMBRYO © 2012 Pearson Education, Inc. After 5 weeks of development, the central nervous system is a hollow tube that runs the length of the body. A series of cartilages appears in the mesenchyme of the head beneath and alongside the expanding brain and around the developing nose, eyes, and ears. These cartilages are shown in light blue. Five additional pairs of cartilages develop in the walls of the pharynx. These cartilages, shown in dark blue, are located within the pharyngeal, or branchial, arches. (Branchial refers to gills—in fish the caudal arches develop into skeletal supports for the gills.) The first arch, or mandibular arch, is the largest. Embryology Summary 28.2 The Development of the Skull (Part 2 of 7) The Development of the Skull Chondrocranium Brain Eye Nasal capsule Vertebrae 8 WEEKS The cartilages associated with the brain enlarge and fuse, forming a cartilaginous chondrocranium (kon-dro-KRAne-um; chondros, cartilage cranium, skull) that cradles the brain and sense organs. At 8 weeks its walls and floor are incomplete, and there is no roof. © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 3 of 7) The Development of the Skull Frontal bone Occipital bone Sphenoid Maxilla The mandible forms as dermal bone develops around the inferior portion of the mandibular arch. Hyoid bone Larynx 9 WEEKS During the ninth week, numerous centers of endochondral ossification appear within the chondrocranium. These centers are shown in red. Gradually, the frontal and parietal bones of the cranial roof appear as intramembranous ossification begins in the overlying dermis. As these centers (beige) enlarge and expand, extensive fusions occur. © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Skull (continued) • 10 weeks • Mandibular arch fuses with the chondrocranium • Hyoid arch begins to form • 12 weeks • Ossification is well under way • Birth • Cranial roof remains incomplete • The anterior and posterior fontanels are present © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 4 of 7) The Development of the Skull The dorsal portion of the mandibular arch fuses with the chondrocranium. The fused cartilages do not ossify; instead, osteoblasts begin sheathing them in dermal bone. On each side this sheath fuses with a bone developing at the entrance to the nasal cavity, producing the two maxillae. Ossification centers in the roof of the mouth spread to form the palatine processes and later fuse with the maxillae. Parietal bone Frontal bone Maxilla Mandible 10 WEEKS The second arch, or hyoid arch, forms near the temporal bones. Fusion of the superior tips of the hyoid with the temporals forms the styloid processes. The ventral portion of the hyoid arch ossifies as the hyoid bone. The third arch fuses with the hyoid, and the fourth and sixth arches form laryngeal cartilages. © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 5 of 7) The Development of the Skull Frontal bone Parietal bone Occipital bone Maxilla Zygomatic arch Mandible Temporal bone 12 WEEKS © 2012 Pearson Education, Inc. After 12 weeks ossification is well under way in the cranium and face. (Compare with Figure 5.6, p. 123.) Embryology Summary 28.2 The Development of the Skull (Part 6 of 7) The Development of the Skull Frontal bone Parietal bone Mandible Occipital bone BIRTH The skull at birth; compare with the situation at 12 weeks. Extensive fusions have occurred, but the cranial roof remains incomplete. (For further details, see Figure 6.19, p. 165.) © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 7 of 7) The Development of the Skull Nasal septum Palatine arch Normal Abnormal Cleft palate or Bilateral cleft lip and palate If the overlying skin does not fuse normally, the result is a cleft lip. Cleft lips affect roughly one birth in a thousand. A split extending into the orbit and palate is called a cleft palate. Cleft palates are half as common as cleft lips. Both conditions can be corrected surgically. © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Vertebral Column • 4 weeks • On either side of the developing spinal cord are blocks of mesenchymal tissue called somites • The medial side of the somites produces the vertebral column • 8 weeks • Vertebral cartilage forms around the spinal cord • The sacrum and coccyx are fused together © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Vertebral Column (continued) • 12 weeks • Ribs and sternum undergo ossification • The tips of the longer ribs do not ossify; they remain as cartilage thus forming the costal cartilages © 2012 Pearson Education, Inc. Embryology Summary 28.3 The Development of the Vertebral Column (Part 1 of 6) The Development of the Vertebral Column Pharyngeal Ear arches Eye Somites Spinal cord Somite Sclerotome Heart Notochord Tail The developing spinal cord lies posterior to a longitudinal rod, the notochord (NO-to-kord; noton, back chorde, cord). In the fourth week of development, mesoderm on either side of the spinal cord and notochord forms a series of mesenchymal blocks called somites (SO-mits). Mesenchyme in the medial portions of each somite, a region known as the sclerotome (SKLER-o-tome; skleros, hard), will produce the vertebral column and contribute to the floor of the cranium. 4-WEEK EMBRYO © 2012 Pearson Education, Inc. Embryology Summary 28.3 The Development of the Vertebral Column (Part 2 of 6) The Development of the Vertebral Column Spinal cord Neural arch Mesenchyme of somite Tubercle of rib Spinal cord in spinal canal Tubercle of rib Spinous process Muscles of back Head of rib Transverse process Centrum of vertebra Cartilaginous rib Ventral body cavity 8 WEEKS The cartilages of the vertebral centra grow around the spinal cord, creating a model of the complete vertebra. In the cervical, thoracic, and lumbar regions, articulations develop where adjacent cartilaginous blocks come into contact. In the sacrum and coccyx, the cartilages fuse together. © 2012 Pearson Education, Inc. Ossification centers 12 WEEKS About the time the ribs separate from the vertebrae, ossification begins. Only the shortest ribs undergo complete ossification. In the rest, the distal portions remain cartilaginous, forming the costal cartilages. Several ossification centers appear in the sternum, but fusion gradually reduces the number. Embryology Summary 28.3 The Development of the Vertebral Column (Part 3 of 6) The Development of the Vertebral Column BIRTH At birth, the vertebrae and ribs are ossified, but many cartilaginous areas remain. For example, the anterior portions of the ribs remain cartilaginous. Additional growth will occur for many years; in vertebrae, the bases of the neural arches enlarge until ages 3–6, and the spinal processes and vertebral bodies grow until ages 18–25. © 2012 Pearson Education, Inc. Embryology Summary 28.3 The Development of the Vertebral Column (Part 4 of 6) The Development of the Vertebral Column Sclerotome Notochord Intersegmental mesenchyme Somites Cartilage of vertebral body 4 WEEKS Cells of the sclerotomal segments migrate away from the somites and cluster around the notochord. © 2012 Pearson Education, Inc. 6 WEEKS The migrating cells differentiate into chondroblasts and produce a series of cartilaginous blocks that surround the notochord. These cartilages, which will develop into the vertebral centra, are separated by patches of mesenchyme. Embryology Summary 28.3 The Development of the Vertebral Column (Part 5 of 6) The Development of the Vertebral Column Intervertebral disc Vertebra Nucleus pulposus 8 WEEKS ADULT Expansion of the vertebral centra eventually eliminates the notochord, but it remains intact between adjacent vertebrae, forming the nucleus pulposus of the intervertebral discs. Later, surrounding mesenchymal cells differentiate into chondroblasts and produce the fibrous cartilage of the anulus fibrosus. © 2012 Pearson Education, Inc. Embryology Summary 28.3 The Development of the Vertebral Column (Part 6 of 6) The Development of the Vertebral Column 8 WEEKS 9 WEEKS Rib cartilages expand away from the developing transverse processes of the vertebrae. At first they are continuous, but by week 8 the ribs have separated from the vertebrae. Ribs form at every vertebra, but in the cervical, lumbar, sacral, and coccygeal regions they remain small and later fuse with the growing vertebrae. The ribs of the thoracic vertebrae continue to enlarge, following the curvature of the body wall. When they reach the ventral midline, they fuse with the cartilages of the sternum. © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Appendicular Skeleton • 4 weeks • Ridges appear along the length of the embryo • An accumulation of ridges at the superior end and the inferior end form pairs of limb buds • 5 weeks • Limb buds develop a core of cartilaginous tissue • Cartilaginous models develop between 5 and 8 weeks • 10 weeks • Ossification occurs © 2012 Pearson Education, Inc. Embryology Summary 28.4 The Development of the Appendicular Skeleton (Part 1 of 4) The Development of the Appendicular Skeleton Cartilage primordia Notochord Cartilaginous core of limb bud Limb buds 4 WEEKS In the