THE MORPHOLOGY OF VALVES AND VALVE-LIKE STRUCTURES IN THE CANINE AND FELINE THORACIC-DUCT

The microanatomy and ultrastructure of the feline and canine thoracic duct and afferent lymphatics were studied by scanning and transmission electron microscopy. We found that the lymphatic vessels were always terminated by ostial valves of two shapes, crescent-and navicular-like , in a ratio of 4:1. Specific regulatory structures along the free edg es of the valves, including marginal thickenings and buttresses, are d escribed. The tissue and cellular organization of the valve endotheliu m showed distinct peculiarities, particularly in the orientation and s hape of the cells and their microrelief. We found that valvular endoth elial cells, especially ''tip cells'', which are situated in unfavoura ble lymphodynamic conditions, were characterized by an increased volum e density of intermediate (probably vimentin-based) filaments, suggest ing an accommodative mechanism involving such filaments.