What are the fetal membranes?

The term fetal membrane is applied to those structures derived from the blastocyst which do not contribute to the embryo. The amnion, the chorion, the yolk sac and the allantois make up the fetal membranes (Fig. 2.2). The amnion lines the amniotic sac and protects the embryo from physical injury. The amniotic sac enlarges rapidly due to an increase in the volume of amniotic fluid. The chorion is a double-layered membrane formed by the trophoblast and the extra-embryonic mesoderm, which eventually will give rise to the fetal part of the placenta. From 12 days until the end of embryonic period the developing embryo is suspended in the chorionic cavity. The expansion of the amniotic sac obliterates the chorionic cavity and the only connection between the embryo and the chorion is via a thick plate of mesoderm called the connecting stalk.

Fig. 2.2 Formation of fetal membranes at implantation sites at 10 days (A) and 15 days (B).

The yolk sac and its diverticulum, the allantois, are the major means of nutritional exchange mechanisms in other mammals. However, in humans the yolk reserves are poor, and part of the yolk sac is incorporated into the embryo to form the gut tube. The allantois, which serves as a reservoir for fetal urine in other mammals, becomes attached to the urinary bladder.

Development of the uteroplacental circulation

Due to the rapid growth of the embryo during the second week, there is a need for a more efficient means of nutritional and gaseous exchange. This is achieved when the embryonic blood vessels of the chorion come into contact with the maternal blood vessels of the decidua.

The umbilical arteries carry the deoxygenated blood from the embryo to the chorion and umbilical veins return the oxygenated blood to the embryo. The uterine vessels supply the maternal blood to the trophoblastic lacunae. When the cytotrophoblast cells grow into the lacunae as the primary chorionic villi, the primitive uteroplacental circulation is established. The initial formation of primary chorionic villi is explained in Chapter 1.

Further development of chorionic villi

From day 15, the primary chorionic villi begin to branch, and the extra-embryonic (chorionic) mesoderm invades the core of the primary villi, thus converting them into secondary chorionic villi (Fig. 2.3). The secondary villi line the entire surface of the chorion, and soon blood vessels develop within the mesenchyme of these villi that connect to the umbilical vessels in the embryo. Villi containing blood vessels are called tertiary villi (Fig. 2.4).

Fig. 2.3 Function of the chorionic villi. (A) By day 14, the primary villi appear as cytotrophoblastic stems. (B) By day 16, the extra-embryonic mesoderm grows into the villi, thus transforming each primary villus into a secondary villus.

Fig. 2.4 By day 21, blood vessels develop within the chorionic mesoderm forming the tertiary villi. (B) Enlarged view of boxed area in (A) showing a tertiary villus.

As the embryo and its coverings grow, the chorionic villi on the abembryonic pole become compressed against the decidua capsularis and become avascular (Figs 2.1 and 2.5). This area of chorion is known as the smooth chorion or chorion laeve. As the chorion laeve regresses, the villi on the embryonic pole increase in size and number. Because of its leafy appearance, this portion of the chorion is known as the chorion frondosum (Figs 2.1 and 2.5).

Fig. 2.5 Fetus at the end of the third month showing the fused amnion and chorion laeve. The placenta is formed by the decidua basalis and chorion frondosum.

Formation of the placenta and placental circulation

The placenta has both maternal and fetal components. The maternal portion is called the basal plate and is derived from the decidua basalis, and the fetal portion is called the chorionic plate, formed by the chorion frondosum (Fig. 2.1). The tertiary villi of the chorion frondosum grow towards the basal plate, and attach to the decidual tissue via the cytotrophoblast shell (Fig. 2.4B). Because these villi secure the attachment of the fetal placenta to the basal plate, they are known as anchoring villi (Fig. 2.6A). The blood-filled spaces between the anchoring villi, which were trophoblastic lacunae, now become the intervillous spaces (Fig. 2.6A).

Fig. 2.6 The full-term placenta showing the maternal and fetal portions (A). The decidual septa divide the maternal portion into cotyledons. Arrows indicate the direction of maternal blood flow. The boxed area is enlarged in (B). Placental membrane (B).

The anchoring villi give rise to numerous side branches called intermediate villi, which in turn produce small rounded terminal or floating villi (Fig. 2.7). The terminal villi develop as sprouts of syncytiotrophoblast. During the third trimester, the terminal villi take over the functions of nutrient and gaseous exchange from the intermediate villi. The continuous branching of the villi into the intervillous spaces therefore results in an extensive placental villous tree. As the placenta grows during the last trimester of pregnancy, the placental villi show maturation changes. The capillaries become larger and come to lie close to a much thinner syncytiotrophoblast. Most of the cells of the cytotrophoblast layer disappear, thus facilitating diffusional exchange mechanisms.

Fig. 2.7 Maturation of placental villi at 7 and 28 weeks.

As the villous trees grow into the intervillous spaces, the decidual tissue sends wedge-like placental septa to divide the placenta into 15–20 irregular convex areas called cotyledons. Since the placental septa do not reach the chorionic plate, the maternal circulation between the cotyledons is maintained (Fig. 2.6A).

