The homogeneous structure of blood vessels in the vascular tree of Macaca mulatta iris

This study reports the results of a systematic analysis of the iris vasculature in Macaca mulatta. Through the application of a variety of morphologic procedures it has been demonstrated that in rhesus monkeys, all iridial vessels, regardless of their diameter, have a similar structure. Their walls consist of (1) endothelial cells in a continuous layer resting on a basal lamina, which are provided with a small number of blunt luminal protrusions and slender basal lamellae; (2) pericytes sandwiched between two layers of the basal lamina, which are characterized by a smooth basal surface and adluminal processes that interdigitate with the basal leaflets of the endothelial cells; and (3) an adventitia of fibroblasts, melanocytes, and occasional macrophages arranged in one or more layers. Typical smooth muscle cells are not found in any vessels of M. mulatta iris. Thus the vessels of the rhesus monkey iris have a remarkably homogeneous morphologic appearance and cannot be classified according to the traditional criteria for arterioles, capillaries, and venules.