Gastrulation: Formation of Embryonic

Gastrulation: Formation of Embryonic
Mesoderm and Endoderm


The most characteristic event occurring during
the thirdweek 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 (Figs. 4.1–4.3A). Initially, the streak
is vaguely defined (Fig. 4.1), but in a 15- to 16-day embryo,
it is clearly visible as a narrow groove with slightly bulging
regions on either side (Fig. 4.2). The cephalic end of the streak,
the primitive node, consists of a slightly elevated area surrounding
the small primitive pit (Fig. 4.3). Cells of the epiblast migrate
toward the primitive streak (Fig. 4.3). Upon arrival in the region
of the streak, they become flask-shaped, detach from the epiblast,
and slip beneath it (Fig. 4.3, B–D). This inward movement is known
as invagination. 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 (Fig. 4.3B), and cells in these layers will give rise to all of the
tissues and organs in the embryo.
As more and more cells move between the epiblast and hypoblast
layers, they begin to spread laterally and cephalad (Fig. 4.3). Gradually,they migrate beyond the margin of the disc and establish contact with the extraembryonic
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 forinduction 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 indentationin the epiblast, the neurenteric canal temporarily connects the amniotic and
yolk sac cavities.