Extra-embryonic membranes and provisory organs

Amnion. In human the amnion is not formed by folding as in birds, but cavitation of the inner cell mass. The bottom of the amniotic vesicle is the epiblast, the amniotic walls are formed by extra-embryonic ectoderm and extra-embryonic mesoderm. The amnion grows with the embryo and fetus development. The amniotic epithelium produces the amniotic fluid that creates watery environment for the fetus and provides its mechanical defence. By 7 weeks the amnion mesoderm comes in contact with the mesoderm of chorion. It exists and functions up to birth as part of the fetal bladder, or the amniochorionic membranes. Besides the amniotic epithelium passes onto the amniotic stalk, and makes contact with the epithelial covering of the embryo. The amnion wall grows and encloses the embryo by thin amniotic membrane (Fig.34,36). By 7 weeks the amnion mesoderm comes in contact with the mesoderm of chorion. Besides the amniotic epithelium passes onto the amniotic stalk, and makes contact with the epithelial covering of the embryo. The amniotic epithelium produces the amniotic fluid. Toward the end of pregnancy the fluid amounts to 1-1,5 liters. It suspends embryo in amniotic fluid that: protects the embryo (foetus) against mechanical injury/shock and adhesions (1), allows for fetal growth and movement (2), and helps to regulate fetal body temperature (3).

Amniotic fluid is produced by dialysis of maternal and fetal blood through blood vessels in the placenta. It contains 200 proteins and cells from embryo used for assessing status of mother & fetus, and in genetic analysis (amniocentesis: e.g., alpha-fetoprotein signals neural tube defects; the detection of sex or genetic defects, for example, Tay Sachs disease). The water content of amniotic fluid turns over every three hours.

Yolk sac. In mammals the yolk sac functions in the early stages of prenatal life. The yolk sac wall is formed by extra-embryonic endoderm and extra-embryonic mesoderm. It arises on the second week and takes part in the embryo nutrition during the period of histiotrophic nutrition. For a short time the yolk sac performs a haematopoeitic function (it gives rise to blood cells and vessels) and after 7-8 weeks it undergoes atrophy and allows development and movement. It remains in umbilical cord as a narrow tube conducting the embryo vessels to the placenta (Fig.36).

Allantois. The allantois grows as a diverticulum from the yolk sac. Its wall consists of extra-embryonic entoderm and visceral layer of mesoderm. Allantois grows into the amniotic stalk. The umbilical blood vessels develop in the wall of the allantois and they vascularize the chorionic part of the human placenta. On the second month allantois is reduced and together with reducing yolk sac it remains in the umbilical cord as a cord of cells (Fig.34, 36).

Chorion. After implantation of the embryo, the endometrium goes through profound changes and is called the decidua. The decidua can be divided into the decidua basalis, situated between the embryo and the myometrium; decidua capsularis, between the embryo and the lumen of the uterus; and the decidua parietalis, the remainder of the decidua (Fig.35). The trophoblast in contact with the decidua capsularis develops only to a slight extent, since its nutrition is deficient. Growth of the trophoblast in the part of the embryo facing the myometrium is ensured by the maternal blood, and its growth is rapid. From this part of the trophoblast, elongated projections, primary villi, are formed (Fig.33, 37b). Their main characteristic is their composition of only cytotrophoblasts and syncytiotrophoblast. During this stage of embryonic development, an extraembryonic mesenchyme appears before the intraembryonic mesenchyme and contributes to the formation of the placenta and the fetal membranes. The extraembryonic mesenchyme and the trophoblast form the chorion. On the side of the decidua basalis, the chorion develops very slightly (smooth chorion); on the side of decidua basalis, the chorion grows extensively and forms the chorion frondosum. The layers of the chorion (beginning at the surface) are the syncytiotrophoblast, cytotrophoblast, and extraembryonic mesenchyme.

When the mesenchyme invades the primary villi, it transforms them into secondary villi (Fig.33, 37c). At the beginning of the 3-th week within the villi, blood vessels are developed gradually and tertiary villi (Fig.37d) are formed. Later the hemocapillaries of the tertiary villi make connection with umbilical (allantoic) vessels from the connecting stalk of the embryo, establishing a circulation and thus allowing exchange of substances and gases between the fetal and maternal blood.

The chorion functions:

· provides exchange between the embryo and the maternal organism;

· secretes enzymes to erode the endometrium;

· provides defense of the embryo, especially immune defense;

· produces hormones.

[Human chorionic gonadotropin (hCG) is secreted by the syncytiotrophoblast in maternal blood and is then excreted with maternal urine. hCG maintains the corpus luteum and stimulates it to continue progesterone production. The detection of hCG in the woman’s urine is a simple, rapid, and a early test to detect pregnancy.]

Further, the chorion will take part in the placenta formation.

Placenta. The placenta is a temporary organ found only in eutherian mammals; it is the site of physiologic exchanges between the mother and the fetus. It consists of a fetal part - chorion - and a maternal part - decidua basalis (Fig.37a). The placenta is the only organ composed of cells derived from 2 different individuals.

A. Fetal Part. The fetal part of the placenta, the chorion, has a chorionic plate at the point where the chorionic villi arise. These villi consist of connective tissue core derived from the extraembryonic mesenchyme surrounded by the syncytiotrophoblast and the cytotrophoblast. The syncytiotrophoblast remains until the end of pregnancy, but the cytotrophoblast disappears gradually during the second half. The chorionic villi may be either free or anchored to the decidua basalis. Both villi have the same structure, but the free ones do not reach the decidua, while the anchored chorionic villi become embedded within the decidua basalis. The stem villous divides into many branches - and resembles a tree (Fig.38). The surfaces of the villi are bathed with blood from the lacunae of the basal decidua; it is here that the exchange of substances between fetal and maternal blood occurs.

B. Maternal Part. The maternal part of the placenta - the decidua basalis - supplies arterial blood to and receive venous blood from the lacunae situated between the secondary and terminal villi. The chorionic villi are submerged in maternal blood because the maternal blood vessels are open during implantation, but fetal blood and maternal blood do not mix. The fetal blood remains isolated by the structures that form the placental barrier (Fig.37e).:

· the endothelium of the fetal capillaries and the basal lamina of these capillaries;

· the mesenchyme in the interior of the villus;

· the basal lamina of the trophoblast;

· the cytotrophoblast;

· the syncytiotrophoblast

The intervillous space of the chorion frondosum soon becomes incompletely divided into cotyledonary bays (Fig.37f). Each bay contains a major villous stem with its branches. As adjacent bays expand, tissue persists between them in form of septa, which project from the basal plate towards the chorionic plate.

At the end of a full-term pregnancy, the placenta has the shape of a disk. The umbilical cord usually arises at the center of the placenta and forms a connection between the fetal and placental circulation.

The placenta is an organ for physiological exchanges between foetus and mother. The placenta is permeable to several substances; it normally transfers oxygen, water, electrolytes, carbohydrates, lipids, proteins, vitamins, hormones, some antibodies, and some drugs from maternal blood to fetal blood. CO₂, water, hormones, and residual products of metabolism are transferred from fetal blood to the maternal blood (Tabl. 4) Thus, the placenta carries out respiratory, trophic, excretory, selective-barrier functions and function of immunologic protection.

The placenta also functions as a complex endocrine gland and as a store for certain metabolites. The placenta produces such hormones as chorionic gonadotropin, chorionic thyrotropin, chorionic corticotropin, estrogens, progesterone, human placental lactogen. All these hormones are synthesized by the syncytiotrophoblast.

Although the placental membrane is referred as placental barrier, harmful substances can cross it to affect the developing embryo (Table 5)

Table 5