Placenta and Fetal Membranes

Decidua

The decidua is the endometrium of the uterus in a pregnant woman. It is the functional layer of the endometrium that separates from the remainder of the uterus after parturition (childbirth). The three regions of the decidua are named according to their relation to the implantation site (see Fig. 7.1 ):

• The decidua basalis is the part of the decidua deep to the conceptus which forms the maternal part of the placenta.

• The decidua capsularis is the superficial part of the decidua overlying the conceptus.

• The decidua parietalis represents the remaining parts of the decidua.

In response to increasing progesterone levels in maternal blood, the connective tissue cells of the decidua enlarge to form decidual cells . These cells enlarge as glycogen and lipid accumulate in their cytoplasm .

The cellular and vascular changes occurring in the endometrium as the blastocyst implants constitute the decidual reaction . Many decidual cells degenerate near the chorionic sac in the region of the syncytiotrophoblast (outer layer of trophoblast), and together with maternal blood and uterine secretions, they provide a rich source of nutrition for the embryo/fetus. It has also been suggested that these cells protect the maternal tissue against uncontrolled invasion by the syncytiotrophoblast, and they may be involved in hormone production . Decidual regions, clearly recognizable during ultrasonography, are important in diagnosing early pregnancy (see Chapter 3 , Fig. 3.7 ).

Development of Placenta

Early development is characterized by rapid proliferation of the trophoblast and development of the chorionic sac and chorionic villi (see Chapters 3 and 4 ). Homeobox genes ( HLX, MSX2, and DLX3 ) expressed in the trophoblast and its blood vessels induce trophoblastic invasion and regulate placental development. By the end of the third week, the anatomical arrangements necessary for physiologic exchanges between the mother and embryo/fetus are established. A complex vascular network is established in the placenta by the end of the fourth week, which facilitates maternal−embryonic exchanges of gases, nutrients, and metabolic waste products.

Chorionic villi cover the entire chorionic sac until the beginning of the eighth week ( Figs. 7.2 and 7.3 , and see Fig. 7.1 C ). As the chorionic sac grows, the villi associated with the decidua capsularis become compressed, so the blood supply to them is reduced; hence, they soon degenerate (see Figs. 7.1 D and 7.3 B ). This produces a relatively avascular bare area, the smooth chorion (chorion laeve). As those villi disappear, the villi associated with the decidua basalis rapidly increase, branch profusely, and enlarge. This forms the bushy area of the chorionic sac, the villous chorion (chorion frondosum).

The uterus, chorionic sac, and placenta enlarge as the embryo/fetus grows. Growth in the size and thickness of the placenta continues rapidly until the fetus is approximately 18 weeks old. The fully developed placenta covers 15% to 30% of the decidua of the endometrium of the uterus and weighs approximately one sixth as much as the fetus. At term, for its own metabolic requirements, the placenta uses 40% to 60% of the oxygen and glucose that reaches the uterus.

The placenta has two parts ( Fig. 7.4 , and see Fig. 7.1 E and F ):

• The fetal part is formed by the villous chorion . The chorionic villi that arise from the chorion project into the intervillous space containing maternal blood (see Fig. 7.1 D ).

• The maternal part is formed by the decidua basalis , the part of the decidua related to the fetal component of the placenta (see Fig. 7.1 C to F ). By the end of the fourth month, the decidua basalis is almost entirely replaced by the fetal part of the placenta.

The fetal part is attached to the maternal part of the placenta by the cytotrophoblastic shell , the external layer of trophoblastic cells on the maternal surface of the placenta ( Fig. 7.5 ). The chorionic villi attach firmly to the decidua basalis through the cytotrophoblastic shell, which anchors the chorionic sac to the decidua basalis. Endometrial arteries and veins pass freely through gaps in the cytotrophoblastic shell and enter the intervillous space.

The shape of the placenta is determined by the persistent area of chorionic villi (see Fig. 7.1 F ). Usually this is a circular area, giving the placenta a discoid shape. As the chorionic villi invade the decidua basalis, decidual tissue is eroded to enlarge the intervillous space (see Fig. 7.4 ). This erosion produces several wedge-shaped areas of decidua, the placental septa , which project toward the chorionic plate , the part of the chorionic wall related to the placenta ( Fig. 7.5 ). The septa divide the fetal part of the placenta into irregular convex areas, or cotyledons . Each cotyledon consists of two or more stem villi and many branch villi ( Fig. 7.6 A , and see Fig. 7.5 ). By the end of the fourth month, the decidua basalis is almost entirely replaced by cotyledons (see Fig. 7.11 ). Expression of kinase genes ( MAP2K1 and MAP2K2 ) and the transcription factor Gcm1 (glial cells missing-1) in trophoblast stem cells regulate the branching process of the stem villi to form the vascular network in the placenta.

The decidua capsularis , the layer of decidua overlying the chorionic sac, forms a capsule over the external surface of the sac (see Fig. 7.1 A to D ). As the conceptus enlarges, the decidua capsularis bulges into the uterine cavity and becomes greatly attenuated. Eventually the decidua capsularis contacts and fuses with the decidua parietalis on the opposite wall, thereby slowly obliterating the uterine cavity (see Fig. 7.1 E and F ). By 22 to 24 weeks, the reduced blood supply to the decidua capsularis causes it to degenerate and disappear.