fourth week of development, ridges appear along the flanks of the embryo, extending from just behind the throat to just before the anus. These ridges form as mesodermal cells congregate beneath the ectoderm of the flank. Mesoderm gradually accumulates at the end of each ridge, forming two pairs of limb buds. © 2012 Pearson Education, Inc. Mesenchyme 5 WEEKS After 5 weeks of development, the pectoral limb buds have a cartilaginous core and scapular cartilages are developing in the mesenchyme of the trunk. Embryology Summary 28.4 The Development of the Appendicular Skeleton (Part 2 of 4) The Development of the Appendicular Skeleton 51/2 WEEKS The formation of the pelvic girdle and legs closely parallels that of the pectoral complex. But as the pelvic limb bud enlarges, the apical ridge rotates medially rather than laterally. As a result, the knee joint faces posteriorly, while the elbow faces anteriorly. Pelvic girdle 5 WEEKS 8 WEEKS By week 8, cartilaginous models of all of the major skeletal components are well formed, and endochondral ossification begins in the future limb bones. Ossification of the hip bones begins at three separate centers that gradually enlarge. © 2012 Pearson Education, Inc. Lower limb 7 WEEKS Embryology Summary 28.4 The Development of the Appendicular Skeleton (Part 3 of 4) The Development of the Appendicular Skeleton Cartilaginous core of scapula Apical ridge Humerus Mesenchyme of pectoral girdle 51/2 WEEKS As the limb bud enlarges, bends develop at the future locations of the shoulder and elbow joints. Two cartilages form in the forearm, and a lteral rotation of the apical ridge places the elbow in its proper orientation. 7 WEEKS The hands originate as paddles, but the death of cells between the phalangeal cartilages produces individual fingers. 5 WEEKS Cartilage Joint cavity Humerus Ossified bone Scapula Joints form where two cartilages are in contact. The surfaces within the joint cavity remain cartilaginous, while the rest of the bones undergo ossification. © 2012 Pearson Education, Inc. Embryology Summary 28.4 The Development of the Appendicular Skeleton (Part 4 of 4) The Development of the Appendicular Skeleton BIRTH The skeleton of a newborn infant. Note the extensive areas of cartilage (blue) in the humeral head, in the wrist, between the bones of the palm and fingers, and in the hips. Notice the appearance of the axial skeleton, with reference to the two previous Embryology Summaries. © 2012 Pearson Education, Inc. 10 WEEKS Ossification in the embryonic skeleton after approximately 10 weeks of development. The shafts of the limb bones are undergoing rapid ossification, but the distal bones of the carpus and tarsus remain cartilaginous. Embryology Summary • The Development of Muscles • 4 weeks • Mesoderm on either side of the notochord forms somites • The medial portion of each somite forms skeletal muscles; this region is called the myotome • 6 weeks • Myotome area develops into posterior muscles called epaxial muscles and anterior muscles called hypaxial muscles © 2012 Pearson Education, Inc. Embryology Summary 28.5 The Development of the Muscles (Part 1 of 4) The Development of the Muscles Near the head, mesoderm forms skeletal muscle associated with the pharyngeal arches. Pharyngeal arches Eye Mesoderm from the parietal portion of the lateral plate and the adjacent myotome forms the limb buds. Myotome Sclerotome Gut Limb bud Migrating mesodermal cells (arrows show directions of movement) Lateral plate mesoderm (parietal layer) Lateral plate (visceral layer) Heart Coelom Somites Somites Umbilical stalk After 4 weeks of development, mesoderm on either side of the notochord has formed somites. The medial portion of each somite will form skeletal muscles; this region is called the myotome. 4 WEEKS © 2012 Pearson Education, Inc. The ventral mesoderm does not form segmental masses, and it remains as a sheet called the lateral plate. A cavity appears within the lateral plate of the chest and abdomen; this cavity is the coelom. Formation of the coelom divides the lateral plate into an inner visceral layer and an other parietal layer. Embryology Summary 28.5 The Development of the Muscles (Part 2 of 4) The Development of the Muscles Eye muscles Hypaxial mesoderm in the trunk grows around the body wall toward the sternum in company with the ribs. This creates a mesodermal layer that extends from the chin to the pelvic girdle. Epaxial muscles Hypaxial muscles Extensors Arm bud Lung Rib Flexors Heart Myotomal muscles organize around the developing vertebral column in two groups, one dorsal (epaxial muscles) and the other ventral (hypaxial muscles). 6 WEEKS © 2012 Pearson Education, Inc. The hypaxial mesoderm near the sacrum migrates caudally to produce the muscles of the pelvic floor. Sternum Each limb bud has a flattened distal tip, with a thickened apical ridge. As cartilages appear in the limb buds, surrounding mesodermal cells from the lateral plate and myotomes differentiate into myoblasts. Embryology Summary • The Development of Muscles (continued) • 7 weeks: muscles of the face, back, and abdomen develop • 8 weeks: limbs enlarge and muscles continue to develop © 2012 Pearson Education, Inc. Embryology Summary 28.5 The Development of the Muscles (Part 3 of 4) The Development of the Muscles Muscles forming at the pharyngeal arches are associated with the head and neck. The muscles of mastication develop from the mesoderm surrounding the mandibular arch. Mesoderm of the hyoid (second) arch migrates over the lateral and ventral surfaces of the neck and the surfaces of the skull to form the muscles of facial expression. Mesoderm of the third, fourth, and sixth pharyngeal arches forms the pharyngeal and intrinsic laryngeal muscles. Epaxial muscles remain arranged in segments. These deep muscles include the intervertebral muscles. Superficial epaxial muscles form the major muscles of the erector spinae group. Intervertebral muscles Erector spinae Extensors Eye muscles Pharyngeal myoblasts form a superficial layer that later subdivides to create the trapezius and sternocleidomastoid muscles. Flexors Migration of myoblasts over the dorsal surface of the trunk creates limb extensors; migration of ventral myoblasts produces the flexors. Quadratus lumborum Transversus abdominis Stomach Internal oblique External oblique Rectus abdominis 7 WEEKS © 2012 Pearson Education, Inc. The oblique, transverse, and rectus muscle groups develop in the hypaxial layer. Embryology Summary 28.5 The Development of the Muscles (Part 4 of 4) The Development of the Muscles Flexors Extensors Flexors 8 WEEKS BIRTH Rotation of the arm and leg buds produces a change in the position of these masses relative to the body axis. © 2012 Pearson Education, Inc. While the limb buds enlarge, additional myoblasts invade the limb from myotomal segments nearby. Lines indicate the boundaries between myotomes providing myoblasts to the limb. Embryology Summary • The Development of the Nervous System • 2 weeks • Somites appear on either side of the notochord • A neural plate is formed • A crease develops along the length of the neural plate • The edges of the crease are the neural folds • The neural folds “fold” to form a tube (neural tube) • 3 weeks • The neural tube becomes the CNS • Axons extending from the neural tube become the PNS © 2012 Pearson Education, Inc. Embryology Summary 28.6 The Development of the Nervous System (Part 1 of 2) The Development of the Nervous System Neural plate Neural plate Notochord Somite Neural groove 20 DAYS Neural fold Neural tube 21 DAYS © 2012 Pearson Education, Inc. After two weeks of development, somites are appearing on either side of the notochord. The ectoderm near the midline thickens, forming an elevated neural plate. The neural plate is largest near the future head of the developing embryo. A crease develops along the axis of the neural plate, creating the neural groove. The edges, or neural folds, gradually move together. They first contact one another midway along the axis of the neural plate, near the end of the third week. Where the neural folds meet, they fuse to form a cylindrical neural tube that loses its connection with the superficial ectoderm. The process of neural tube formation is called neurulation; it is completed in less than a week. The formation of the axial skeleton and that of the musculature around the developing neural tube were described on pages 770 and 774. Embryology Summary 28.6 The Development of the Nervous System (Part 2 of 2) The Development of the Nervous System Neurocoel Head Neural crest Cells at the tips of the neural folds do not participate in neural tube formation. These cells of the neural crest at first remain between the dorsal surface of the neural tube and the ectoderm, but they later migrate to other locations. The neural tube becomes the CNS. Axons from neurons within the neural tube and the axons of neural crest cells form the PNS. Schwann cell Sensory neurons Somites Neural crest Autonomic motor neurons Ependymal layer Mantle layer Marginal layer 23 DAYS Ependymal cells CNS neurons The first