Epiblast cells

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, epiblast cells move inward (invaginate) to form new cell layers,
endoderm and mesoderm. Hence, epiblast gives rise to all three germ
layers in the embryo. Cells of the intraembryonic mesodermal germ layer
migrate between the two other germ layers until they establish contact with
the extraembryonic mesoderm covering the yolk sac and amnion (Figs. 4.3
and 4.4).
Prenotochordal cells invaginating in the primitive pit move forward until
they reach the prechordal plate. They intercalate in the endoderm as the notochordal
plate (Fig. 4.4). With further development, the plate detaches from the
endoderm, and a solid cord, the notochord, is formed. It forms a midline axis,
84 Part One: General Embryology
Figure 4.18 Stem villi (SV) extend from the chorionic plate (CP) to the basal plate (BP).
Terminal villi (arrows) are represented by fine branches from stem villi.
which will serve as the basis of the axial skeleton (Fig. 4.4). 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 anterior visceral endoderm that expresses head-forming genes, including
OTX2, LIM1, and HESX1 and the secreted factor cerberus. Nodal, a member
of the TGF-β family of genes, is then activated and initiates and maintains the
integrity of the node and streak. BMP-4, in the presence of FGF, ventralizes
mesoderm during gastrulation so that it forms intermediate and lateral plate
mesoderm. Chordin, noggin, and follistatin antagonize BMP-4 activity and
dorsalize mesoderm to form the notochord and somitomeres in the head region.
Formation of these structures in more caudal regions is regulated by the
Brachyury (T) gene. Left-right asymmetry is regulated by a cascade of genes;
first, FGF-8, secreted by cells in the node and streak, induces Nodal and Lefty-2
expression on the left side. These genes upregulate PITX2, a transcription factor
responsible for left sidedness.
Epiblast cells moving through the node and streak are predetermined by
their position to become specific types of mesoderm and endoderm. Thus, it is
possible to construct a fate map of the epiblast showing this pattern (Fig. 4.11).
Chapter 4: Third Week of Development: Trilaminar Germ Disc 85
By the end of the third week, three basic germ layers, consisting of ectoderm,
mesoderm, and endoderm, are established in the head region, and
the process continues to produce these germ layers for more caudal areas of
the embryo until the end of the 4th week. Tissue and organ differentiation has
begun, and it occurs in a cephalocaudal direction as gastrulation continues.
In the meantime, the trophoblast progresses rapidly. Primary villi obtain
a mesenchymal core in which small capillaries arise (Fig. 4.17). When these
villous capillaries make contact with capillaries in the chorionic plate and connecting
stalk, the villous syste