The exchange of nutrients, respiratory gases and waste products between the maternal and fetal blood takes place across the placental membrane (see below) within the intervillous spaces. Maternal blood enters these spaces from the spiral arteries, branches of the uterine artery, bringing nutrients and oxygen for the embryo and fetus, and the endometrial veins drain the waste products to the maternal circulation (Figs 2.4A and 2.6A). The oxygenated fetal blood from the placenta passes into the umbilical vein, and the deoxygenated blood from the fetus is returned to the placenta by way of the umbilical arteries.

Placental membrane and placental functions

The partition between the maternal and fetal circulations is the placental membrane, often called the placental barrier (Fig. 2.6B). This membrane is composed of fetal tissues and is initially four layers thick: (1) the fetal capillary endothelium, (2) the connective tissue of the villus, (3) the cytotrophoblast and (4) the syncytiotrophoblast. From the fourth month the placental membrane becomes progressively thinner as the fetal capillaries enlarge, the amount of connective tissue in the core of the villus is reduced, and the cytotrophoblast largely disappears.

The most important function of the placenta is to provide exchange of nutrients and waste products between the mother and fetus. The surface of the syncytiotrophoblast facing the intervillous spaces shows numerous microvilli, and these increase the area available for physiological exchange. Smaller molecules are transmitted by simple diffusion, but for larger molecules mechanisms of either active transport or pinocytosis are involved. Maternal antibodies cross the placental membrane by pinocytosis, and these give passive immunity to the newborn. Another important function of the placenta is that of secretion of hormones. The trophoblast secretes HCG, which maintains the corpus luteum. The placenta also produces large amounts of progesterone and oestrogen.

Structure of the full-term placenta

At term the placenta appears as a disc, and has fetal and maternal surfaces (Fig. 2.8). The fetal surface is smooth, and the umbilical cord is inserted into the centre of this surface. The amnion invests the umbilical cord, and covers the fetal surface of the placenta. The umbilical vessels radiate from the umbilical cord, and run between the transparent amnion and smooth chorion before supplying the chorionic villi. At birth the amnion and chorion are torn from the margins of the placenta. The maternal surface is covered by a thin layer of decidua basalis torn from the uterine wall during birth. Beneath this layer, the maternal surface shows 15–20 roughened cotyledons (Fig. 2.8A).

Fig. 2.8 Photograph of a full-term placenta. (A) Maternal surface. (B) Fetal surface.

Following the delivery of the child at birth, the placenta separates from the uterine wall and is expelled by uterine contractions through the cervix.

The umbilical cord

The umbilical cord suspends the embryo in the amniotic cavity. The cord contains two umbilical arteries and one umbilical vein through which blood passes between the embryo and the placenta. The umbilical vessels are surrounded by mucoid tissue, called Wharton’s jelly. Within the umbilical cord the arteries coil around the single vein, giving the umbilical cord a spiral twist appearance.

Twins and their fetal membranes

The organization of fetal membranes in twins depends on the type of twins and on the stage of development at which the original single embryo separates. Two types of twins occur: dizygotic and monozygotic. Dizygotic or non-identical twins result when the two released oocytes fertilize with two different spermatozoa. Twins that form by the splitting of a single zygote are called monozygotic or identical twins.

Dizygotic twins implant separately and develop separate placentae and fetal membranes (Fig. 2.9A). Sometimes, however, because of their proximity a secondary fusion between the two amniotic and chorionic sacs may occur.

Fig. 2.9 (A) Dizygotic twins. (B) Monozygotic twinning resulting from the splitting of inner cell mass. (C) Monozygotic twinning resulting from the splitting of embryonic disc.

The sharing of placenta and fetal membranes in monozygotic twins depends on the stage at which a single zygote divides. If the splitting occurs at the two-cell stage of the embryo, the resulting twins will implant separately with their own fetal membranes and placentae (Fig. 2.9B). In such cases, although the arrangement of fetal membranes is the same as that of the non-identical dizygotic twins, their identity as monozygotic twins can only be revealed by conducting blood and other diagnostic tests. In most cases of identical twinning, the inner cell mass splits within a single blastocyst. Subsequently, two embryos will share a single chorionic sac and placenta but will be enclosed by separate amniotic sacs (Fig. 2.9B). If the splitting occurs after the formation of the bilaminar embryonic disc, the twins will occupy a single amnion and chorionic sac, and share the same placenta (Fig. 2.9C). Such twins usually fail to survive, as the umbilical cords may get entangled within the amniotic sac thus blocking the fetal circulation.

Summary box

The placenta develops from both maternal and fetal tissues to provide nutrients and oxygen to the fetus, and to carry waste products to the mother.

The maternal and fetal circulations are separated by a placental membrane derived from fetal tissues.

The umbilical cord connects the placenta to the embryo.

The chorion and amnion become fused around the fully formed placenta.

The extent to which the fetal membranes are shared in twins depends on their derivation and the time at which the separation of embryonic cells occurs.