cells to appear in the mantle differentiate into neurons, while the last cells to arrive become astrocytes and oligodendrocytes. Further development of the CNS and PNS will be found in the Embryology Summaries later in this chapter. Astrocytes and oligodendrocytes The neural tube increases in thickness as its epithelial lining undergoes repeated mitoses. By the middle of the fifth developmental week, there are three distinct layers. The ependymal layer lines the enclosed cavity, or neurocoel. The ependymal cells continue their mitotic activities, and daughter cells create the surrounding mangle layer. Axons from developing neurons form a superficial marginal layer. © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Spinal Cord • 22 days • The neural tube is closed thus producing three layers: • Ependymal layer • Mantle layer • Marginal layer • Neurons develop in the mantle layer • Axons grow from the mantle layer to the marginal layer © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Spinal Cord (continued) • 23 days • Axons form bundles and tracts in the spinal cord • 7 weeks • • • • Ventral roots and dorsal roots develop Glial cells develop Spinal and cranial meninges develop Spinal nerves develop © 2012 Pearson Education, Inc. Embryology Summary 28.7 The Development of the Spinal Cord, Part I (Part 1 of 2) The Development of the Spinal Cord, Part I Ectoderm Neural crest Neural tube Neurocoel Ependymal layer Mantle layer Marginal layer 22 DAYS By the end of the fifth developmental week, the neural tube is almost completely closed. In the spinal cord the mantle layer that contains developing neurons and neuroglial cells will produce the gray matter that surrounds the neurocoel. As neurons develop in the mantle layer, their axons grow toward central or peripheral destinations. The axons leave the mantle layer and travel toward synaptic targets within a peripheral marginal layer. © 2012 Pearson Education, Inc. Neuroepithelial (ependymal) layer 23 DAYS Eventually, the growing axons will form bundles, or tracts, in the marginal layer, and these tracts will crowd together in the columns that form the white matter of the spinal cord. Mantle layer Marginal layer 28 DAYS Embryology Summary 28.7 The Development of the Spinal Cord, Part I (Part 2 of 2) The Development of the Spinal Cord, Part I By this time, cells of the neural crest have migrated to either side of the spinal cord and have formed the dorsal root ganglia. The neural crest cells become sensory neurons and glial cells (Schwann cells and satellite cells). Processes from these sensory neurons grow both into the periphery, to contact receptors, and into the CNS via the dorsal roots. Roof plate Dorsolateral plate Dorsal root Dorsal root ganglion Ventrolateral plate In each segment the axons of developing motor neurons form a pair of ventral roots that grow away from the spinal cord. Distal to each dorsal root ganglion, the motor efferents of the ventral root and the sensory afferents of the dorsal root are bound together into a single spinal nerve. Along much of the spinal cord, these nerves share a stereotyped pattern of peripheral branches, and the pattern accounts for the distribution of dermatomes. © 2012 Pearson Education, Inc. Floor plate 7 WEEKS As the mantle enlarges, the neurocoel becomes laterally compressed and relatively narrow. The relatively thin roof plate and floor plate will not thicken substantially, but the dorsolateral and ventrolateral plates enlarge rapidly. Neurons developing within the dorsolateral plate will receive and relay sensory information, while those in the ventrolateral region will develop into motor neurons. Embryology Summary 28.8 The Development of the Spinal Cord, Part II (Part 1 of 3) The Development of the Spinal Cord, Part II In addition to forming dorsal root ganglia and associated glial cells, neural crest cells migrate around the central nervous system and develop into the spinal and cranial meninges. Larynx Teeth Dorsal root ganglia Suprarenal medulla Meninges Autonomic ganglia Melanocytes 7 WEEKS (Distribution of neural crest cells) Neural crest cells aggregate to form autonomic ganglia near the vertebral column and in peripheral organs. Migrating neural crest cells contribute to the formation of teeth and form the laryngeal cartilages, melanocytes of the skin, the skull, connective tissues around the eye, the intrinsic muscles of the eye, Schwann cells, satellite cells, and the suprarenal medullae. © 2012 Pearson Education, Inc. Embryology Summary 28.8 The Development of the Spinal Cord, Part II (Part 2 of 3) The Development of the Spinal Cord, Part II Cranial nerves and ganglia Eye Cervical plexus Brachial plexus Spinal nerves Lumbosacral plexus 7 WEEKS (Peripheral nerve distribution) Several spinal nerves innervate each developing limb. When embryonic muscle cells migrate away from the myotome, the nerves grow right along with them. If a large muscle in the adult is derived from several myotomal blocks, connective tissue partitions will often mark the original boundaries, and the innervation will always involve more than one spinal nerve. © 2012 Pearson Education, Inc. Embryology Summary 28.8 The Development of the Spinal Cord, Part II (Part 3 of 3) The Development of the Spinal Cord, Part II DEVELOPMENTAL ABNORMALITIES Spina bifida Spina bifida (BI-fi-da) results when the developing vertebral laminae fail to unite due to abnormal neural tube formation at that site. The neural arch is incomplete, and the meninges bulge outward beneath the skin of the back. The extent of the abnormality determines the severity of the defects. In mild cases, the condition may pass unnoticed; extreme cases involve much of the length of the vertebral column. © 2012 Pearson Education, Inc. Neural tube defect A neural tube defect (NTD) is a condition that is secondary to a developmental error in the formation of the spinal cord. Instead of forming a hollow tube, a portion of the spinal cord develops as a broad plate. This is often associated with spina bifida. Neural tube defects affect roughly one individual in 1000; prenatal testing can detect the existence of these defects with an 80–85 percent success rate. Embryology Summary • The Development of the Brain • 23 days • The cephalic extension forms three brain vesicles • Prosencephalon • Mesencephalon • Rhombencephalon © 2012 Pearson Education, Inc. Embryology Summary 28.9 The Development of the Brain, Part I (Part 1 of 3) The Development of the Brain, Part I Before proceeding, briefly review the summaries of skull formation, vertebral column development, and development of the spinal cord in the previous Embryology Summaries. Mesencephalon Rhombencephalon Neurocoel Prosencephalon Cephalic area Neural tube The initial cephalic expansion occurs as the neurocoel enlarges, forming three distinct brain vesicles: (1) the prosencephalon (pros-enSEF-a-lon) or ―forebrain,‖ (2) the mesencephalon or ―midbrain,‖ and (3) the rhombencephalon (rom-ben-SEF-a-lon) or ―hindbrain.‖ The prosencephalon and rhombencephalon will be subdivided further as development proceeds. Even before neural tube formation has been completed, the cephalic portion begins to enlarge. Major differences in brain versus spinal cord development include (1) early breakdown of mantle (gray matter) and marginal (white matter) organization; (2) appearance of areas of neural cortex; (3) differential growth between and within specific regions; (4) appearance of characteristic bends and folds; and (5) loss of obvious segmental organization. 23 DAYS © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Brain (continued) • 4 weeks • The rhombencephalon subdivides to form: • Metencephalon • Myelencephalon • The prosencephalon subdivides to form: • Telencephalon • Diencephalon © 2012 Pearson Education, Inc. Embryology Summary 28.9 The Development of the Brain, Part I (Part 2 of 3) The Development of the Brain, Part I Mesencephalon Metencephalon Myelencephalon Diencephalon Telencephalon The rhombencephalon first subdivides into the metencephalon (meten-SEF-a-lon; meta, after) and the myelencephalon (mi-el-en-SEF-a-lon; myelon, spinal cord). The prosencephalon forms the telecephalon (tel-en-SEF-a-long; telos, end enkephalos, brain) and the diencephalon. The telencephalon begins as a pair of swellings near the rostral, dorsolateral border or the prosencephalon. 4 WEEKS © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Brain (continued) • 5 weeks • Mesencephalon develops • Cranial nerves begin to develop • 8 weeks • Choroid plexus develops • Cerebral hemispheres expand • 11 weeks • Cerebellum develops © 2012 Pearson Education, Inc. Embryology Summary 28.9 The Development of the Brain, Part I (Part 3 of 3) The Development of the Brain, Part I N IV Cranial nerves develop as sensory ganglia and link peripheral receptors with the brain, and motor fibers grow out of developing cranial nuclei. Special sensory neurons of cranial nerves I, II, and VIII develop in association with the developing receptors. The somatic motor nerves III, IV, and VI grow to the eye muscles; the mixed nerves (V, VII, IX, and X) innervate the pharyngeal arches (page 768). 