Wall structures of the arteries in the corpora cavernosa penis were observed in rabbits. The deep (central) arteries in the proximal part of the penis have five or more independent ridges of the wall which are formed by the internal layer of the media consisting of almost longitudinal muscle cell layers. Electron microscopy of these regions revealed that the endothelial cells are connected to each other by many maculae and zonulae adherentes, and that smooth muscle cells of the media are separated from one another by a wide intercellular space embedding abundant collagenous fibers. The epithelioid cells constituting the media of the helicine branches of the deep artery vary in their fine structure from close resemblance to muscle cells, to cells with easily identified special features. Frankly epithelioid cells have few scattered cytoplasmic filaments of a single type, 60-90A in diameter, abundant ribosomes, active Golgi, and other features different from the typical smooth muscle cells. There is no suggestion that these epithelioid cells are either degenerative or undifferentiated forms of smooth muscle cells, as postulated by light microscopists. Transitions from muscle to epithelioid cells are consistent with, but not proved by, our data.

Scanning electron microscopic investigations by others have revealed "hair-like" projections from the endothelial surface of the rabbit aorta. We have confirmed by transmission electron microscopy that endothelial microvilli are found, in a limited number of areas in the aortic arch, particularly at the presumed site of fetal origin of the ductus arterious and near the origin of the subclavian artery. The microvilli are almost uniform in diameter (about 0.1 mum) and are as long as 3 mum in length. The outer leaflet of the limiting membrane shows a fuzzy appearance, and thin filaments are recognized in their interiors. A profuse array of endothelial microvilli was also found over the entire surface of the large arteries of human fetuses. The possible hemodynamic significance of these processes is discussed.

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