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
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.