5 WEEKS © 2012 Pearson Education, Inc. NV Developing ear N VII N III Myelencephalon Development of the mesencephalon produces a small mass of neural tissue with a constricted neurocoel, the aqueduct of the midbrain. N IX Pharyngeal arches NX N XI N XII As differential growth proceeds and the position and orientation of the embryo change, a series of bends, or flexures (FLEK-sherz), appears along the axis of the developing brain. Embryology Summary 28.10 The Development of the Brain, Part II (Part 1 of 2) The Development of the Brain, Part II Cephalic flexure N III The roofs of the diencephalon and myelencephalon fail to develop, leaving a thin ependymal layer in contact with the developing meninges. Blood vessels invading these regions create areas of the choroid plexus. N IV Pontine flexure NX N XI N XII Cervical flexure NI 8 WEEKS © 2012 Pearson Education, Inc. N II N VI N VII N VIII N IX As growth continues and the pontine flexure develops, the brain becomes more compact. The expanding cerebral hemispheres now dominate the superior and lateral surfaces of the brain. Migrating neuroblasts create the cerebral cortex, and underlying masses of gray matter develop into the basal nuclei. Embryology Summary 28.10 The Development of the Brain, Part II (Part 2 of 2) The Development of the Brain, Part II Cerebral hemisphere (telencephalon) Diencephalon Mesencephalon Cerebral hemisphere Cerebellum Pons Pons Medulla oblongata Cerebellum Medulla oblongata Spinal cord Cranial nerve XI After 11 weeks, the expanding cerebral hemispheres have overgrown the diencephalon. At the metencephalon, cortical formation and expansion produce the cerebellum, which overlies the nuclei and tracts of the pons. 11 WEEKS © 2012 Pearson Education, Inc. CHILD Embryology Summary • The Development of Special Sense Organs • 4 weeks • Optic bulges appear in the lateral walls of the prosencephalon • The bulges indent forming the optic cup • The epidermis covering the optic cup forms the lens • 5 weeks • A pair of thickened areas in the front of the prosencephalon form the olfactory receptors • When nerves innervate the tongue, the epithelial cells differentiate to form gustatory cells © 2012 Pearson Education, Inc. Embryology Summary • The Development of Special Sense Organs (continued) • 3 weeks • Otic regions begin to appear on the lateral sides of the rhombencephalon • 4 weeks • Deep pockets form in the otic regions, creating otic vesicles • 6 weeks • The vesicles form the bony labyrinth • This process continues until the end of the third month © 2012 Pearson Education, Inc. Embryology Summary 28.11 The Development of Special Sense Organs, Part I (Part 1 of 3) The Development of Special Sense Organs, Part I All special sense organs develop from the interaction between the epithelia and the developing nervous system of the embryo. VISION Neurocoel Prosencephalon Epidermis Optic vesicle Optic cup Choroid Retina Lens N II Lens placode Optic stalk Sclera Lens vesicle The first indication of optic development appears as a pair of bulges called optic vesicles in the lateral walls of the prosencephalon. These extend to either side like a pair of dumbbels, each containing a cavity continuous with the neurocoel. © 2012 Pearson Education, Inc. These bulges become indented, forming a pair of optic cups, which remain connected to the diencephalon by optic stalks. The epidermis overlying the optic cup responds by forming a lens placode, which thickens and creates another vesicle. This lens vesicle becomes the lens. Mesoderm aggregating around this complex forms the choroid and scleral coats. The anterior and posterior chambers develop as cavities appear within the mesoderm. Embryology Summary 28.11 The Development of Special Sense Organs, Part I (Part 2 of 3) The Development of Special Sense Organs, Part I OLFACTION Nasal placode Eye External nares Nasal placode © 2012 Pearson Education, Inc. 5 WEEKS Olfactory receptors begin as a pair of thickened areas in front of the prosencephalon during the fifth developmental week. The thickenings are called nasal placodes. 10 WEEKS Over time, the nasal placodes are enfolded and protected by developing facial structures. (Development of the face was discussed in the previous Embryology Summary of the Skull.) Embryology Summary 28.11 The Development of Special Sense Organs, Part I (Part 3 of 3) The Development of Special Sense Organs, Part I GUSTATION Epithelial cells Sensory neuron Gustatory receptors are the least specialized of any of the special sense organs. Taste buds develop as sensory fibers grow into the developing mouth and pharynx. © 2012 Pearson Education, Inc. Taste buds When the nerve endings contact epithelial cells, the epithelial cells differentiate into gustatory cells. If the sensory nerves are cut, the taste buds degenerate; if the sensory nerve is moved, it will stimulate the development of new taste buds at its new location. Embryology Summary 28.12 The Development of Special Sense Organs, Part II (Part 1 of 2) The Development of Special Sense Organs, Part II Neural groove EQUILIBRIUM AND HEARING Late in the third week of development, a pair of otic placodes appears on either side of the rhombencephalon. Otic placode Pharynx Otic placode 3 WEEKS The otic placodes form deep pockets that subsequently lose their connection with the epidermis, creating hollow otic vesicles. Neural tube Otic vesicle Epidermis Tail 4 WEEKS These vesicles gradually change shape, forming the membranous labyrinth. This process has essentially been completed by the end of the third developmental month. Developing membranous labyrinth Ganglia of N VIII External pharyngeal groove Pharyngeal pouch 6 WEEKS © 2012 Pearson Education, Inc. Embryology Summary 28.12 The Development of Special Sense Organs, Part II (Part 2 of 2) The Development of Special Sense Organs, Part II Developing ossicles Vestibular ganglion Thickened portions of the otic vesicles differentiate into the spiral and vestibular ganglia, and their sensory terminals grow toward the developing hair cells. Spiral ganglion External acoustic meatus Cartilage Middle ear cavity Auditory tube 7 WEEKS Semicircular ducts Auricle Auditory ossicles External acoustic meatus Cochlea Tympanic membrane Temporal bone Middle ear cavity FULL TERM © 2012 Pearson Education, Inc. As these developments are underway, the surrounding mesenchyme begins to differentiate into cartilage. This cartilage will later ossify to form the bony labyrinth. Embryology Summary • The Development of the Endocrine System • Week 5 • Five pharyngeal pouches are formed • The pouches form the thymus gland, parathyroid gland, and thyroid gland • The pituitary gland forms as a pocket in the pharyngeal area; this ectodermal pocket breaks away from the pharyngeal area and develops just superior to the thyroid gland area © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Endocrine System • Week 5 (continued) • After the formation of the neural tube, some neural cells migrate away from the CNS and form an aggregate of cells that become the suprarenal glands © 2012 Pearson Education, Inc. Embryology Summary 28.13 The Development of the Endocrine System, Part I (Part 1 of 2) The Development of the Endocrine System, Part I As noted in Chapter 3, all secretory glands, whether exocrine or endocrine, are derived from epithelia. Endocrine organs develop from epithelia (1) covering the outside of the embryo, (2) lining the digestive tract, and (3) lining the coelomic cavity. PARATHYROID GLANDS AND THYMUS Ectoderm Neural tube First pharyngeal pouch I First pharyngeal cleft Pharyngeal arches Pharyngeal clefts Pharynx The dorsal masses of the third and fourth pouches form the parathyroid glands. The ventral masses move toward the midline and fuse to create the thymus gland. II III IV V–VI Endoderm WEEK 5 The pharyngeal region of the embryo plays a particularly important role in endocrine development. After 4–5 weeks of development, the pharyngeal arches are well formed. Human embryos develop five or six pharyngeal arches, not all visible from the exterior. (Arch 5 may not appear or may form and degenerate almost immediately.) The five major arches (I–IV, VI) are separated by pharyngeal clefts, deep ectodermal grooves. © 2012 Pearson Education, Inc. Developing ear Pharynx Parathyroids Cells originating in the walls of the small fifth pouch will be incorporated into the thyroid gland (see below), where they will differentiate into C thyrocytes. In sectional view, five pharyngeal pouches extend laterally toward the pharyngeal clefts. The first pouch lies caudal to the first (mandibular) arch. Pharyngeal pouches 5 and 6 are very small and are interconnected. Endoderm lining the third, fourth, and fifth pairs of pharyngeal pouches forms dorsal and ventral masses of cells that migrate beneath the endodermal epithelium. Thyroid Thymus Embryology Summary 28.13 The Development of the Endocrine System, Part I (Part 2 of 2) The Development of the Endocrine System, Part I THYROID GLAND Endoderm Ectoderm Ventral pocket WEEK 