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Lecture Outline • Terminology review • Fertilization • First week of development • Week 2 of development • Week 3-4 of development • Endochondral and intramembranous bone formation The second week of development is significant for: 1. The formation of the bilaminar disc (two-layers) – this will give rise to all the tissues and organs of the body 2. The completion of implantation Syncytiotrophoblast cells displace the endometrium around the implantation site The endometrium has undergone changes Cells have become filled with glycogen and lipids The nutrients spill into the connective tissue This is called the decidua reaction The syncytiotrophoblast is responsible for hormone production. hCG maintains the corpus luteum in the ovary, allowing it to continue to produce P+E. Ovary Uterus hCG Produces P+ E to maintain the pregnancy The cells of the embryoblast will also differentiate into 2 layers: 1. The epiblast- a layer of high, columnar cells adjacent to the amniotic cavity. 2. The hypoblast- A layer of small cuboidal cells adjacent to the blastocyst cavity. Together these layers form a flat disc. Amnioblasts (derived from the epiblast) separate and form the lining of the amniotic cavity. Cells from the hypoblast form a membrane that lines the inner surface of the cytotrophoblast. This forms the exocoelomic cavity or primitive yolk sac The cavities allow movement of the disc The primordial uteroplacental circulation is established The yolk sac contains no yolk- the embryo is nourished from the lacunar networks- but may have a role in selective transfer of nutrients New cells appear between the yolk sac and the cytotrophoblast They form a layer of loose connective tissue: extraembryonic mesoderm. Cavities or spaces appear in the extra-embryonic mesoderm The cavities form a new space- the chorionic cavity The primitive yolk sac is pinched off- a secondary or definitive yolk sac is formed The cavity divides the extraembryonic mesoderm into the 1. Extraembryonic somatic mesodermlining trophoblast and amnion 2. Extraembryonic splanchnic mesoderm- lines the yolk sac The chorion is formed by 1. Extraembryonic somatic mesoderm 2. Cytotrophoblast 3. Syncytiotrophoblast The chorion forms the wall of the chorionic cavitythe amniotic cavity and yolk sac are suspended in the chorionic cavity by the connecting stalk. 3 2 1 By day 14 the embryo has the form of a flat, bilaminar disc, ovoid in shape. In a localized area of the hypoblast, the cells become more columnar and form a thickened circle area, the prechordal plate. At the end of the second week: Trophoblast has had a period of growth- greater than the embryoblast. The 2 layer bilaminar disc is formed and will give rise to other tissues and structures. Lecture Outline • Terminology review • Fertilization • First week of development • Week 2 of development • Week 3-4 of development • Endochondral and intramembranous bone formation The third week is Floor of the amniotic cavity or epiblast significant for the conversion of the bilaminar disc to the trilaminar disc Gastrulation: The formation of all 3 germ layers. • Begins with the formation of the primitive streak. • Cells of the epiblast proliferate and migrate to the median plane of the embryonic disc. • Primitive node surrounds the primitive pit. The primitive groove and pit form from migration of epiblast cells towards the primitive streak. Upon arrival to the region of the streak, they detach from the epiblast and slip beneath it. Once the cells have invaginated: • Some displace the hypoblast = embryonic endoderm. • Cells between the epiblast and newly created endoderm form mesoderm. • Cells remaining in the epiblast then form ectoderm. Section Is Here The endoderm and ectoderm are closely adhered at the prechordal plate and the cloacal membrane. The prechordal plate is the primordium of the oropharyngeal membrane, the cloacal membrane is the primordium of the anus. Prechordal plate Cloacal membrane View looking down on the dorsal surface Longitudinal Section staff.um.edu Mesenchymal cells invaginating in the primitive pit move cranially forming a median process -the notochordal process. The notochordal process grows cranially between the ectoderm and the endoderm until it reaches the prechordal plate. The notochord is a cellular rod that develops by transformation of the notochordal process. The notochord serves as a basis of development of the axial skeleton. The notochordal process eventually becomes the notochordal plate. The cells proliferate and the ends of the plates fold to form the notochord. The notochord is