Terminal endothelial cells of lymph capillaries as active transport structures involved in the formation of lymph in rat skin.

Electron microscopic examination of lymph capillaries of the dermal papillae of rat scalp skin revealed continuous extension of the lymph collection system into 4 to 10 micron diameter lumen capillaries with thin walls, scant basement lamellae (membranes), and blind-endings of 1 to 4 micron lumen diameter within endothelial-type cells. These terminal endothelial cells also displayed intracytoplasmic channels and pinocytotic vesicles, extensive cytoplasmic processes and a high cytoplasmic volume-percent of mitochondria suggesting active transport capabilities of lymphatic endothelia. The mitochondrial cytoplasmic volume-percent (mean 14.5%) exceeded that present in blood capillary endothelial cells of the rat brain (the anatomic substrate of the blood-brain barrier), that have a volume-percent of mitochondria of 10 to 12% (1). Active transport processes centered in such endothelial cells could account for a portion of lymph formation, and explain the continued accumulation of lymphedema distal to blocked lymphatic collection ducts when lymphatic intraluminal pressure is greatly increased. The small lumen diameter capillaries, which correspond spatially to the prelymphatics of other authors (2), typically converge in groups of three to form larger diameter lymph capillaries corresponding to the lymph "initials" previously described (2,3).