Anatomy and Pathology of the Umbilical Cord

Abstract

The development of the umbilical cord is closely related to that of the amnion (see Chap. 11). Throughout the last days of the second week p.c., the blastocystic cavity is filled by a loose meshwork of mesoderm cells, the extraembryonic mesoblast, which surrounds the embryoblast (Fig. 12.1, day 13). The embryoblast at that time is composed of two vesicles: the amnionic vesicle and the primary yolk sac. When these two vesicles are in contact with each other, they form the double-layered embryonic disk. During the following days, the extraembryonic mesoderm cells are rearranged in such a way that they line the inner surface of the trophoblastic shell as chorionic mesoderm. They also cover the surface of the two embryonic vesicles (Fig. 12.1, day 18). Between the two mesoderm layers the exocoelom cavity forms. It largely separates the embryo and its mesodermal cover from the chorionic mesoderm. The exocoelom is bridged by the mesoderm in only one place, which lies basal to the amnionic vesicle. This mesenchymal connection is referred to as the connecting stalk (Fig. 12.1, day 18). It fixes the early embryo to the membranes and is the forerunner of the umbilical cord. During the same period (around day 18 p.c.), a duct-like extension of the yolk sac, originating from the future caudal region of the embryo, develops into the connecting stalk. This structure is the transitory allantois, the primitive extraembryonic urinary bladder. Heifetz (1996) estimated that some 20% of umbilical cords at term contain remnants of allantoic or omphalomesenteric duct, primarily near the fetal end.