[Structure and function of the fetal umbilical vessels].

A morphological study carried out using electron microscopy has shown that there is an endothelium in the umbilical blood vessels consisting of intercellular spaces which interdigitate with one another and that are not specialization in junction complex. The endothelial cells are rich in their typical organ structure and have pinocytotic vesicles. There media contains a muscular coat which is thicker in the artery than in the veins. The smooth muscle cells also show pinocytotic vesicles. This morphological state leads the authors to two groups of thought: There is active diffusion by a process of "plasma perfusion" from the lumen of the vessels through the vascular wall which forms a physiological system for taking up fluids which in turn is capable of providing for the energy and plastic requirements of the umbilical vessels: The umbilical arteries, because of their thick muscular tunic and because of the ability of the lumen to dilate as there is an absence of an internal elastic membrane, can function not only as tubes for conducting blood but also as organs that regulate the blood flow. On the other hand the umbilical vein, which cannot stretch and cannot retract because its wall is so thin, behaves as an organ to return the blood flow, particularly because it does have an inner limiting layer preventing over-stretching.