The Mammalian Placenta and Early Mammalian Development

The Mammalian Placenta and Early Mammalian Development
Because mammals are descendants of one of three lineages that originated with a common amniote ancestor, they inherited the amniotic egg. However, rather than developing within the egg shell like other amniotes, most mammalian embryos evolved the propitious strategy of developing within the mother’s body. We have already seen that mammalian gastrulation closely parallels that of the egg-laying amniotes. The earliest mammals were egg layers, and even today some mammals retain this primitive character; the monotremes (duck-billed platypus and spiny anteater) lay large yolky eggs that closely resemble bird eggs. In marsupials (pouched mammals such as opossums and kangaroos), the embryos develop for a time within the mother’s uterus. But the embryo does not “take root” in the uterine wall, and consequently it receives little nourishment from the mother before birth. The young of marsupials are therefore born immature and are sheltered in a pouch in the mother’s abdominal wall and nourished with milk.

All other mammals, composing 94% of class Mammalia, are placental mammals. These mammals have evolved a placenta, a remarkable fetal structure through which the embryo is nourished. Evolution of this fetal organ required substantial restructuring, not only of the extraembryonic membranes to form the placenta but also of the maternal oviduct, part of which had to expand into long-term housing for the embryo, the uterus. Despite these modifications, development of the extraembryonic membranes in placental mammals is remarkably similar to their development in egg-laying amniotes (compare Figures 8-20 and 8-22).

Figure 8-23 Early development of the human embryo and its extraembryonic membranes.

The early stages of mammalian cleavage, shown in Figure 8-7E, occur while the blastocyst is traveling down the oviduct toward the uterus, propelled by ciliary action and muscular peristalsis. When a human blastocyst is about six days old and composed of about 100 cells, it contacts the uterine endometrium (uterine lining) (Figure 8-23). On contact, the trophoblast cells proliferate rapidly and produce enzymes that break down the epithelium of the uterine endometrium. These changes allow the blastocyst to implant in the endometrium. By the eleventh or twelfth day the blastocyst is completely buried and surrounded by a pool of maternal blood. The trophoblast thickens, sending out thousands of tiny, fingerlike projections, the chorionic villi. These projections sink like roots into the uterine endometrium after the embryo implants. As development proceeds and embryonic demands for nutrients and gas exchange increase, the great proliferation of chorionic villi vastly increases the total surface area of the placenta. Although the human placenta at term measures only 18 cm (7 inches) across, its total absorbing surface is approximately 13 square meters—50 times the surface area of the skin of the newborn infant.

One of the most intriguing questions the placenta presents is,why is it not immunologically rejected by the mother? Both placenta and embryo are genetically alien to the mother because they contain proteins (called major histocompatibility proteins) that differ from those of the mother.We would expect uterine tissues to reject the embryo just as the mother would reject an organ transplanted from her own child.The placenta is a uniquely successful foreign transplant, or allograft, because it has evolved measures for suppressing the immune response that normally would be mounted against it and the fetus by the mother.Recent experiments suggest that the chorion produces proteins and lymphocytes that block the normal immune response by suppressing formation of specific antibodies by the mother.

Since the mammalian embryo is protected and nourished through the placenta rather than with stored yolk, what becomes of the four extraembryonic membranes it has inherited from the early amniotes? The amnion remains unchanged, a protective water jacket in which the embryo floats. A fluid-filled yolk sac is also retained, although it contains no yolk. It has acquired a new function: during early development it is the source of stem cells that give rise to blood and lymphoid cells. These stem cells later migrate into the developing embryo. The two remaining extraembryonic membranes, the allantois and the chorion, are recommitted to new functions. The allantois is no longer needed for the storage of metabolic wastes. Instead it contributes to the umbilical cord, which links the embryo physically and functionally with the placenta. The chorion, the outermost membrane, forms most of the placenta itself. The rest of the placenta is formed by the adjacent uterine endometrium.

The embryo grows rapidly, and all of the major organs of the body have begun their formation by the end of the fourth week of development. The embryo is now about 5 mm in length and weighs approximately 0.02 g. During the first two weeks of development (the germinal period) the embryo is quite resistant to outside influences. However, during the next eight weeks, when all of the major organs are being established and body shape is forming (the embryonic period), the embryo is more sensitive to disturbances that might cause malformations (such as exposure to alcohol or drugs taken by the mother) than at any other time in its development. The embryo becomes a fetus at approximately two months after fertilization. This ushers in the fetal period, which is primarily a growth phase, although some of the organ systems (especially the nervous and endocrine systems) will continue to develop. The fetus grows from approximately 28 mm and 2.7 g at 60 days to approximately 350 mm and 3000 g at term (nine months).