Ultrastructure of the human dermal microcirculation. III. The vessels in the mid- and lower dermis and subcutaneous fat.

This paper describes the ultrastructure of the microcirculatory vessels in the mid- and lower dermis and subcutaneous fat. Reconstruction of vessel walls, tracing out the courses of individual vessels, and survey examination of vessels were carried out by various combinations of routine and serial, ultrathin, and 1-micron sections. Intracellular myofilamentous bundles associated with extracellular filaments were a characteristic feature of the endothelial cells in arterioles possessing an internal elastic lamina, but were only rarely seen in endothelial cells of venules. The ultrastructural features of these bundles and filaments suggested both contractile and anchoring functions. The elastic lamina of the arterioles was shown to be composed of individual elastic fibers oriented in the long axis of the blood vessel under the endothelium, rather than being a continuous sheet as in larger arteries and arterioles. The capillaries in the fat had walls of normal thickness (0.1-0.3 micron) similar to capillaries in other organs, in contrast to the dermal capillaries whose walls are 2-3 micron thick. The arterioles, capillaries, and venules in the fat were frequently devoid of veil cells in contrast to those in the dermis. Venous capillaries with bridged fenestrations were found in close proximity to eccrine sweat glands and hair bulbs in the forearm, trunk, and buttock skin.