5, Mid-sagittal section The boundary between ectoderm and endoderm lies along the line formed by the circumvallate papillae of the tongue (see Figure 18.7, p. 477). This line roughly corresponds to the middle of the mandibular (first) arch. The thyroid gland forms here in the ventral midline. © 2012 Pearson Education, Inc. Thyroid gland The thyroid gland begins as a pocket in the ventral midline. As this pocket branches slightly, its walls thicken, and the paired masses lose their connection with the surface. Thyroid C thyrocytes As the embryo enlarges and changes shape, the thyroid shifts caudally to a position near the thyroid cartilage of the larynx. On its way, the thyroid gland incorporates C thyrocytes from the walls of the fifth pouch. Embryology Summary 28.14 The Development of the Endocrine System, Part II (Part 1 of 2) The Development of the Endocrine System, Part II PITUITARY GLAND WEEK 5, Mid-sagittal section The pituitary gland forms in the dorsal midline above the forming thyroid gland. © 2012 Pearson Education, Inc. Developing pituitary gland Hypothalamus Ectodermal pocket The pituitary gland has a compound origin. The first step is the formation of an ectodermal pocket in the dorsal midline of the pharynx. This pocket loses its connection to the pharynx, creating a hollow ball of cells that lies inferior to the floor of the diencephalon posterior to the optic chiasm. Anterior lobe Posterior lobe As these cells undergo division, the central chamber gradually disappears. This endocrine mass will become the adenohypophysis (anterior lobe) of the pituitary gland. The neurohypophysis (posterior lobe) of the pituitary gland begins as a depression in the hypothalamic floor and grows toward the developing adenohypophysis. Embryology Summary 28.14 The Development of the Endocrine System, Part II (Part 2 of 2) The Development of the Endocrine System, Part II SUPRARENAL GLANDS Neural crest cell mass Pharyngeal arches Mesothelium Lining of coelomic cavity Sympathetic preganglionic fibers Spinal cord Suprarenal medulla Migrating neural crest cells Suprarenal cortex Dorsal root ganglion Sympathetic chain ganglion Future suprarenal medulla Overlying epithelial cells respond by undergoing division, and the daughter cells surround the neural crest cells to form a thick suprarenal cortex. Digestive tube WEEK 5 Each suprarenal gland also has a compound origin. Shortly after the formation of the neural tube, neural crest cells migrate away from the CNS. This migration leads to the formation of the dorsal root ganglia and autonomic ganglia. On each side of the coelomic cavity, neural crest cells aggregate in a mass that will become a suprarenal medulla. For additional details concerning the development of other endocrine organs, refer to the subsequent Embryology Summaries on the Lymphoid, Digestive, Urinary, and Reproductive systems. © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Heart • Week 2 • The heart begins as a pair of tubes in the pharynx area • Week 3 • • • • The heart begins to pump The tubes fuse forming a one-chambered heart Two large veins bring blood to the heart One large artery carries blood away from the heart; the artery is called the truncus arteriosus © 2012 Pearson Education, Inc. Embryology Summary 28.15 The Development of the Heart (Part 1 of 3) The Development of the Heart Mesoderm Pharynx Pharynx Future pericardial cavity Neural groove Heart tubes WEEK 2 LATERAL VIEW During the second developmental week, the heart consists of a pair of thin-walled, muscular tubes beneath the floor of the pharynx. © 2012 Pearson Education, Inc. Future pericardial cavity WEEK 3 Truncus arteriosus Future pericardial cavity The lateral plate mesoderm Ventricle in this region has already Left atrial split into parietal and primordia visceral layers, creating a VENTRAL VIEW space that will eventually form the pericardial cavity. By the third week, the heart is pumping and circulating blood. The cardiac tubes have fused, producing a heart with a single central chamber. Two large veins bring blood to the heart, and a single large artery, the truncus arteriosus, carries blood to the general circulation. Embryology Summary • The Development of the Heart (continued) • Week 5 • The interatrial and interventricular septa begin to form, ultimately forming the four chambers of the heart • The foramen ovale and ductus arteriosum form • 1 year • The foramen ovale and ductus arteriosum have closed (at birth) • The remnant of the foramen ovale is the fossa ovalis • The remnant of the ductus arteriosum is the ductus ligamentum © 2012 Pearson Education, Inc. Embryology Summary 28.15 The Development of the Heart (Part 2 of 3) The Development of the Heart Pharynx Right atrium Left atrium Future interatrial septum Truncus arteriosus Aortic arches Ventricle Opening of sinus venosus Atrium Sinus venosus Future interventricular septum © 2012 Pearson Education, Inc. WEEK 5 In week 5, the interatrial and interventricular septa begin to subdivide the interior of the heart. WEEK 4 The heart elongates as the embryo grows larger. It curves back upon itself, forming an S-curve that gradually becomes more pronounced. The atrial and ventricular regions already differ in thickness. Embryology Summary 28.15 The Development of the Heart (Part 3 of 3) The Development of the Heart Interatrial septa Foramen ovale Fossa ovalis Right atrium Two interatrial septa develop, one overlapping the other. A gap between the two, called the foramen ovale, permits blood flow from the right atrium to the left atrium. Backflow from left to right is prevented by a flap that acts as a one-way valve. Until birth, this atrial short circuit diverts blood from the pulmonary circuit. © 2012 Pearson Education, Inc. Right ventricle AGE 1 YEAR Left atrium Left ventricle At birth, the foramen ovale closes, separating the pulmonary and systemic circuits in the heart. A shallow depression, the fossa ovalis, remains through adulthood at the site of the foramen ovale. (Other cardiovascular changes at birth are detailed in Figure 22.27, p. 600.) Embryology Summary • The Development of the Cardiovascular System • Week 4 • Several aortic arches form and fuse together to form the dorsal aorta • Some of the aortic arches disintegrate leaving ultimately just one aortic arch © 2012 Pearson Education, Inc. Embryology Summary 28.16 The Development of the Cardiovascular System (Part 1 of 6) The Development of the Cardiovascular System THE AORTIC ARCHES An aortic arch carries arterial blood through each of the pharyngeal arches. In the dorsal pharyngeal wall, these vessels fuse to create the dorsal aorta, which distributes blood throughout the body. The arches are usually numbered from I to VI, corresponding to the pharyngeal arches. I II III IV V VI VENTRAL VIEW Dorsla aorta Aortic arches Yolk sac 4 WEEKS We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.) © 2012 Pearson Education, Inc. Aortic arches Left dorsal aorta Right dorsal aorta Fused dorsal aorta Embryology Summary 28.16 The Development of the Cardiovascular System (Part 2 of 6) The Development of the Cardiovascular System External carotid arteries Right common carotid artery Internal carotid artery Brachiocephalic trunk Common carotid arteries Aortic arch As development proceeds, some of these arches disintegrate. The ductus arteriosus provides an external short-circuit between the pulmonary and systemic circuits. Most of the blood entering the right atrium bypasses the lungs, passing instead through the ductus arteriosus or the foramen ovale in the heart. Ductus arteriosus Pulmonary artery Dorsal aorta Aortic arches Yolk sac 4 WEEKS We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.) © 2012 Pearson Education, Inc. Right subclavian artery The left half of arch IV ultimately becomes the aortic arch, which carries blood away from the left ventricle. Left common carotid artery Left subclavian artery Ligamentum arteriosum Pulmonary artery Descending aorta Embryology Summary • The Development of the Cardiovascular System • Week 4 (continued) • Venous circulation begins with the following vessels draining the tissues: • Anterior cardinal veins • Posterior cardinal veins • Subcardinal veins • These veins are paired and each pair fuses to form: • Superior vena cava • Inferior vena cava © 2012 Pearson Education, Inc. Embryology Summary 28.16 The Development of the Cardiovascular System (Part 3 of 6) The Development of the Cardiovascular System Dorsal aorta THE VENAE CAVAE Aortic arches Anterior cardinal veins Heart Posterior cardinal veins Subcardinal veins Yolk sac 4 WEEKS We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.) © 2012 Pearson Education, Inc. DORSAL VIEW The early venous circulation draining the tissues of the body wall, limbs, and head centers around the paired anterior cardinal veins, posterior cardinal veins, and subcardinal veins. Embryology Summary 28.16 The Development of the Cardiovascular System (Part 4 of 6) The Development of the Cardiovascular System Posterior cardinal vein Right internal and external jugular veins Inferior vena cava Superior