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Enbryology / All About Embryology
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Extra-embryonic Membranes |
The way in which the encapsulating membrane of the blastocyst becomes the chorion, and the most deeply embedded part of it becomes the fetal placenta, has already been described. There are still other important membranes that develop from those portions of the inner cell mass of the blastocyst that are not directly involved in becoming an embryo.
  Cells split off from the inner cell mass of the blastocyst and fashion themselves into a primitive yolk sac (Figure 1I, Ja). The roof of the sac then folds into a tubular gut, whereas the remainder becomes a vascularized bag that attains the size of a small pea (Figure 1M). In other vertebrates, such as amphibians and birds, the yolk sac is large and contains a store of nutritive yolk. But in man and other true mammals there is practically none. A slender neck, the yolk stalk, soon connects the rapidly elongating gut with the fast growing yolk sac proper. The stalk detaches from the intestine early in the second month, but the shrunken sac commonly persists and can be found in the expelled afterbirth.
  A cleft separates the outermost cells of the inner cell mass of the blastocyst from the remainder, which then becomes the embryonic disk (Figure 1G). The split-off, thin upper layer is the amnion, which remains attached to the periphery of the embryonic disk. As the disk folds into a cylindrical embryo, the amniotic margin follows the underfolding, and its line of union becomes limited to the ventral (frontward) body wall, where the umbilical cord attaches (Figure 1K, O). The amnion becomes a tough, transparent, nonvascular membrane that gradually fills the chorionic sac and then fuses with it (Figure 1N, P). At the end of the third month of pregnancy, the nonplacental extent of this nearly exposed double membrane comes into contact with the lining of the uterus elsewhere. Fusion then obliterates the uterine cavity, which has been undergoing progressive reduction in size. For the remainder of pregnancy the only cavity within the uterus is that of the fluid-filled amniotic sac.
Clear, watery fluid fills the amniotic sac. The embryo is suspended in this fluid and thus can maintain its shape and mold its body form without hindrance. Throughout pregnancy the amniotic sac serves as a water cushion, absorbing jolts, equalizing pressures, and permitting the fetus to change posture. At childbirth it acts as a fluid wedge that helps dilate the neck of the uterus. When the sac ruptures, about a quart of fluid escapes as the “waters.” If the sac does not rupture or if it covers the head at birth, it is known as a caul.
The allantois, a tube of endoderm (the innermost germ layer), grows out of the early yolk sac in a region that soon becomes the hindgut. The tube extends into a bridge of mesoderm (the middle germ layer) that connects embryo with chorion and will become incorporated into the umbilical cord (Figure 1K, O above). The human allantoic tube is tiny and never becomes a large sac with important functions, as it does in reptiles, birds, and many other mammals. In the second month it ceases to grow, and it soon is obliterated. Blood vessels, however, develop early in its mesodermal sheath, and these spread into the chorion and vascularize it. Throughout pregnancy they will keep the embryo in close relationship with the mother's uterine circulation.
As the ventral body wall closes in, the yolk stalk and allantois are brought together, along with their mesodermal sheaths and blood vessels (Figure 1K, O). Enclosing everything is a wrapping of amnion. In this manner a cylindrical structure, the umbilical cord, comes to connect the embryo with the placenta (Figure 1N, P above). It will serve the embryo and fetus as a physiological lifeline throughout the period of pregnancy. The mature cord is about 1.3 centimetres (0.5 inch) in diameter, and it attains an average length of nearly 50 centimetres (two feet).
The inner cell mass, attached to the deep pole of the implanted blastocyst, is sometimes called the embryoblast, since it supplies the materials used in the formation of an embryo. The cellular mass flattens and enters into the process of gastrulation, through which the three primary germ layers segregate and the gastrula stage, the next advance after the blastula, begins to take form. First, cells facing the cavity of the blastocyst arrange into a layer named the endoderm (Figure 1G, H). The thick residual layer, temporarily designated as epiblast, is the source of a definitive uppermost sheet, the ectoderm, and an intermediate layer, the mesoderm. In this second phase of gastrulation, some cells of the epiblast migrate to the midline position, then turn downward and emerge beneath as mesoderm. Such cells continue to spread laterally, right and left, between the endoderm and the residue of epiblast, which is now definitive ectoderm (Figure 1Jb above).
The site where the migratory mesodermal cells leave the epiblast is an elongated, crowded seam known as the primitive streak (Figure 1Ja above). Similar migrating cells produce a thick knob at one end of the primitive streak. Their continued forward movement from this so-called primitive knot produces a dense band that becomes the rodlike notochord.
The germ layers are not segregated sheets whose cells have predetermined, limited capacities and inflexibly fixed fates in carrying out organ-building activities. Rather, the layers represent advantageously located assembly grounds out of which the component parts of the embryo emerge normally, according to a master constructional plan that assigns different parts to definite spatial positions and local sites. Thus, although the germ layers have developmental potencies in excess of their normal developmental fates, their ordinary participation in organ forming does not deviate from a definite, standard program.
The derivatives of the primary germ layers can best be presented in tabular form. In naming the germ-layer origin of an organ, only the principal functional tissue is designatedm. In a few instances, such as the suprarenal (adrenal) glands and the teeth, a compound organ has important parts of different origin. Take this preliminary to see if your condition could respond to treatment.
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