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Chapter 5 Third Week of Development: Trilaminar Germ Disc 61

Maternal vessels Outer cytotrophoblast shell Syncytiotrophoblast

Endometrium

Intervillous space

Cytotrophoblast

Mesoderm core with capillaries

Chorionic plate

Chorionic cavity

Connecting stalk

Figure 5.13 Longitudinal section through a villus at the end of the fourth week of development. Maternal vessels pen- etrate the cytotrophoblastic shell to enter intervillous spaces, which surround the villi. Capillaries in the villi are in contact with vessels in the chorionic plate and in the connecting stalk, which in turn are connected to intraembryonic vessels.

Here they establish contact with similar exten- sions of neighboring villous stems, forming a thin outer cytotrophoblast shell (Figs. 5.12 and 5.13).This shell gradually surrounds the tropho- blast entirely and attaches the chorionic sac firmly to the maternal endometrial tissue (Fig. 5.12). Villi that extend from the chorionic plate to the decidua basalis ( decidual plate : the part of the endometrium where the placenta will form; see Chapter 8) are called stem or anchoring villi . Those that branch from the sides of stem villi are free (terminal) villi , through which exchange of nutrients and other factors will occur. The chorionic cavity, meanwhile, becomes larger, and by the 19th or the 20th day, the embryo is attached to its trophoblastic shell by a narrow connecting stalk (Fig. 5.12). The con- necting stalk later develops into the umbilical cord , which forms the connection between the placenta and embryo. Summary The most characteristic event occurring during the third week is gastrulation , which begins with the appearance of the primitive streak , which has at its cephalic end the primitive node . In the region of the node and streak, epi- blast cells move inward (invaginate) to form new cell layers, endoderm and mesoderm . Cells that do not migrate through the streak but

remain in the epiblast form ectoderm. Hence, epiblast gives rise to all three germ layers in the embryo, ectoderm, mesoderm , and endo- derm , and these layers form all of the tissues and organs (Figs. 5.2 and 5.3). Prenotochordal cells invaginating in the primitive pit move forward until they reach the prechordal plate. They intercalate in the endo- derm as the notochordal plate (Fig. 5.3).With further development, the plate detaches from the endoderm, and a solid cord, the notochord , is formed. It forms a midline axis, which will serve as the basis of the axial skeleton (Fig. 5.3). Cephalic and caudal ends of the embryo are established before the primitive streak is formed. Thus, cells in the hypoblast (endoderm) at the cephalic margin of the disc form the AVE, which expresses head-forming genes, including OTX2, LIM1 , and HESX1 and the secreted factor cer- berus . Nodal, a member of the TGF- b family of genes, is then activated and initiates and main- tains the integrity of the node and streak. In the presence of FGF , BMP4 ventralizes mesoderm during gastrulation so that it forms intermediate and lateral plate mesoderm. Chordin, noggin , and follistatin antagonize BMP4 activity and dorsal- ize mesoderm to form the notochord and somi- tomeres in the head region. Formation of these structures in more caudal regions is regulated by the Brachyury (T) gene (Fig. 5.4 A ). Laterality (left–right asymmetry) is regulated by a cascade of signaling molecules and genes. FGF8 , secreted

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