vena cava Inferior vena cava Right common iliac vein Interconnections form among these veins, and a combination of fusion and disintegration produces moredirect, larger-diameter connections to the right atrium. Dorsal aorta Aortic arches Yolk sac 4 WEEKS We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.) © 2012 Pearson Education, Inc. This process continues, ultimately producing the superior and inferior venae cavae. Embryology Summary • The Development of the Cardiovascular System • Week 4 (continued) • Umbilical arteries carry blood to the placenta • Umbilical veins carry blood to the liver • Week 12 • Right umbilical vein disintegrates • Ductus venosus allows some blood to bypass the liver • Veins draining the digestive tract fuse to form the hepatic portal vein © 2012 Pearson Education, Inc. Embryology Summary 28.16 The Development of the Cardiovascular System (Part 5 of 6) The Development of the Cardiovascular System Dorsal aorta Aortic arches We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.) Yolk sac 4 WEEKS THE HEPATIC PORTAL AND UMBILICAL VESSELS Heart Liver Heart Ductus venosus Liver Hepatic portal vein Umbilical veins Right umbilical vein Umbilical arteries 4 WEEKS Paired umbilical arteries deliver blood to the placenta. At 4 weeks, paired umbilical veins return blood to capillary networks in the liver. Veins running along the length of the digestive tract have extensive interconnections. © 2012 Pearson Education, Inc. Left umbilical vein Digestive tract 12 WEEKS By week 12, the right umbilical vein disintegrates, and the blood from the placenta travels along a single umbilical vein. The ductus venosus allows some venous blood to bypass the liver. The veins draining the digestive tract have fused, forming the hepatic portal vein. Embryology Summary 28.16 The Development of the Cardiovascular System (Part 6 of 6) The Development of the Cardiovascular System Dorsal aorta Aortic arches We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.) Yolk sac 4 WEEKS Foramen ovale Pulmonary artery Descending aorta Inferior vena cava Pulmonary vein Descending aorta Hepatic portal vein Umbilical vein Liver Umbilical arteries FULL TERM Shortly before birth, blood returning from the placenta travels through the liver in the ductus venosus to reach the inferior vena cava. Much of the blood delivered by the venae cavae bypasses the lungs by traveling through the foramen ovale and the ductus arteriosus. © 2012 Pearson Education, Inc. Lung Ductus arteriosus NEWBORN At birth, pressures drop in the pleural cavities as the chest expands and the infant takes its first breath. The pulmonary vessels dilate, and blood flow to the lungs increases. Pressure falls in the right atrium, and the higher left atrial pressures close the valve that guards the foramen ovale. Smooth muscles contract the ductus arteriosus, which ultimately converts to the ligamentum arteriosum, a fibrous strand. Embryology Summary • The Development of the Lymphoid System • Week 7 • The thymus gland forms from a pharyngeal pouch • The cells lose their attachment and form just inferior to the thyroid gland area • Week 8 • Numerous lymphatic sacs form throughout the body © 2012 Pearson Education, Inc. Embryology Summary 28.17 The Development of the Lymphoid System (Part 1 of 4) The Development of the Lymphoid System Jugular lymph sac The development of the lymphatic vessels is closely tied to the formation of blood vessels. Paired jugular lymph sacs form from the fusion of small, endothelium-lined pockets in the mesoderm of the neck. By week 7, these sacs become connected to the venous system. Primordial lymph sacs Median lymph sac © 2012 Pearson Education, Inc. 7 WEEKS Primordial lymph sacs form parallel with veins of the trunk, and a large median lymph sac marks the future location of the cisterna chyli. Embryology Summary 28.17 The Development of the Lymphoid System (Part 2 of 4) The Development of the Lymphoid System Parathyroid The thymus forms from cells of the third pharyngeal pouch. These cells lose their connection with the epithelium and divide repeatedly. As the embryo changes shape, the thymic lobes are brought together near the midline of the chest. At birth, the thymus is relatively large, filling much of the anterior mediastinum. Third pharyngeal pouch Pharynx Thyroid 6 WEEKS Larynx Pharynx Thymus Thyroid 7 WEEKS Larynx Parathyroid Thyroid Esophagus Thymus Trachea 8 WEEKS © 2012 Pearson Education, Inc. Embryology Summary 28.17 The Development of the Lymphoid System (Part 3 of 4) The Development of the Lymphoid System Right lymphatic duct As growth continues, the isolated lymphatic sacs fuse, forming the thoracic duct and right lymphatic duct. As the limb buds enlarge, lymphatic vessels grow into the area along with developing arteries and veins. Thoracic duct Cisterna chyli © 2012 Pearson Education, Inc. 8 WEEKS Embryology Summary 28.17 The Development of the Lymphoid System (Part 4 of 4) The Development of the Lymphoid System Lymphatic sac Lymphocyte cluster Small blood vessels grow into areas where lymphocytes cluster within developing lymphatic sacs. Connective tissue capsules form, and the internal organization of a lymph node gradually appears. Lymph vessel Capsule © 2012 Pearson Education, Inc. Lymph node Embryology Summary • The Development of the Respiratory System • Week 3 • A pulmonary groove forms in the pharynx area • Week 4 • This groove forms deeper and then forms two blind pouches at the end of the groove • The groove forms a tube, which becomes the trachea, and the two blind pouches become the lungs • Week 9 • The diaphragm forms © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Respiratory System (continued) • 3 Months • The lung buds continue to branch numerous times • 6 Months • There are a million branches • All the bronchioles are formed • 6–9 Months • Alveoli begin to form © 2012 Pearson Education, Inc. Embryology Summary 28.18 The Development of the Respiratory System, Part I (Part 1 of 2) The Development of the Respiratory System, Part I THE LUNGS Pharyngeal pouches Pulmonary groove Lung buds Heart Yolk sac 3 WEEKS A shallow pulmonary groove appears in the midventral floor of the pharynx after roughly 31/2 weeks of development. This groove, which lies near the level of the last pharyngeal arch, gradually deepens. © 2012 Pearson Education, Inc. 4 WEEKS By week 4, the groove has become a blind pocket that extends caudally, anterior to the esophagus. This tube will become the trachea. At its tip, the tube branches, forming a pair of lung buds. Embryology Summary 28.18 The Development of the Respiratory System, Part I (Part 2 of 2) The Development of the Respiratory System, Part I The lung buds continue to elongate and branch repeatedly. Bronchioles Alveoli Over the next three months, each of the bronchioles gives rise to several hundred alveoli. This process continues for a variable period after birth. © 2012 Pearson Education, Inc. By the end of the sixth fetal month, there are around a million terminal branches, and the conducting passageways are complete to the level of the bronchioles. 3 MONTHS Embryology Summary 28.19 The Development of the Respiratory System, Part II (Part 1 of 3) The Development of the Respiratory System, Part II THE PLEURAL CAVITIES Digestive tube Lung buds Elongation of the tube carries it into the mediastinum, and as the branching proceeds, the lung buds project into the ventral cavity dorsal to the developing heart. 4 WEEKS © 2012 Pearson Education, Inc. Heart Embryology Summary 28.19 The Development of the Respiratory System, Part II (Part 2 of 3) The Development of the Respiratory System, Part II Esophagus Developing lung Pleuropericardial membrane 6 WEEKS © 2012 Pearson Education, Inc. The pericardial sac begins forming in week 6 as a thin pleuropericardial membrane forms between the heart and the developing lungs. Heart Embryology Summary 28.19 The Development of the Respiratory System, Part II (Part 3 of 3) The Development of the Respiratory System, Part II Pleural cavity Lung Heart Heart Pericardial cavity Pericardium Left lung Diaphragm Liver 9 WEEKS By week 9, the diaphragm completes its formation, forming a transverse sheet superior to the liver. © 2012 Pearson Education, Inc. 8 WEEKS By week 8, the pericardial sac is complete and the pericardial cavity is isolated from the rest of the ventral body cavity. The diaphragm then completes its formation, attaching to the pericardial sac and tissues of the mediastinum. This attachment separates the abdominopelvic cavity from the pleural cavities. Embryology Summary • The Development of the Digestive System • Week 3 • Yolk sac is formed • The layer between the yolk sac and the amniotic sac develops a hindgut and a foregut • The hindgut and the foregut form the digestive tube • Week 4 • Lateral to the digestive tube are cavities called the coelomic cavities • The digestive tube is held in position within the midline of the coelomic cavities by the dorsal mesentery and ventral mesentery © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Digestive System • Week 4 (continued) • The