The ultrastructure of lymphatic valves in rabbits and mice

The ultrastructure of lymphatic valves was studied in rabbits and mice. The lymphatic valves usually consist of two cusps but three or four are sometimes present. The cusps are covered by endothelium. Along the free edge of the cusps are endothelial cells which can differentiate morphologically from other endothelial cells. They are named “tip‐cells”; they have pseudopod‐like projections and abundant cytoplasmic filaments 60–90 Å in diameter. Vesicles occur in endothelial cells of both lymphatic vessels and their valves; they are on the luminal or connective tissue side and are never provided with a diaphragm like that frequently observed in blood vessels. Joining endothelial cells are zonulae occludentes but desmosomes are not observed. No open intercellular junctions are encountered along the valvular endothelium. A basement membrane (basal lamina) is more frequently observed in valves than in walls of lymphatic vessels. Connective tissue in the cusps consists of collagenous fibrils, fine filaments and fibroblasts.