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 edges of the valves, including marginal thickenings and buttresses, are described. The tissue and cellular organization of the valve endothelium showed distinct peculiarities, particularly in the orientation and shape of the cells and their microrelief. We found that valvular endothelial cells, especially ¿tip cells¿, which are situated in unfavourable lymphodynamic conditions, were characterized by an increased volume density of intermediate (probably vimentin-based) filaments, suggesting an accommodative mechanism involving such filaments.