Nearly all mammals form a placenta and give birth to a sexually-differentiated offspring. The placenta of eutherian mammals forms the interface between the microcirculatory systems of the mother and fetus during the gestational period; it functions in the exchange of nutrients, respiratory gases, and metabolic waste, protects the growing fetus, and is a source of hormones.

Anatomy. The placenta is composed of three layers. The innermost placental layer surrounding the fetus is called the amnion (Figure 5-30). The allantois is the middle layer of the placenta (derived from the embryonic hindgut); blood vessels originating from the umbilicus traverse this membrane. The outermost layer of the placenta, the chorion, comes into contact with the endometrium; it is composed of two layers of cells - inner cytotrophoblast and outer syncytiotrophoblast. The chorion and allantois fuse to form the chorioallantoic membrane. An allantoic cavity is significant in quadrupeds (Figure 5-31). In primates the amnion and chorioallantois eventually fuse.

The structure of the placenta varies among species. Differences in placental anatomy are due to gross arrangement in sites of attachment (Table 5-10) and endometrial tissue layers interposed between the circulatory systems of the mother and fetus (Table 5-11). Endometrial tissues, especially those of invasive implanters, are broken down; the fetal side (chorion) remains intact.

Transport processes. Oxygen, carbon dioxide, lipid-soluble vitamins (A, D, E, K), some electrolytes (eg., sodium), and many drugs are transferred across the placenta by simple diffusion. Simple carbohydrates and certain trace minerals (eg., iron) are transported to the fetus by facilitated diffusion. Amino acids, water soluble vitamins (B complex, C), and other ions (eg., calcium and phosphorus) are brought to the fetus by active transport. Proteins are most readily transferred (by pinocytosis or diffusion) to the fetal circulation in those species with a scant placental barrier. Flow of blood through the placenta (ie., hydrostatic pressure) increases as pregnancy advances.

Transplacental leakage of blood cells can actually occur in those species with a hemochorial placenta. In primates this can lead to Rh (an antigen expressed on the surface of erythrocytes that was first discovered in Rhesus monkeys) incompatibility. If an Rh-negative mother has an Rh-positive baby (of paternal inheritance; a potential complication in about 10% of marriages), then the presence of fetal Rh antigen is recognized as foreign by the mother - maternal lymphocytes become sensitized to the antigen and produce antibodies against the Rh factor. An Rh incompatibility is usually not a problem with the immediate pregnancy. However, in subsequent Rh-positive pregnancies (ie., when rechallenged with antigen), large quantities of lytic antibody are produced, which cross the placenta, and destroy fetal red blood cells (erythroblastosis fetalis). Severe hemolytic anemia within the newborn (necessitating complete tranfusion) can cause death. Injection of the mother with Rhogam (antibodies to Rh factor) soon before delivery of the first infant (or after miscarriage/abortion) will destroy Rh-positive cells that may have entered her circulation (especially as occurs at birth), and thereby negate a sustained immune (memory) response that would otherwise be carried over into future pregnancies.

Hormones. The placenta serves an important role as an endocrine organ. De novo placental production of progesterone from cholesterol occurs throughout pregnancy. The main estrogen produced by the placenta is estriol - a metabolite of estrone lacking significant biological activity. The preparturient placenta lacks 17a-hydroxylase; estrogens are synthesized from androgens supplied by the fetal and maternal adrenal glands. The placenta produces estradiol just before the birth process.

The chorionic gonadotropins (PMSG and hCG) have been the most studied of the placental protein hormones. The placenta has also been shown to synthesize relaxin, lactogens, hypothalamic-like peptides, and opiates.

Decidualization. In response to an implanting embryo stromal cells of the endometrium proliferate and enlarge - the decidual cell reaction (DCR). The DCR is accompanied by interstitial edema. Decidualization begins along the margins of the implantation site and then quickly progresses throughout the endometrium. In some species (eg., murine rodents) decidual tissue produces luteotropic hormones. Decidual tissue is said to constitute the maternal placenta; this tissue is shed at parturition, and thus uterine bleeding is substantial. Farm animals do not exhibit a DCR.

A DCR also can be caused by nonspecific mechanical trauma (eg., scratching) to the endometrium (if it has been sensitized by steroid hormones); this artificial decidual tissue is called a deciduoma.