important because: • The vertebral column and base of skull develop around it • It will degenerate, only adult remnant is the nucleus pulposus • It will induce the ectoderm to form the neural plate (when one population of cells influences the development of another population of cells -INDUCTION) Lateral mesoderm Neurulation The appearance of the notochord induces the overlying ectoderm form the neural plate. Cells of the neural plate make up the neuroectoderm. The induction of these cells and the formation of the neural tube is called neurulation. • The nervous system develops as a thickening of the ectoderm • The thickening is called the neuroectoderm and constitutes the neural plate • From this, the brain and spinal cord will develop Oropharyngeal membrane Neural plate Newly added cells Cloacal membrane Approximately 18 days The neural plate invaginates along the central axis to form a median neural groove surrounded by neural folds. By the end of the 3rd week the neural folds approach each other in the midline where they fuse. Fusion begins in the cervical region and proceeds cranially and caudally. As a result the primordium of the CNS, the neural tube, is formed. Approximately 20 days The neural tube separates from the surface ectoderm (which differentiates into the epidermis). Until fusion is complete, the cephalic and caudal ends of the neural tube communicate with the amniotic cavity by way of the cranial and caudal neuropores. Closure of the anterior neuropore occurs day 25, posterior day 27. As the neural folds elevate and fuse, cells at the lateral border of crest of the neuroectoderm begin to disassociate from their neighbors. These neural crest cells will undergo epithelial to mesenchymal transformation as they leave the neuroectoderm by active migration and displacement to enter the underlying mesoderm. Summary Neuroectoderm consists of : 1. Neural plate> neural tube>>brain and spinal cord. 2. Neural crest>>consists of pluripotent cells which migrate to all areas and give rise to formation of many organs and tissues. Intraembryonic mesoderm As the neural tube forms, the embryonic mesoderm on each side of it proliferates forming 3 distinct longitudinal columns of cells. The paraxial mesoderm (most medial) gives rise to the somites The intermediate mesoderm gives rise to the urogenital system The lateral mesoderm is continuous with the extraembryonic mesoderm covering the yolk sac and the amnion. Segmented blocks, called somites, first appear in the cephalic region- their formation proceeds cephalocaudally. The somites are located on each side of the neural tube. www.neoucom.edu During this stage of development the age of the embryo (approximate number of days) is correlated to the number of somites The somites eventually shift their position around the notochord and give rise to: Sclerotome- form vertebral column and base of the skull Myotome- segmental muscle component Dermatome- gives rise to the dermis and subcutaneous connective tissue www.neuro.wustl.edu Paraxial mesoderm cells arrange in concentric whorls around a small cavity Cells of the ventral/medial wall become less compact and migrate to the direction of the somite- these cells form the sclerotome Cells of the dorsal lateral aspect migrate as the dermomyotome The myotome cells are precursors to limb and body wall musculature The dermotome cells spread out under the ectoderm/epider mis to form the dermis epidermis Folding of the Embryo Folding occurs in 2 planes- longitudinal (cephalocaudal) and transverse planes (lateral folding). Longitudinal folding: due to brain development. Lateral folds: due to growth of the somite. Folding begins day 24usually complete by day 28 Longitudinal folding or cephalocaudal fold because it is most pronounced in the head and tail regions Head folding : • septum transversum, • the primordial heart, • pericardial cavity • oropharyngeal membrane • The endoderm (which is ventral and lines the yolk sac) is incorporated into the embryo and forms the foregut Head region The head fold forms the stomatodeum – the primitive oral cavity Thus ectoderm lines the oral cavity and is separated from the foregut by the oropharyngeal membrane Oropharyngeal membrane stomatodeum Amniotic cavity Tail folding: • allantois • primitive streak • cloacal membrane • connecting stalk The endoderm layer and a portion of the yolk sac is incorporated as the hindgut Tail fold Lateral Folding In the transverse plane As a result of the rapid growth of somites Embryo forms a round appearance Forms the ventral body wall and incorporates the midgut , which remains in communication with the yolk sac