Chapter 5
Third Week of Development: Trilaminar Germ Disc
61
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
Maternal
vessels
Outer
cytotrophoblast
shell
Syncytiotrophoblast
Cytotrophoblast
Mesoderm core
with capillaries
Chorionic plate
Chorionic cavity
Connecting stalk
Intervillous
space
Endometrium
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.
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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