After the disappearance of the decidua capsularis, the smooth part of the chorionic sac (smooth chorion) fuses with the decidua parietalis (see Fig. 7.1 F ). This fusion can be separated and usually occurs when blood escapes from the intervillous space (see Fig. 7.4 ). The collection of blood (hematoma) pushes the chorionic membrane away from the decidua parietalis, thereby reestablishing the potential space of the uterine cavity.

Initially, when trophoblastic cells invade the spiral arteries, these cells create plugs within the arteries. These plugs allow only maternal plasma to enter the intervillous space. As a result, there is a net negative oxygen gradient created; it has been shown that elevated oxygen levels during the early stages of development can cause complications. However, by 11 to 14 weeks, the plugs begin to break down, maternal whole blood begins to flow, and oxygen concentrations increase.

The intervillous space of the placenta , which by 11 to 14 weeks contains maternal blood, is derived from the lacunae (small spaces) that developed in the syncytiotrophoblast during the second week of development (see Chapter 3 , Fig. 3.2 A and B ). This large blood-filled space results from the coalescence and enlargement of the lacunar networks . The intervillous space is divided into compartments by placental septa; however, there is free communication between the compartments because the septa do not reach the chorionic plate (see Fig. 7.5 ).

Maternal blood enters the intervillous space from the spiral endometrial arteries in the decidua basalis (see Figs. 7.4 and 7.5 ). The spiral arteries pass through gaps in the cytotrophoblastic shell and discharge blood into the intervillous space. This large space is drained by endometrial veins , which also penetrate the cytotrophoblastic shell. These veins are found over the entire surface of the decidua basalis.

The numerous branch villi , arising from stem villi , are continuously showered with maternal blood that circulates through the intervillous space (see Figs. 7.4 and 7.5 ). The blood in this space carries oxygen and nutritional materials that are necessary for fetal growth and development. The maternal blood also contains fetal waste, carbon dioxide, salts, and products of protein metabolism.

The amniotic sac enlarges faster than the chorionic sac. As a result, the amnion and smooth chorion fuse to form the amniochorionic membrane (see Figs. 7.4 and 7.5 ). This composite membrane fuses with the decidua capsularis and, after the disappearance of the latter, adheres to the decidua parietalis (see Figs. 7.1 F , 7.4, and 7.5 ). It is the amniochorionic membrane that ruptures during labor. Preterm membrane rupture (i.e., at less than 37 weeks’ gestation) is the most common event leading to premature labor. Membrane rupture allows amniotic fluid to escape through the vagina.

Placental Circulation

The branch chorionic villi of the placenta provide a large surface area where materials may be exchanged across the very thin placental membrane interposed between the fetal and maternal circulations (see Figs. 7.5 to 7.6 ). It is through the branch villi, which arise from stem villi, that the main exchange of material between the mother and fetus takes place. Fetal and maternal circulations are separated by the placental membrane consisting of extrafetal tissues ( Fig. 7.7 , and see Fig. 7.6 B and C ).

Placental Membrane

The placental membrane is a composite structure that consists of extrafetal tissues separating the maternal and fetal blood. Until approximately 20 weeks, the placental membrane consists of four layers (see Figs. 7.6 and 7.7 ): syncytiotrophoblast , cytotrophoblast , connective tissue of the villi , and endothelium of fetal capillaries . After the 20th week, cellular changes occur in the branch villi that result in the cytotrophoblast, in many of the villi, becoming attenuated.

Eventually cytotrophoblastic cells disappear over large areas of the villi, leaving only thin patches of syncytiotrophoblast. As a result, the placental membrane consists of three layers in most places (see Fig. 7.6 C ). In some areas, the placental membrane becomes markedly thinned and attenuated. At these sites, the syncytiotrophoblast comes into direct contact with the endothelium of the fetal capillaries to form a vasculosyncytial placental membrane .

Sometimes the placental membrane is called the placental barrier ; this is an inappropriate term because there are only a few substances, endogenous or exogenous, that are unable to pass through the membrane in detectable amounts. The placental membrane acts as a barrier only when a molecule is of a certain size, configuration, and charge, such as that of heparin (a compound occurring in the liver, lungs, and mast cells that inhibits blood coagulation). Some metabolites, toxins, and hormones, although present in the maternal circulation, do not pass through the placental membrane in sufficient concentrations to affect the embryo/fetus. Most drugs and other substances in the maternal blood plasma pass through the placental membrane and enter the fetal blood plasma (see Fig. 7.7 ). The syncytiotrophoblast free surface has many microvilli that increase the surface area for exchange between the maternal and fetal circulations. As pregnancy advances, the placental membrane becomes progressively thinner, and thus blood in many fetal capillaries is extremely close to the maternal blood in the intervillous space (see Figs. 7.6 C and 7.7 ).

During the third trimester, numerous nuclei in the syncytiotrophoblast aggregate to form multinucleated protrusions, syncytial knots (see Fig. 7.6 C ). These aggregations regularly break off and are carried from the intervillous space into the maternal circulation. Some knots lodge in capillaries of the maternal lungs, where they are rapidly destroyed by local enzyme action. Toward the end of pregnancy, eosinophilic fibrinoid material thickens on the surfaces of villi (see Fig. 7.6 C ), which appears to reduce placental transfer.

Functions of Placenta

The placenta has several main functions:

• Metabolism (e.g., synthesis of glycogen)

• Transport of gases and nutrients

• Endoc r ine secretion (e.g., human chorionic gonadotropin [hCG])

• Protection

• Excretion (fetal waste products)

These comprehensive activities are essential for maintaining pregnancy and promoting normal fetal development.