Presentation on theme: "Gastrulation, Neurulation and Folding"— Presentation transcript:
1Gastrulation, Neurulation and Folding Dr Rania Gabr
2OBJECTIVES By the end of this lecture , the student should be able to: Define GastrulationDescribe the formation of the primitive streakDescribe the formation of the intraembryonic mesoderm and the Trilaminar discExplain the formation , function and fate of the notochordDefine NeurulationDescribe the formation of the neural plate, groove, fold , crest and canalUnderstand the process of folding its timing and results
3The significant event of third week is Gastrulation The Third WeekThe significant event of third week is GastrulationGastrulation: is the process of formation of the 3 germ layers (ectoderm, mesoderm & endoderm).
4GastrulationThe process by which the bilaminar disc is converted into a trilaminar discIt is the beginning of morphogenesis (formation of body form)Consists of formation of the primitive streak, the three germ layers & the notochordEmbryo is referred to as a Gastrula
5Primitive StreakThe primitive streak results from proliferation of the epiblastic cells in the median plane, in the caudal half of the epiblast, and lies along the cranio-caudal axis.Its cranial end forms the primitive nodeA groove, primitive groove, appears in the primitive streak, which continues with a small depression, primitive pit, in the primitive node.
6A circular thickening appears in the hypoblast near the cranial end, in the midline, to form the prechordal plate ( oropharyngeal membrane), that marks the future site of mouthA circular thickening appears in the hypoblast caudal to primitive streak in the midline to form the cloacal membrane, the future site of the anus
7Formation of Intraembryonic Mesoderm The epiblastic cells from the primitive streak (groove) proliferate to form mesenchymal tissueThe newly formed cells invaginate and migrate ventrally, laterally & cranially between the epiblast and hypoblast & organize to form the intraembryonic mesoderm
8Formation of Intraembryonic Mesoderm cont’d Intraembryonic mesoderm merges with the extra- embryonic mesoderm at the periphery of the embryonic discBy the end of 3rd week, mesoderm lies between embryonic ectoderm and endoderm everywhere EXCEPT in the region of prechordal plate and cloacal membrane, as the embryonic ectoderm & endoderm are fused at these regions
10Formation of Intraembryonic Mesoderm cont’d Some mesenchymal cells displace the hypoblasts forming the embryonic endodermCells remaining in the epiblast form the embryonic ectoderm
11Thus the EPIBLAST gives rise to all three germ layers, Ectoderm, Mesoderm, Endoderm in the embryo Each of the three germ layers gives rise to specific tissues and organs
12Fate of Primitive Streak Actively forms mesoderm until the early part of 4th weekThen it starts regressing and becomes an insignificant structure in the sacrocooccygeal regionsNormally it degenerates and disappears by the end of 4th weekRemnants may persist and give rise to a large tumor called Sacrococcygeal Teratomas
13NotochordA rod of mesenchymal cells located cranially, in the midline, extending between the primitive node and the prechordal plate
14Formation of Notochord Mesenchymal cells migrate cranially from the primitive pit towards the prechordal plate, and form a rod like notochordal processThe notochordal process becomes canalized forming a hollow tube, the notochordal canal, communicating with the primitive pit.
15Formation of Notochord cont’d The floor of the tube and the underlying endoderm break down, forming a notochordal plateThe notochordal plate becomes continuous with the endodermal layer.
16Formation of Notochord cont’d A temporary communication is established between the amniotic cavity and the yolk sac, termed the neurenteric canal.
18Functions of Notochord Defines primordial axis of the embryoProvides rigidity to the embryoServes as a basis for the development of the axial skeletonIndicates the future site of the vertebral bodies/columnRegulates differentiation of surrounding structures including the overlying ectoderm (neural plate) and mesoderm (somites).
19Fate of NotochordDegenerates and disappears as the bodies of the vertebrae develop, but it persists as the nucleus pulposus of each intervertebral discRemnants of notochordal tissue give rise to tumors called Chordomas
20Ectodermal Derivatives The Neurulation It is the process by which the neural tube is formed.The stages of neurulation include the formation of:Neural plateNeural grooveNeural folds & their fusionNeural crest cellsNeural tubeBegins during early part of the 4th week ( days)Ends by the end of 4th week (27 days)Is induced by the notochord
21The NeurulationUnder the inducing effect of the developing notochord, the overlying ectodermal cells thickens to form the neural plate
22The neural plate first appears: Cranial to the primitive node andDorsal to the developing notochord & the mesoderm adjacent to it
23As the notochord forms & elongates: The embryonic disc elongates and becomes club-shapedThe neural plate broadens and extends cranially as far as the buccopharyngeal membrane, and later on grows beyond it
24Neural foldOn 18th day: the neural plate invaginates to form neural groove & neural folds
25Some neuroectodermal cells along the crest of the neural fold differentiate as the neural crest cells.Neural crest cellsNeural fold
26By the end of 3rd week, the neural folds move to the midline and fuse to form the neural tube The fusion begins in the future cervical region and then extends both in cranial and caudal direction
27The neural tube separates from the surface ectoderm, lies in the midline, dorsal to the notochord
28Neural tube is open at both ends, communicating freely with the amniotic cavity. The cranial opening, the rostral neuropore closes at about 25th day & the caudal neuropore closes at about the 27th day
29The cranial ⅓ of the neural tube represent the future brain The caudal ⅔ represents the future spinal cord
30Folding Of EmbryoFolding means conversion of the flat trilaminar embryonic disc into a cylindrical embryo. Time:Folding of the embryo begins by the end of the 3rd week. It is completed by the 4th week.
31Folding of the embryo is due to rapid growth of the embryo specially the nervous system. The head folds first then the tail . At the same time, side to side folding occurs.