Ultrastructure of the human dermal microcirculation. IV. Valve-containing collecting veins at the dermal-subcutaneous junction

Valve-containing collecting veins 70-120 micron in diameter were found at the interface of the dermis and fat. Two cusped valves with sinuses were found at most points where 25- to 50-micron venules entered these larger veins as well as in the lumens of the larger veins unrelated to branch points. The walls of these valve-containing veins in their narrowest portions were composed of smooth muscle cells, collagen fibers, and homogeneous-appearing basement membrane material. As the vessel became wider, elastic fibers first appeared just inside the most peripheral layer of smooth muscle cells, and gradually occupied the spaces between all the layers of smooth muscle cells. However the subendothelial zone was frequently devoid of elastic fibers. The valves were always pointed in the direction of the larger vessel as would be expected in a collecting vein. Recent experiments by other workers have demonstrated that the capillary blood flow in human nail-fold capillaries is pulsatile, thereby providing physiologic reasons for the presence of valve-containing veins at the dermis-fat interface.