pancreas is formed within the dorsal mesentery area • The liver is formed within the ventral mesentery area • Part of the ventral mesentery becomes the falciform ligament of the liver • The remaining mesenteric tissue becomes the greater omentum and lesser omentum © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Digestive System (continued) • Week 6 • The intestines elongate and coil • The yolk sac and the body stalk fuse to form the umbilical cord © 2012 Pearson Education, Inc. Embryology Summary 28.20 The Development of the Digestive System, Part I (Part 1 of 3) The Development of the Digestive System, Part I Foregut Endoderm Amniotic cavity Amniotic cavity Somite Hindgut Hindgut Yolk sac 3 WEEKS By week 3, endodermal cells have migrated around the inside of the blastocyst, completing a pouch known as the yolk sac. © 2012 Pearson Education, Inc. Yolk sac Yolk stalk As the embryo forms on the embryonic shield, two pockets of endoderm are created: the foregut and hindgut. A broad connection between these pockets and the yolk sac remains within the yolk stalk. Developing coelom Mesoderm In sectional view, the embryonic gut is a simple endodermal tube surrounded by mesoderm. Cavities appearing within the mesoderm create the coelom (ventral body cavity). Embryology Summary 28.20 The Development of the Digestive System, Part I (Part 2 of 3) The Development of the Digestive System, Part I Neural tube Notochord Dorsal mesentery Digestive tube Coelomic cavity Yolk stalk Body stalk © 2012 Pearson Education, Inc. 4 WEEKS Ventral mesentery The digestive tube remains suspended in the coelom by a dorsal mesentery and a ventral mesentery. The ventral mesentery disintegrates everywhere except where major vessels or visceral organs have grown into it. It remains intact along the path of the umbilical arteries and where the umbilical vein and liver develop. Embryology Summary 28.20 The Development of the Digestive System, Part I (Part 3 of 3) The Development of the Digestive System, Part I Pancreas Neural tube Greater omentum Aorta Liver Lesser omentum Pancreas Liver As the embryo enlarges, the stomach and liver rotate toward the right, creating two pockets. The mesenteries that form these pockets are the greater omentum and the lesser omentum. Pancreas Liver Coelom Stomach Falciform ligament © 2012 Pearson Education, Inc. The pancreas and liver begin as epithelial pockets that grow away from the digestive tract and into the dorsal and ventral mesenteries, respectively. Embryology Summary 28.21 The Development of the Digestive System, Part II (Part 1 of 3) The Development of the Digestive System, Part II Liver Stomach The intestines begin to elongate, and with the breakdown of the ventral mesentery, they push outward into the umbilical stalk. Further elongation and coiling occur outside the body of the embryo. See sectional view at 4 weeks in Part I. 6 WEEKS Pancreas Allantois Cloaca Umbilical stalk The hindgut extends into the tail, where it forms a large chamber, the cloaca. A tubular extension of the cloaca, the allantois (a-LAN-to-is; allantos, sausage), projects away from the body and into the body stalk. Fusion of the yolk stalk and body stalk will create the umbilical stalk, also known as the umbilical cord. © 2012 Pearson Education, Inc. Embryology Summary 28.21 The Development of the Digestive System, Part II (Part 2 of 3) The Development of the Digestive System, Part II Entrance to trachea Esophagus Liver Pancreas Small intestine 8 WEEKS A partition grows across the cloaca, dividing it into a posterior rectum and an anterior urogenital sinus that retains a connection to the allantois. Urogenital sinus Rectum © 2012 Pearson Education, Inc. Embryology Summary 28.21 The Development of the Digestive System, Part II (Part 3 of 3) The Development of the Digestive System, Part II Heart 10 WEEKS Stomach Gallbladder Small intestine Umbilical cord Urinary bladder © 2012 Pearson Education, Inc. By week 10, the intestines have begun moving back into the coelomic cavity, although they continue to grow longer. Embryology Summary • The Development of the Urinary System • Week 3.5 • The kidneys begin as seven pairs of tubules called pronephric tubes • Week 6 • Branches of the aorta extend toward the pronephric tubes • Week 8 • The allantois becomes the urinary bladder • Week 12 • Kidneys begin producing filtrate even though there aren’t any waste products in it © 2012 Pearson Education, Inc. Embryology Summary 28.22 The Development of the Urinary System, Part I (Part 1 of 3) The Development of the Urinary System, Part I The kidneys develop in stages along the axis of the urogenital ridge, a thickened area beneath the dorso-lateral wall of the coelomic cavity. Pronephros Mesonephros Metanephros Cloaca Urogenital ridge © 2012 Pearson Education, Inc. Kidney development proceeds along the cranial/caudal axis of this ridge, beginning with the formation of the pronephros, continuing along the mesonephros, and ending with the development of the metanephros. Embryology Summary 28.22 The Development of the Urinary System, Part I (Part 2 of 3) The Development of the Urinary System, Part I Neural tube The pronephros consists of a series of tubules (generally 7 pairs) that appears within the nephrotome, the narrow band of mesoderm between the somites and the lateral plate. Notochord Somite Pronephric tubule Pronephric duct Nephrotome Lateral plate mesoderm The pronephric tubules are very small and nonfunctional, and they disintegrate almost at once. The only significant contribution of the pronephros is the formation of a pair of pronephric ducts that grow caudally until they connect to the cloaca. 31/2 WEEKS © 2012 Pearson Education, Inc. Embryology Summary 28.22 The Development of the Urinary System, Part I (Part 3 of 3) The Development of the Urinary System, Part I Pronephros Developing aorta Mesonephric duct Mesonephros Mesonephric duct Mesonephric tubule Metanephros Nephrotomal mesoderm of the metanephros forms a dense mass without a trace of segmental organization. This will become the functional adult kidney. 4 WEEKS © 2012 Pearson Education, Inc. After approximately 4 weeks of development, the mesoderm midway along the urogenital ridge begins organizing into the mesonephros. On either side of the midline, approximately 70 tubules develop within these segments. These tubules grow toward the adjacent pronephric duct and fuse with it. From this moment on, the duct is called the mesonephric duct. Embryology Summary 28.23 The Development of the Urinary System, Part II (Part 1 of 3) The Development of the Urinary System, Part II A ureteric bud, or metanephric diverticulum, forms in the wall of each mesonephric duct, and this blind tube elongates and branches within the adjacent metanephros. Tubules developing within the metanephros then connect to the terminal branch of the ureteric bud. Glomerulus Mesonephros Allantois Mesonephric duct Cloaca Metanephros Ureteric bud Most of the metabolic wastes produced by the developing embryo are passed across the placenta to enter the maternal circulation. The small amount of urine produced by the kidneys accumulates within the cloaca and the allantois, an endoderm-lined sac that extends into the umbilical stalk. 6 WEEKS © 2012 Pearson Education, Inc. Mesonephric duct Renal corpuscle In each segment, a branch of the aorta grows toward the nephrotome, and the tubules form large nephrons with enormous glomeruli. Like the pronephros, the mesonephros does not persist, and when the last segments of the mesonephros are forming, the first are already beginning to degenerate. Embryology Summary 28.23 The Development of the Urinary System, Part II (Part 2 of 3) The Development of the Urinary System, Part II Mesonephric duct Degenerating mesonephros Developing metanephros Urinary bladder Urogenital sinus Rectum 8 WEEKS Near the end of the second developmental month, the cloaca is subdivided into a dorsal rectum and a ventral urogenital sinus. The proximal portions of the allantois persist as the urinary bladder, and the connection between the bladder and an opening on the body surface will form the urethra. © 2012 Pearson Education, Inc. Embryology Summary 28.23 The Development of the Urinary System, Part II (Part 3 of 3) The Development of the Urinary System, Part II Nephron Collecting tubule Collecting duct Collecting system Major calyx Metanephros Ureteric bud Ureter 12 WEEKS The kidneys begin producing filtrate by the third developmental month. The filtrate does not contain waste products, as they are excreted at the placenta for removal and elimination by the maternal kidneys. The sterile filtrate mixes with the amniotic fluid and is swallowed by the fetus and reabsorbed across the lining of the digestive tract. © 2012 Pearson Education, Inc. The ureteric bud branches within the metanephros, creating the calyces and the collecting system. The nephrons, which form within the mesoderm of the metanephros, tap into the collecting tubules. Embryology Summary • The Development of the Reproductive System • Weeks 3–6 • Endodermal cells of the yolk sac migrate