Epiblast And Hypoblast

Gastrulation: Formation of Embryonic Mesoderm and Endoderm

Prechordal MesodermEpiblast Und Hypoblast

Figure 4.1 A. Implantation site at the end of the second week. B. Representative view of the germ disc at the end of the second week of development. The amniotic cavity has been opened to permit a view on the dorsal side of the epiblast. The hypoblast and epiblast are in contact with each other and the primitive streak forms a shallow groove in the caudal region of the embryo.

Figure 4.1 A. Implantation site at the end of the second week. B. Representative view of the germ disc at the end of the second week of development. The amniotic cavity has been opened to permit a view on the dorsal side of the epiblast. The hypoblast and epiblast are in contact with each other and the primitive streak forms a shallow groove in the caudal region of the embryo.

they migrate beyond the margin of the disc and establish contact with the ex-traembryonic mesoderm covering the yolk sac and amnion. In the cephalic direction, they pass on each side of the prechordal plate. The prechordal plate itself forms between the tip of the notochord and the buccopharyngeal membrane and is derived from some of the first cells that migrate through the node in a cephalic direction. Later, the prechordal plate will be important for

Primitive Streak

Figure 4.2 A. Dorsal aspect of an 18-day embryo. The embryo has a pear-shaped appearance and shows the primitive streak and node at its caudal end. B. Photograph of an 18-day human embryo, dorsal view. Note the primitive node and the notochord extending cranially. The yolk sac has a somewhat mottled appearance. The length of the embryo is 1.25 mm and the greatest width is 0.68 mm.

Figure 4.2 A. Dorsal aspect of an 18-day embryo. The embryo has a pear-shaped appearance and shows the primitive streak and node at its caudal end. B. Photograph of an 18-day human embryo, dorsal view. Note the primitive node and the notochord extending cranially. The yolk sac has a somewhat mottled appearance. The length of the embryo is 1.25 mm and the greatest width is 0.68 mm.

induction of the forebrain (Figs. 4.3A and 4.4A). The buccopharyngeal 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 pit move forward cephalad until they reach the prechordal plate (Fig. 4.4). 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 (Fig. 4.4, B and C). As the hypoblast is replaced by endoderm cells moving in at the streak, cells of the notochordal plate proliferate and detach from the endoderm. They then form a solid cord of cells, the definitive notochord (Fig. 4.4, D and E), which underlies the neural tube and serves as the basis for the axial skeleton. Because elongation of the notochord is a dynamic process, the cranial end forms first, and caudal regions are added as the primitive streak assumes a more caudal position. The notochord and prenotochordal cells extend cranially to the prechordal plate (an area just caudal to the buccopharyngeal membrane) and caudally to the primitive pit. At the point where the pit forms an indentation f

Primitive node

Primitive streak

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