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• An important organizer, the notochord, develops between the ectoderm and endoderm extending from the primitive node cranially to the prechordal plate. • As the notochord develops and extends cranially, the primitive streak shrinks caudally. • Caudal to the primitive streak the ectoderm and endoderm are fused. This is the site of the future anus. • The embryonic disc enters the third week as an oval but slowly becomes narrower and longer by the end of the third week. • The mesoderm lies between the ectoderm and endoderm except in the • midline cranially where the ectoderm and endoderm are fused at the prochordal plate; • where the notochord lies between the ectoderm and endoderm; and • caudally at the cloacal membrane. • The notochord induces the overlying ectoderm to form the neural plate. • The notochord • The notochord is the structure around which the vertebral column forms. • As each vertebral body forms the notochord is replaced. • It persists in the spaces between adjacent vertebrae forming the intervertebral discs. • During development the notochord develops from the notochordal process which passes through a phase in which it forms a hollow rod which subsequently opens to both the amniotic and yolk sac cavities, followed by reversion to a solid rod. • Neurulation • The notochord induces changes in the overlying ectoderm resulting in the formation of the neural plate. • The neural plate ectoderm forms the central nervous system. •. • By about day 18 the neural plate extends from the primitive node to the oropharyngeal membrane (future mouth). • At this time a groove develops in the midline running from caudal to cranial, causing the neural plate to develop a fold on each side of the midline, the neural folds • The neural folds move together to form the neural tube leaving the cranial and caudal ends open. • As the folds move together to form the tube, the tube sinks below the ectoderm. • At this stage some of the cells which were on the edge of each fold separate from the neural tube as the neural crest. • The openings in the tube, the cranial and caudal neuropores remain open until the fourth week when the cranial neuropore closes followed by the caudal neuropore. • The cells of the neural crest migrate laterally to form various tissues including the peripheral ganglia, the adrenal medulla, the meninges covering the brain (pia and arachnoid), and skeletal and muscular components of the head. • Somites • The body of the developing embryo forms structural units, the somites. • The mesoderm lying lateral to the neural tube and notochord is formed into these units, beginning at the head end and moving caudally. • Each somite will have associated with it a strip of ectoderm, a block of mesoderm and eventually a spinal nerve. • The blood vessels will come to lie between adjacent blocks. The formation of somites starts by the end of the third week close to where the future occipital region will be. • The process continues caudally so that by the end of the fifth week there are about 43 pairs of somites. • Body cavities • In the adult there are four separate body cavities lined by a simple squamous epithelium, the mesothelium. • These cavities are invaginated by organs. The cavity contains a small amount of fluid to lubricate the moving surfaces and reduce friction. • The cavities are, the two pleural cavities, the pericardium (for the heart) and the peritoneal cavity (abdomen). • Between the second and third weeks of development a space expands within the mesoderm running up both sides and joining in the middle at the cranial end of the embryo to form a U shape. • As development proceeds the bend in the U is folded underneath and comes to be related to the heart. • The two limbs of the U form the pleural cavities at the cranial end, and fuse together to form the peritoneal cavity caudally.