Third Week Of Development:Trilaminar Germ Disc
• The most characteristic event occurring during the third week of gestation is gastrulation, the process that establishes all three germ layers( ectoderm, mesoderm, and endoderm) in the embryo.• Gastrulation begins with formation of the primitive streak on the surface of the epiblast.
• Initially, the streak is vaguely defined but in a 15- to 16-day embryo, it is clearly visible as a narrow groove with slightly bulging regions on either side.
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Gastrulation:
Cells of the epiblast migrate toward the primitive streak. The cephalic end of the streak, the primitive node, consists of a slightly elevated area surrounding the small primitive pit .
• Upon arrival in the region of the streak, they become flask-shaped, detach from the epiblast, and slip beneath it. This inward movement is known as invagination. Cell migration and specification are controlled by fibroblast growth factor 8 (FGF8), which is synthesized by streak cells themselves.
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• Once the cells have invaginated, some displace the hypoblast, creating the embryonic endoderm, and others come to lie between the epiblast and newly created endoderm to form mesoderm. Cells remaining in the epiblast then form ectoderm.
• Thus, the epiblast , through the process of gastrulation, is the source of all of the germ layers and cells in these layers will give rise to all of the tissues and organs in the embryo.
• The prechordal plate itself forms between the tip of the notochord and the oropharyngeal membrane and is derived from some of the first cells that migrate through the node in the midline and move in a cephalic direction.
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• Later, the prechordal plate will be important for induction of the forebrain. The oropharyngeal membrane at the cranial end of the disc consists of a small region of tightly adherent ectoderm and endoderm cells that represents the future opening of the oral cavity.
• Formation Of The Notochord
• Prenotochordal cells invaginating in the primitive node move forward cranially in the midline until they reach the prechordal plate.These prenotochordal cells become intercalated in the hypoblast so that for a short time, the midline of the embryo consists of two cell layers that form the notochordal plate. Cells of the notochordal plate proliferate and detach from the endoderm. They then form a solid cord of cells, the definitive notochord which underlies the neural tube and serves as the basis for the axial skeleton.
• The cloacal membrane is formed at the caudal end of the embryonic disc This membrane, which is similar in structure to the oropharyngeal membrane, consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm. When the cloacal membrane appears, the posterior wall of the yolk sac forms a small diverticulum that extends into the collecting stalk. This diverticulum, the allantoenteric diverticulum, or allantois, appears around the 16th day of development.
The embryonic disc ,initially flat and almost round ,gradually becomes elongated ,with a broad cephalic and a narrow caudal end. Expansion of the embryonic disc occurs mainly in the cephalic region, thus gastrulation continues in the caudal segments while cranial structures are differentiating causing the embryo to develop cephalocaudally.
Further development of the placenta
By the beginning of the third week, the trophoblast is characterized by primary villi that consist of a cytotrophoblastic core covered by a syncytial layer. During further development, mesodermal core penetrate the core of primary villi and grow toward the decida. The newly formed structure is known as a secondary villus.
By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system . The villus is now known as a tertiary villus or a definitive placental villus.
