4Formation of Amniotic Cavity As implantation of the blastocyst progresses, changes appear in the inner cell mass (embryoblast)A cavity, amniotic cavity appears separating embryoblast from the trophoblast, which soon becomes lined by amnioblasts derived from inner cell massThe cavity gradually increases in size and is filled with amniotic fluid
5Formation of Embryonic Disc The inner cell mass becomes flattened forming a circular bilaminar plate, the embryonic disc, consisting of two layers:Epiblast (ectoderm), thicker layer, consists of high columnar cells, and is related to the amniotic cavityHypoblast (endoderm), consists of small cuboidal cells, and lies adjacent to the blastocyst cavity
6Formation of Primitive Yolk Sac The blastocyst cavity becomes lined with exocelomic membrane and is called exocelomic cavityThe hypoblastic cells soon replace the exocelomic membrane and the cavity is then named as the primitive (primary) yolk sac
7At this stage the embryonic disc is: A circular bilaminar disc, that lies between the amniotic cavity and the primitive yolk sacThe epiblast forms the floor of the amniotic cavity & the hypoblast lies in the roof of the primitive yolk sac
8Formation of Extraembryonic Mesoderm Endoderm of the yolk sac gives rise to a layer of loosely arranged connective tissue, extraembryonic mesoderm (EEM), which surrounds the amniotic cavity and the yolk sac.
9Formation of Extraembryonic Celome Isolated spaces appear in the EEMThese spaces rapidly fuse to form a large fluid filled, C-shaped cavity, the extraembyonic celome surrounding the amniotic cavity and the yolk sac
10Formation of Connecting Stalk The region where no cavity has appeared, forms the connecting stalk, that connects the amniotic cavity, yolk sac and the embryonic disc to the outer wallThe site of the connecting stalk determines the caudal pole of the embryonic disc
11With the formation of extraembryonic celome: The extraembryonic mesoderm splits into two layers:an outer extraembryonic parietal (somatic) mesoderman inner extraembryonic visceral (splanchnic) mesoderm
12The primary yolk sac decreases in size and becomes the secondary (definitive) yolk sac Wall of the yolk sac, amnion & chorion are formed
13Wall of the yolk sac: Two layers: Amnion: Two layers:AmnioblastsExtraembronic splanchnic mesodermWall of the yolk sac: Two layers:EndodermChorion: Three layers:Extraembryonic somatic mesodermCytotrophoblastSyncytiotrophoblast
14Extraembryonic celome is now called the chorionamnionbilaminar discwall of the yolk sacConnecting stalkEECExtraembryonic celome is now called theCHORIONIC CAVITY
15Trilaminar Embryonic Disc The Third WeekThe significant event of third week is Gastrulation
16GastrulationThe 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
17Primitive 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 primitive nodeA groove, primitive groove, appears in the primitive streak, which continues with a small depression, primitive pit, in the primitive node.
18A circular thickening appears in the hypoblast near the cranial end, in the midline, to form the prechordal plate, 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
19By this stage of development, it is possible to identify the embryo’s: craniocaudal axiscranial and caudal endsdorsal and ventral surfacesright and left sides.Connecting stalk
20Formation of Intraembryonic Mesoderm The epiblastic cells from the primitive streak (groove) proliferate to form mesenchymal tissueThe newly formed cells invaginate, migrate ventrally, laterally & cranially between the epiblast and hypoblast & organize to form the intraembryonic mesoderm
21Formation 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
22Formation of Intraembryonic Mesoderm cont’d Some mesenchymal cells displace the hypoblasts forming the embryonic endodermCells remaining in the epiblast form the embryonic ectoderm
23Thus 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
24Fate 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
25NotochordA rod of mesenchymal cells located cranially, in the midline, extending between the primitive node and the prechordal plate
26Formation of Notochord Mesenchymal cells migrate cranially from primitive pit toward 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.
27Formation 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.
28Formation of Notochord cont’d A temporary communication is established between the amniotic cavity and the yolk sac, termed the neurenteric canal.
30Functions 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).
31Fate 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
32Differentiation of the Intraembryonic Mesoderm Induced by the notochordDifferentiates (in the region of notochord) into:Paraxial mesodermIntermediate cell massLateral plate mesoderm