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Lecture 4 Development Introductory Terminology Sexual reproduction - the process by which organisms produce offspring by making sex cells called gametes Male gametes are called sperm (spermatozoa) Female gametes are called secondary oocytes The organs that produce gametes are called gonads, (testes in the male, ovaries in the female) Pregnancy is a sequence of events that begins with fertilization, proceeds to implantation, embryonic development, and fetal development, and normally ends with birth about 38 weeks later Developmental biology is the study of the sequence of events from the fertilization of a secondary oocyte to the formation of an adult organism. Embryology is the study of the developing embryo From fertilization through the eighth week of development, the developing human is called an embryo and the stage is called the embryonic period The fetal period begins at week nine and continues until birth During this time the developing human is called a fetus Prenatal development includes both embryonic and fetal periods It is divided into three periods of three calendar months each called trimesters (first, second, and third trimesters). 2 The Path to Fertilization Fertilization: the genetic material from the haploid spermatozoon and haploid secondary oocyte merges into a single nucleus creating a diploid cell called a zygote The oocyte is viable for 12 to 24 hours Sperm is viable 24 t0 72 hours For fertilization to occur, coitus must occur no more than: Three days before ovulation 24 hours after ovulation Fertilization normally occurs in the uterine (fallopian) tube about 12 to 24 hours after ovulation Sperm swim (due to the actions of their tails) from the vagina into the cervical canal; the journey through the rest of the uterus and then into the uterine tubes results mainly from contractions of the walls of these organs 3 Fertilization Sperm undergo maturation in the epididymis, they are not able to fertilize an oocyte until they undergo capacitation in the female reproductive tract During this process, the sperm’s tail beats more vigorously and its plasma membrane’s ability to fuse with the oocyte’s plasma membrane is enhanced Binding of sperm cells to zona pellucida receptor molecules triggers the acrosomal reaction in which acrosomal enzymes are released to help the sperm cells penetrate the corona radiata and the zona pellucida Normally only one spermatozoon penetrates and enters a secondary oocyte- this event is called syngamy and it triggers events that prevent polyspermy 4 Fertilization When a spermatozoon enters a secondary oocyte: The oocyte completes meiosis II The nucleus of the ovum develops into a female pronucleus The nucleus of the sperm develops into a male pronucleus The two pronuclei fuse to form a single diploid nucleus that contains 46 chromosomes (23 from each pronucleus) The fertilized ovum is called a zygote 5 First Week of Development Cleavage: rapid mitotic cell divisions that occur immediately after fertilization Cell division occurs without cell growth producing progressively smaller cells called Blastomeres A morula is a solid sphere of blastomeres From about 16 cells until a central cavity forms Remains about the same size as the original zygote Still surrounded by the zona pellucida Blastocyst formation: After 4 or 5 days, the morula enters the uterine cavity It becomes bathed by glycogen-rich uterine milk secreted by the endometrium At about the 32-cell stage, this fluid enters the morula to form a fluid-filled blastocyst cavity 6 Fertilization The blastocyst hatches from the zona pellucida A blastocyst has the following components: An embryoblast or inner cell mass which develops into the embryo An outer covering of cells called the trophoblast which ultimately forms the outer chorionic sac about 5 days after fertilization Twins Dizygotic (fraternal) twins are produced from the independent release of two secondary oocytes and the subsequent fertilization of each by different spermatozoa Monozygotic (identical) twins develop from a single fertilized ovum that splits at an early stage in development, usually within 8 days after fertilization, into two embryos Conjoined twins identical twins are joined together and share some body structures from separations that occur later than 8 days 7 Implantation Implantation is the process by which the blastocyst attaches to and embeds itself within the endometrium; Occurs about 6 days after fertilization Following implantation, the endometrium is called the decidua As the blastocyst implants, usually on the posterior wall of the fundus or body of the uterus, it is oriented so that the inner cell mass faces the endometrium Different regions of the decidua are named based on their positions relative to the site of the implanted blastocyst Decidua basalis is the portion of the endometrium located between the embryo and the stratum basalis; it becomes the maternal part of the placenta Decidua capsularis is the portion of the endometrium located between the embryo and the uterine cavity Decidua parietalis is the remaining modified endometrium that lines the noninvolved areas of the rest of the uterus 8 Second Week of Development The trophoblast develops two layers in the region of contact between the blastocyst and endometrium: Outer syncytiotrophoblast which secretes enzymes that enable the blastocyst to penetrate into the endometrium Inner cytotrophoblast that is composed of distinct cells the trophoblast produces human chorionic gonadotropin (hCG) Bilaminar embryonic disc: a flat disc formed from the inner cell mass that differentiates into the: