Ultrastructure of human intramuscular blood vessels in development.

Muscles from human foetuses of nine weeks to nine months development were examined by electron microscopy. Capillaries, arteries and veins are frequent in the human foetal muscle at all stages of development. At nine weeks the vessels have the appearance of capillaries and basement membrane lies around the endothelial cells forming them. The capillaries are of continuous type which do not have apertures in their walls. Tight junctions are seen at some zones of adjacent endothelial cells of the capillaries. Sometimes, pericytes are also seen. At sixteen weeks vessels having the features of veins and of arteries can be identified between the muscle cells. Regarding the arteries, some of the endothelial cells are united to the smooth muscle cells and the intimal elastic lamina is interrupted where these cells approximate. The significance of this junction may be to anchor the intima to the media.