to the genital ridges in the abdominal cavity • Each ridge consists of columns of cells called primary sex cords; these will further develop into gonads © 2012 Pearson Education, Inc. Embryology Summary • The Development of the Reproductive System • Weeks 3–6 (continued) • If the gonad-developing cells are exposed to androgens, a male will develop, regardless of the genetic sex of the embryo • If they are not exposed to androgens, a female will develop, regardless of the genetic sex of the embryo • The male embryo will begin producing testosterone after week 6, which then directs the development of the external genitalia © 2012 Pearson Education, Inc. Embryology Summary 28.24 The Development of the Reproductive System: Sexually Indifferent Stages (Part 1 of 3) The Development of the Reproductive System SEXUALLY INDIFFERENT STAGES (WEEKS 3–6) DEVELOPMENT OF THE GONADS Aorta Gut Yolk sac Genital ridge Allantois Primary sex cords Mesonephric duct Paramesonephric (Müllerian) duct 3 WEEKS During the third week, endodermal cells migrate from the wall of the yolk sac near the allantois to the dorsal wall of the abdominal cavity. These primordial germ cells enter the genital ridges that parallel the mesonephros. © 2012 Pearson Education, Inc. Each ridge has a thick epithelium continuous with columns of cells, the primary sex cords, that extend into the center (medulla) of the ridge. Anterior to each mesonephric duct, a duct forms that has no connection to the kidneys. This is the paramesonephric (Müllerian) duct; it extends along the genital ridge and continues toward the cloaca. At this sexually indifferent stage, male embryos cannot be distinguished from female embryos. Embryology Summary 28.24 The Development of the Reproductive System: Sexually Indifferent Stages (Part 2 of 3) The Development of the Reproductive System SEXUALLY INDIFFERENT STAGES (WEEKS 3–6) DEVELOPMENT OF DUCTS AND ACCESSORY ORGANS Paramesonephric duct Mesonephric duct Gonad Both sexes have mesonephric and paramesonephric ducts at this stage. Unless exposed to androgens, the embryo—regardless of its genetic sex—will develop into a female. In a normal male embryo, cells in the core (medulla) of the genital ridge begin producing testosterone sometime after week 6. Testosterone triggers the changes in the duct system and external genitalia that are detailed on the following page. © 2012 Pearson Education, Inc. Kidney Cloacal opening Embryology Summary 28.24 The Development of the Reproductive System: Sexually Indifferent Stages (Part 3 of 3) The Development of the Reproductive System SEXUALLY INDIFFERENT STAGES (WEEKS 3–6) DEVELOPMENT OF EXTERNAL GENITALIA Urethral fold Genital tubercle Urogenital membrane Cloacal fold Genital swelling Cloacal membrane Anal fold 4 WEEKS 6 WEEKS After 4 weeks of development, there are mesenchymal swellings called cloacal folds around the cloacal membrane (the cloaca does not open to the exterior). The genital tubercle forms the glans of the penis in males and the clitoris in females. Two weeks later, the cloaca has been subdivided, separating the cloacal membrane into a posterior anal membrane, bounded by the anal folds, and an anterior urogenital membrane, bounded by the urethral folds. A prominent genital swelling forms lateral to each urethral fold. © 2012 Pearson Education, Inc. Embryology Summary • Development of the Male Reproductive System • Week 7 • The primary sex cords proliferate and eventually form the seminiferous tubules within the developing testes • Week 10 • The urethral folds begin to fuse to form the spongy urethra of the penis • This line of fusion can be seen in the adult as the raphe • 7 Months • The testes descend into the scrotum © 2012 Pearson Education, Inc. Embryology Summary 28.25 The Development of the Reproductive System: Development of the Male Reproductive System (Part 1 of 3) The Development of the Reproductive System DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM DEVELOPMENT OF THE TESTES Degenerating mesonephric tubule Testis cords 7 WEEKS In the male, the primary sex cords proliferate and the germ cells migrate into the sex cords. The resulting testis cords will form the seminiferous tubules. © 2012 Pearson Education, Inc. Tunica albuginea Rete testis Testis cords (seminiferous tubules) 12 WEEKS Connections form between the arching testis cords and the adjacent mesonephric nephrons. Although these nephrons later degenerate, the seminiferous tubules remain connected to the mesonephric duct. Embryology Summary 28.25 The Development of the Reproductive System: Development of the Male Reproductive System (Part 2 of 3) The Development of the Reproductive System DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM DEVELOPMENT OF MALE DUCTS AND ACCESSORY ORGANS Seminal gland Rete testis Testis cord Developing testis Paramesonephric duct degenerates Prostate Paramesonephric duct Testis cords Mesonephric duct (becomes ductus deferens) Mesonephros Mesonephric duct © 2012 Pearson Education, Inc. Testis Epididymis Urogenital sinus 4 MONTHS A view of the testis and ducts of the left side as seen in frontal section. Note the location and orientation of the mesonephros relative to the developing testis. Ductus deferens After four months of development, the testis cords are connected to the remnants of the mesonephric tubules by the rete testis. The paramesonephric (Müllerian) duct has degenerated. 7 MONTHS Definitive organization after the testis has descended into the scrotum (see Figure 27.2, p. 719). Note the relationships between the definitive sex organs and the embryonic structures. Embryology Summary 28.25 The Development of the Reproductive System: Development of the Male Reproductive System (Part 3 of 3) The Development of the Reproductive System DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM DEVELOPMENT OF MALE EXTERNAL GENITALIA Urethral folds External urethral orifice Spongy urethra Glans of penis Urethral folds Line of fusion Scrotal swelling Scrotum Anus 10 WEEKS At 10 weeks, the genital tubercle has enlarged, the tips of the urethral folds are moving together to form the spongy urethra (see sectional views), and paired scrotal swellings have developed from the genital swellings. © 2012 Pearson Education, Inc. BIRTH In the newborn male, the line of fusion between the urethral folds is quite evident. Embryology Summary • Development of the Female Reproductive System • Weeks 7–12 • Primary sex cords degenerate • Primordial germ cells migrate to the genital ridge • The urethral folds do not fuse; they become the labia minora • Week 10 • The ovaries and uterus develop © 2012 Pearson Education, Inc. Embryology Summary 28.26 The Development of the Reproductive System: Development of the Female Reproductive System (Part 1 of 3) The Development of the Reproductive System DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM DEVELOPMENT OF THE OVARIES Primordial germ cells Uterine tube Mesonephric duct Primary sex cords Cortex 7 WEEKS © 2012 Pearson Education, Inc. In the female embryo, the primary sex cords degenerate and the primordial germ cells migrate into the outer region (cortex) of the genital ridge. Degenerating primary sex cords 12 WEEKS Embryology Summary 28.26 The Development of the Reproductive System: Development of the Female Reproductive System (Part 2 of 3) The Development of the Reproductive System DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM DEVELOPMENT OF FEMALE DUCTS AND ACCESSORY ORGANS Ovary Degenerating mesonephric tubules Cortex of ovary Peritoneal opening of uterine tube Mesonephric tubule remnants Ovary Ovarian ligament Mesonephros Paramesonephric (Müllerian) duct Uterus Uterus Uterune tube (from paramesonephric duct) Vagina Urogenital sinus 7 WEEKS 10 WEEKS The mesonephric tubules and duct degenerate; the paramesonephric (Müllerian) duct develops a broad opening into the peritoneal cavity. Note the fusion of the ducts and the separation of the common chamber, which will form the uterus, from the urogenital sinus. © 2012 Pearson Education, Inc. BIRTH Embryology Summary 28.26 The Development of the Reproductive System: Development of the Female Reproductive System (Part 3 of 3) The Development of the Reproductive System DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM COMPARISON OF MALE AND FEMALE EXTERNAL GENITALIA Males Females Penis Corpora cavernosa Corpus spongiosum Proximal shaft of penis Spongy urethra Bulbo-urethral glands Scrotum Clitoris Erectile tissue Vestibular bulbs Labia minora Vestibule Greater vestibular glands Labia majora DEVELOPMENT OF FEMALE EXTERNAL GENITALIA Clitoris Genital tubercle Labia minora Genital swelling Urethra Labia majora Urethral fold Urogenital membrane Opening to vagina Hymen Anus 7 WEEKS BIRTH In the female, the urethral folds do not fuse; they develop into the labia minora. The genital swellings will form the labia majora. The genital tubercle develops into the clitoris. The urethra opens to the exterior immediately posterior to the clitoris. The hymen remains as an elaboration of the urogenital membrane. © 2012 Pearson Education, Inc.