Epiblast (primitive ectoderm) Hypoblast (primitive endoderm) A small cavity appears in the epiblast and eventually enlarges to form the amniotic cavity 9 Second Week of Development Amnion: a thin, protective membrane that forms from the epiblast and initially overlies the bilaminar embryonic disc Comes to completely surround the embryo, creating an amniotic cavity Filled with amniotic fluid Serves as a shock absorber for the fetus Helps regulate fetal body temperature Prevents adhesions between the fetal skin and surrounding tissues Embryonic cells sloughed off into amniotic fluid may be examined via amniocentesis Usually ruptures just before birth and with its fluid constitutes the “bag of waters” Yolk sac: a thin, exocoelomic membrane that forms from the hypoblast, formerly called the blastocyst cavity In humans, is small, relatively empty, and progressively decreases in size Several important functions include supplying nutrients during the second and third weeks of development, providing blood cells, differentiating into primitive germ cells, forming part of the gut, acting as a shock absorber, and preventing dehydration of the embryo 10 Second Week of Development Sinusoids: Develop from small spaces within the syncytiotrophoblast called lacunae Lacunae fuse to form larger, interconnected lacunar networks Endometrial capillaries expand to form sinusoids Maternal blood and endometrial secretions enter the lacunar networks to provide embryonic nutrition and to serve as a disposal site for embryonic wastes Extraembryonic coelom: Extraembryonic mesoderm develops around the amnion and yolk sac Cavities develop in the extraembryonic mesoderm, which then form a single larger cavity called the extraembryonic coelom 11 Second Week of Development Chorion develops from the trophoblast and the extraembryonic mesoderm Surrounds the embryo and, later, the fetus Eventually becomes the major embryonic component of the placenta Protects the embryo and fetus from maternal immune responses and produces human chorionic gonadotropin (hCG) Inner layer of the chorion fuses with the amnion and the extraembryonic coelom is now called the chorionic cavity The bilaminar embryonic disc becomes connected to the trophoblast by the connecting (body) stalk, the future umbilical cord 12 Third Week of Development Gastrulation: the bilaminar embryonic disc is transformed into a trilaminar embryonic disc consisting of three primary germ layers Gastrulation begins with formation of the primitive streak, a faint groove on the dorsal surface; at the head end, a rounded primitive node develops Invagination results in formation of the three primary germ layers: ectoderm, mesoderm and endoderm 13 Third Week of Development A hollow tube called the notochordal process develops; then later forms a solid cylinder called the notochord, which plays a very important role in tissue induction The oropharyngeal membrane develops, but subsequently degenerates to connect the oral cavity to the throat and the remainder of the GI tract The cloacal membrane develops, but subsequently degenerates to form the openings of the anus, urinary, and reproductive tracts The allantois develops from an outpouching of the yolk sac that functions in early formation of blood and blood vessels and is associated with development of the urinary bladder 14 Neurulation and Development of Somites The embryonic process by which the neural plate, neural folds and neural tube form is called neurulation The head end of the neural tube develops into the brain The mesoderm segments into cube-shaped structures called somites Each somite differentiates into a: Myotome, which develops into skeletal muscles of the neck, trunk and limbs Dermatome, which forms connective tissue, including the dermis Sclerotome, which develops into vertebrae and ribs 15 Development of the Intraembryonic Coelom Small spaces appear in the lateral plate mesoderm and merge into a larger cavity called the intraembryonic coelom Consequently, the lateral plate mesoderm is split into the splanchnic mesoderm and the somatic mesoderm, each of which develops into specific structures 16 Development of the Cardiovascular System Copyright © 2004 Pearson Education, Inc. Angiogenesis, the formation of blood vessels, begins when Mesodermal cells differentiate into hemangioblasts Which then develop into angioblasts Which then aggregate to form blood islands; Which develop into blood vessels Blood cells arise from pluripotent stem cells The heart develops when: The cardiogenic area forms a pair of endocardial tubes Which form a single primitive heart tube The latter becomes S-shaped, begins to beat, and then joins with blood vessels 17 Placentation The process of forming the placenta, the site of exchange of nutrients and wastes between the mother and fetus Shaped like a pancake, the placenta consists of a fetal portion formed by chorionic villi and a maternal portion formed by the decidua basalis As embryonic tissue invades the uterine wall, maternal uterine vessels are eroded and maternal blood fills spaces called lacunae Fingerlike projections of the chorion called chorionic villi grow and blood capillaries develop within them Maternal and fetal blood do not normally mix Exchange of substances occurs between the maternal blood in the intervillous spaces and fetal blood flowing through capillaries within the chorionic villi 18 Placenta The placenta is a protective barrier, most microorganisms cannot pass through it Certain viruses such as those that cause AIDS, German measles, chickenpox, measles, encephalitis, and poliomyelitis can cross the placenta Certain drugs and alcohol can cause birth defects as