Extraembryonic organs.

Umbilical cord. The umbilical cord is vascular cable (~fifty-five centimetres) that connects the embryo to placenta. The umbilical cord of the fetus is covered by the amniotic epithelium and contains two umbilical arteries and one umbilical vein embedded into the Wharton’s jelly. It is first formed during the fifth week of embryonic life and initially contains the yolk sac and body stalk with the enclosed allantois.

Amnion. The thin, transparent, tough membrane lining the fluid-filled cavity which contains the embryo.

Allantois. One of the extraembryonic membranes, providing respiratory exchange. It is covered by a conjoined vascular layer containing umbilical or allantoic vessels which are the vascular connections with the placenta.

Vitelline sac. An extraembryonic structure, attached to the umbilical cord and lined by endoderm. Vitelline sac is a site of hematopoiesis.

Chorion. The outermost of the fetal membranes of mammals, formed from extraembryonic somatopleuric mesoderm and the overlying trophoblast. The trophoblast divides into an outer syncytiotrophoblast and inner cytotrophoblast. The chorion develops villi, engaged in fetal-maternal exchange.

Placenta.The blastocyst normally attached to the uterine mucosa about six days after fertilization. The outer layer of the blastocyst proliferates to form the trophoblastic cell mass or chorionic shell. Trophoblastic cells derived from this mass penetrate the columnar epithelial cells of the maternal endometrium. The invasion of the endometrium begins on the seventh day after fertilization and is complete by about the twelfth day. The primitive trophoblast rapidly differentiates into two layers. The inner layer is composed of mononuclear cells and is known as the cytotrophoblastic layer, whereas the outer layer consists of multinucleated cells and is known as the syncytiotrophoblast. Intercommunicating fluid-filled spaces (lacunae) appear in the rapidly enlarging trophoblastic mass from the eighth day after fertilization.

The end of the second week is characterized by the first appearance of chorionic villi. The layer of the placenta from which the villi project is called the chorionic plate. Primary villous stems have a central core of cells derived by the proliferation of the cytotrophoblast. The primary villi gradually develop mesenchymal cores, which convert them into the secondary villi. The mesenchymal cells within the villi differentiate into blood capillaries, thus forming the tertiary villi. The vessels from the villi soon become connected with the embryonic heart via vessels that differentiate in the mesenchyme of the chorion and in the connecting stalk.

By the end of the third week, embryonic blood is circulating through the capillaries of the villi. As the growth continues, the villi on the decidua capsularis (abembryonic) pole degenerate to form the chorion laeve, while the villi adjacent to the decidual plate rapidly grow and expand to form the chorion frondosum. The chorion frondosum forms up the placenta together with the decidual plate. By the end of the fourth month of the pregnancy, the placenta has attained its definitive form and undergoes no further anatomical modifications. It has two components: the maternal portion, formed by the decidual plate, and a fetal portion, made by the chorion frondosum. Growth continues by further ramification of the stem villi into the surrounding intervillous spaces. On the maternal side decidual septa extend into the intervillous spaces, dividing placenta into ten to thirty eight cotyledons.

The villous surface area continues to increase until term, although the rate of the increase slows from approximately thirty four to thirty six weeks of gestation. The placenta has a circular shape and measures about fifteen to twenty centimetres in diameter, weighing five hundred to six hundred grammes at full term.

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