Third Week of Development:
Trilaminar Germ Disc
Gastrulation: Formation of Embryonic Mesoderm
and Endoderm
The most characteristic event
occurring during the third week of gestation is gastrulation,the process that
establishes all three germ layers (ectoderm,mesoderm ,andendoderm)in the
embryo. Gastrulation begins with formation of the primitive streak on the surface
of the epiblast. Initially, the streak is vaguely defined, but in a 15- to
16-day embryo,it is clearly visible as a narrow groove with slightly bulging regions
on either side. The cephalic end of the streak,the primitive node,consists of a
slightly elevated area surrounding the small primitive pit. Cells of the
epiblast migrate toward the primitive streak. Upon arrival in the region of the
streak, they become flask-shaped, detach from the epiblast,and slip beneath it.
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, 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. Gradually, 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 theprechordal plate.The prechordal plate itself forms between the tip
of the notochord and the buccopharyngeal membraneand 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 induction of the forebrain. The
buccopharyngeal mem-brane 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 cellsinvaginating in
the primitive pit move forward cephalad until they reach theprechordal plate.
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. As the hypoblast is replaced by endodermcells 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,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 in the
epiblast, the neurenteric canal temporarily connects the amniotic and yolk sac
cavities.The cloacal membrane is formed at the caudal end of the embryonic disc.This
membrane, which is similar in structure to the buccopharyngeal membrane,
consists of tightly adherent ectodermand endodermcells with no intervening
mesoderm. When the cloacal membrane appears, the posterior wall of the yolk sac
forms a small diverticulum that extends into the connecting stalk. This diverticulum,
the allantoenteric diverticulum, orallantois,appears around the 16th day of
development. Although in some lower vertebrates the allantois serves as a
reservoir for excretion products of the renal system, in humans it remains
rudimentary but may be involved in abnormalities of bladder development. Establishment
of the Body Axes Establishment of the body axes, anteroposterior, dorsoventral,
and left-right,takes place before and during the period of gastrulation. The
anteroposterior axis is signaled by cells at the anterior (cranial) margin of
the embryonic disc.This area, theanterior visceral endoderm (AVE),expresses
genes essential for head formation, including the transcription factorsOTX2,
LIM1,andHESX1 and the secreted factorcerberus.These genes establish the cranial
end of the embryo before gastrulation. The primitive streak itself is initiated
and main-tained by expression ofNodal,amember of thetransforming growth factorβ
(TGF-β)family. Once the streak is formed, a number of genes reg-ulate formation
of dorsal and ventral mesoderm and head and tail structures.Another member of
theTGF-βfamily, bone morphogenetic protein-4 (BMP-4)is secreted throughout the
embryonic disc. In the presence of this protein andfibroblast growth factor
(FGF),mesoderm will be ventralized to contribute to kidneys (intermediate mesoderm),
blood, and body wall mesoderm (lateral plate mesoderm). In fact, all mesoderm
would be ventralized if the activity of BMP-4 were not blocked by other genes
expressed in the node.For this reason, the node is the organizer.It was given
that designation by Hans Spemann, who first described this activity in the
dorsal lip of the blasto-pore, a structure analogous to the node, in Xenopusembryos.
Thus,chordin (activated by the transcription factor Goosecoid), noggin,and follistatinan-tagonize
the activity of BMP-4.As a result, cranial mesoderm is dorsalized into notochord,
somites, and somitomeres. Later, these three genes are expressed in the
notochord and are important in neural induction in the cranial
region.
As mentioned,Nodalis involved in
initiating and maintaining the primitive streak. Similarly, HNF-3βmaintains the
node and later induces regional specificity in the forebrain and midbrain
areas. Without HNF-3β, embryos fail to gastrulate properly and lack forebrain
and midbrain structures. As mentioned previously,Goosecoid activates inhibitors
ofBMP-4 and contributes to regulation of head development. Over expression or
under expression of this gene results in severe malformations of the head
region, including duplications.
Regulation of dorsal mesoderm
formation in mid and caudal regions of the embryo is controlled by the Brachyury
(T)gene. Thus, mesoderm formation in these regions depends on this gene
product, and its absence results in shortening of the embryonic axis (caudal
dysgenesis).The degree of shortening depends upon the time at which the protein
becomes deficient.Left-right sidedness, also established early in development,
is orchestrated by a cascade of genes. When the primitive streak appears,fibroblast
growth factor 8 (FGF-8) is secreted by cells in the node and primitive streak
and induces expression ofNodalbut only on the left side of the embryo . Later,
as the neural plate is induced, FGF-8 maintains Nodal expression in the lateral
plate mesoderm, as well as Lefty-2,and both of these genes up regulate PITX2,a
transcription factor responsible for establishing left sidedness.
Simultaneously, Lefty-1 is expressed on the left side of the floor plate of the
neural tube and may act as a barrier to prevent left-sided signals from
crossing over.Sonic hedgehog(SHH) may also function in this role as well as
serving as a repressor for left sided gene expression on the right. The Brachyury(T)gene,
another growth factor secreted by the notochord, is also essential for
expression of Nodal, Lefty-1,and Lefty-2. Genes regulating right-sided
development are not as well defined, although expression of the transcription
factor NKX 3.2is restricted to the right lateral plate mesoderm and probably
regulates effector genes responsible for establishing the right side.Why the
cascade is initiated on the left remains a mystery, but the reason may Involve
cilia on cells in the node that beat to create a gradient of FGF-8 toward the
left. Indeed, abnormalities in cilia-related proteins result in laterality
defects in mice and some humans with these defects have abnormal ciliary
function.
Growth of the Embryonic Disc
The embryonic disc,
initially flat and almost round, gradually becomes elongated, with a broad
cephalic and a narrow caudal end. Expansion of the embryonic disc occurs mainly
in the cephalic region; the region of the primitive streak remains more or less
the same size. Growth and elongation of the cephalic part of the disc are caused
by a continuous migration of cells from the primitive streak region in a
cephalic direction. Invagination of surface cells in the primitive streak and
their subsequent migration forward and laterally continues until the end of the
fourth week. At that stage, the primitive streak shows regressive changes,
rapidly shrinks, and soon disappears.That the primitive streak at the caudal
end of the disc continues to supply new cells until the end of the fourth week
has an important bearing on development of the embryo. In the cephalic part, germ
layers begin their specific differentiation by the middle of the third week ,
whereas in the caudal part, differentiation begins by the end of the fourth
week. Thus gastrulation,or formation of the germ layers, continues in caudal
segments while cranial structures are differentiating, causing the embryo to
develop cephalocaudally.
Further Development of the
Trophoblast
By the beginning of the third week,
the trophoblast is characterized by primary villi that consist of a cytotrophoblastic
core covered by a syncytial layer. During further development, mesodermal cells
penetrate the core of primary villi and grow toward the decidua. The newly
formed structure is known as a secondary villus.By the end of the third week,
mesodermal cells in the core of the villus begin to differentiate into blood
cells and small blood vessels, forming the villous capillary system. The villus
is now known as atertiary villus.