they pass freely through the placenta The umbilical cord forms the actual connection between the placenta and embryo, which develops from the connecting stalk; it consists of: Two umbilical arteries carry deoxygenated fetal blood and wastes from the fetus to the placenta One umbilical vein carries oxygenated blood and nutrients from the placenta into the fetus Supporting mucous connective tissue is called Wharton’s jelly After birth, the placenta detaches from the uterus and is called the afterbirth The area where the umbilical cord was attached to the infant becomes scar tissue called the umbilicus (navel or belly button) 19 Fourth Week of Development Organogenesis, the development of body organs and systems, begins during the fourth through eighth weeks of development Embryonic folding produces formation of: A head fold, tail fold, and two lateral folds A primitive gut, Future pharyngeal region (throat) Future ears and eyes Upper limb buds and lower limb buds A visible heart prominence 20 Fifth Through Eighth Weeks of Development Rapid development of the brain and head Development of distinct limb regions Further development of the eyes and ears The tail disappears The external genitals begin to differentiate By the end of the eighth week, the embryo has clearly human characteristics 21 Fetal Development The remaining months of development are the fetal period, during which time the fetus continues to develop and grows at a remarkable rate The fetus takes on a human appearance Organs grow rapidly By the end of the third month, the placenta is functioning Third month Second trimester • A time of growth • Bone formation occurs • Hair and body are covered with fine hair called lanugo • By the end of the 6th month, the fetus is 30 cm (1 foot) long Development is essentially complete Except for lungs and brain Developing human is now called a fetus It carries out primitive reflexes like sucking The developmental biology of the various body systems will be described in their respective textbook chapters Fetal Development & Birth After birth, the umbilical cord is tied off and severed The short remnant of the cord withers and falls away usually within 12-15 days after birth Third trimester The area where the cord was attached develops a scar called the umbilicus (navel) Pace of growth accelerates Premature infant - “Preemie” Weight of fetus more than doubles Nutrients are still provided by mother’s blood via the placenta Most major nerve tracts are formed in the brain A baby weighing less than 2500 g at birth; The body of such a baby is not yet ready to sustain some critical functions Thus survival is uncertain without medical assistance After Birth Apgar Score At 1-5 minutes after birth, the infant’s physical status is assessed based on five signs: heart rate, respiration, color, muscle tone, and reflexes Each observation is given a score of 0 to 2 Apgar score = the total score of the above assessments 8-10 indicates a healthy baby Lower scores reveal problems Post Natal Development Babies typically double birth weight within a few months Allometric growth: Different body parts grow at different rates Nerve cells produced at an average rate of > 250,000 per minute At 6 months, neuron production ceases permanently 24 Labor Obstetrics is the medical specialty that deals with the management of pregnancy, labor, and the neonatal period (the first 28 days after birth) Labor is the process by which the fetus is expelled from the uterus through the vagina to the outside of the body parturition also means giving birth. True labor begins when uterine contractions (spreading as waves from the top of the uterus and moving downward) occur at regular intervals The contractions usually produce pain As the interval between contractions shortens, the contractions intensify There may be localization of pain in the back, which is intensified by walking Evidence of “show”, a discharge of blood-containing mucus that accumulates in the cervical canal during labor Dilation of the cervix 25 True Labor - Three Stages Stage of dilation is the time (6-12 hours) from the onset of labor to the complete dilation (to 10 cm) of the cervix There are regular contractions of the uterus The amniotic sac usually ruptures spontaneously Stage of expulsion is the time (10 minutes to several hours) from complete cervical dilation to delivery of the baby Placental stage is the time (5-30 minutes or more) after delivery until the placenta or “afterbirth” is expelled by uterine contractions These contractions constrict blood vessels that were torn during delivery which decreases the likelihood of hemorrhage 26 Maternal Changes During Pregnancy Anatomical Changes Chadwick’s sign – the vagina develops a purplish hue Breasts enlarge and their areolae darken The uterus expands, occupying most of the abdominal cavity and pushes against abdominal organs Lordosis is common due to the change of the body’s center of gravity Relaxin causes pelvic ligaments and the pubic symphysis to relax Typical weight gain is about 29 pounds Specific changes in the following systems: cardiovascular, respiratory, digestive, urinary, integumentary, and reproductive Structure of Lactating Mammary Glands Modified sweat glands consisting of 15-25 lobes that radiate around and open at the nipple Breast size is determined by the amount of adipose tissue, not the number of lobes and alveolar glands Areola – pigmented skin surrounding the nipple Suspensory ligaments attach the breast to underlying muscle fascia Lobes contain glandular alveoli that produce milk in lactating women Compound alveolar glands pass milk to lactiferous ducts, which open to the outside