Initiation of folding- approximately 24 days Transverse section through the midgut showing the connection between the midgut and the yolksack Amniotic cavity ectoderm Connection b/w Gut and yolksac Intra embryonic cavity Section below the midgut to show the closed ventral abdominal wall ectoderm Intraembryonic cavity Amniotic cavity Lecture Outline • Terminology review • Fertilization • First week of development • Week 2 of development • Week 3-4 of development • Endochondral and intramembranous bone formation Development of Bone and Cartilage Mesenchyme is embryonic connective tissue Mesenchymal cells migrate to various areas and differentiate into several different cells, including fibroblasts, chondroblasts, osteoblasts The paraxial mesoderm forms segmented blocks on each side of the neural tube: Somitomeres in the head region Somites from the occipital region down Bone formation is NOT restricted to the sclerotome It also occurs in somatic mesoderm of the body wall and to form the pelvic and shoulder girdles and long bones of the limbs • NCC’s also differentiate into mesenchyme and participate in the formation of the bones of face • In some bones, primarily flat bones, mesenchyme differentiates directly into bone (intramembranous ossification) • In most bones, mesenchymal cells first give rise to hyaline cartilage models which ossify by endochondral ossification Membranous neurocranium Cartilaginous neurocranium Mesenchyme derived from neural crest cells (viscerocranium) • • • • • Development of Cartilage First appears during 5th week of development Mesenchyme condenses to form chondrification centers The cells loose their processes, become rounded and cluster together to form chondrification centers Chondroblasts (cartilage forming cells) secrete collagen and ground substance There are 3 types of cartilage- hyaline, fibrocartilage, and elastic cartilage Mesenchymal cells Chondrification Center Secreting the Fibers and Ground subs. Chondrocytes In lacunae Three types of cartilage are distinguished based on the make-up of the extracellular matrix: • Hyaline cartilage – most abundant in body, lies the template for endochondral bone formation. Articular cartilage is a specialized hyaline cartilage. • Elastic cartilage – elastic fibers added to matrix increase flexibility • Fibrocartilageincreased tensile strength Development of Bone Initial bone formation in two ways: Intramembranous bone formation – bone develops in well vascularized mesenchyme. Most flat bones develop this way (scapula, flat bones of the head) Intracartilaginous (endochondral) bone formation- bone forms in a cartilage model (limb bones) These names refer only to how the bone is initially formed The initial bone is replaced by remodeling that occurs later Intramembranous Ossification Bone is formed by differentiation of mesenchymal cells into (osteoprogenitor) osteoblasts Occurs in the mesenchyme where mesenchymal cells aggregate in the specific area that bone is to be formed A. The newly organized tissue becomes more vascularized- the cells more rounded B. The now differentiated osteoblasts secrete collagen and bone matrix C. The osteoblasts become separated as more bone matrix is produced D. When the matrix calcifies the cells are termed osteocytes, still interconnected by channels- osteoclasts are apparent- participate in bone modeling Endochondral Ossification • • • • (intracartilaginous) Occurs in preexisting cartilage In long bones this occurs in the diaphysis- the long part between the ends (shaft) This is the primary center of ossification Initially, a hyaline cartilage model is formed in the shape of bone The first sign of ossification is the formation of a bone collar around the cartilage model The cartilage in this area no longer gives rise to chondrocytes- instead osteoblasts (this part ONLY is intramembranous development) After the bone collar forms, the cells in the midregion of the cartilage become hypertrophic The cartilage matrix is resorbed and the cells are on a scaffolding of thin matrix The hypertrophic cells secrete a substance that causes the remaining cartilage matrix to calcify- this impairs diffusion and the chondrocytes die Blood vessels penetrate the bony collar and vascularize the cavity The vessels carry osteoprogenitor cells that differentiate into osteoblasts and lay down bone Ossification of limb bones begins at the end of the embryonic period- thus there is more demand on the maternal supply of calcium At birth the shafts are ossified, but the ends or the epiphyses are still cartilage The secondary ossification centers are in the epiphyses and are a postnatal event