Ultrastructure and organization of the cutaneous microvasculature in normal and pathologic states.

The cutaneous microvasculature is organized into upper and lower horizontal plexuses with the dermal capillary loops arising from the upper plexus. The arteriolar and venular sides of the microvasculature can be identified by the ultrastructure of the mural basement membrane material. Collecting venules present in the lower dermis contain valves. Periadventitial cells (veil cells) are present around all microvessels. Their size and number appear to correlate with the quantity of mural basement membrane material found in cutaneous vessels in diabetes, actinic damage, and chronological aging. The contractile cells of the vascular wall surround the endothelial cell tube in a manner suggesting specific functions. The smooth muscle cells in the arteriolar segment form a sleeve, whereas each pericyte in the postcapillary venular simultaneously makes many contacts with several underlying endothelial cells. The common telangiectases can be explained by abnormalities in this organization and ultrastructure rather than by neovascularization or random anastomoses. The macular telangiectases seen in scleroderma, generalized essential telangiectasia, and nevus flammeus are produced by dilatation of the postcapillary venules of the upper horizontal plexus. Cherry angiomas are produced by spherical and tubular dilatations of capillary loops in dermal papillae with tortuous cross-connections between individual loops. Angiokeratomas of Fabry and Fordyce have the ultrastructure of collecting venules that contain valves, and appear to represent the ectopic development or placement of small valve-containing collecting veins. The cutaneous lesions of hereditary hemorrhagic telangiectasia represent arteriovenous communications.