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 pa pillae of rat scalp skin revealed continuous extension of the lymph co llection system into 4 to 10 micron diameter lumen capillaries with th in walls, scant basement lamellae (membranes), and blind-endings of 1 to 4 micron lumen diameter within endothelial-type cells. These termin al endothelial cells also displayed intracytoplasmic channels and pino cytotic vesicles, extensive cytoplasmic processes and a high cytoplasm ic volume-percent of mitochondria suggesting active transport capabili ties of lymphatic endothelia. The mitochondrial cytoplasmic volume-per cent (mean 14.5%) exceeded that present in blood capillary endothelial cells of the rat brain (the anatomic substrate of the blood-brain bar rier), that have a volume-percent of mitochondria of 10 to 12% (1). Ac tive transport processes centered in such endothelial cells could acco unt for a portion of lymph formation, and explain the continued accumu lation 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 con verge in groups of three to form larger diameter lymph capillaries corresponding to the lymph ''initial s '' previously described (2,3).