The aim of our study was to examine the lymphatic vascularization of the diaphragm in hibernating bats (Pipistrellus pipistrellus, Rhinolophus ferrumequinum) and the ultrastructural characteristics of the absorbing peripheral lymphatic vessels of the pleural and peritoneal surface, and also to determine how far and with which mechanisms these vessels participate in the mobilization of liquids and particles (China ink and polystyrene latex sferules) from the abdominal cavity. Deep lymphatic vessels forming networks and superficial lymphatic vessels were observed draining at the diaphragm's peripheral margin into valved pre-lymph node precollectors and collectors; the ventral precollectors principally terminate into the sternal lymph nodes or into the anterior mediastinal lymph nodes, while the dorsal precollectors terminate into the interrenal or para-aortic lymph nodes. The superficial lymphatic vessel undergoes significant seasonal changes, and as in other organs, has peculiar absorbing properties characterized by the membrane diffusioon phenomena, the 'vesicular pathway', and the 'intraendothelial channel pathway'. We demonstrated and discussed the ultrastructural aspects of the mesothelial stoma and the lower mesothelial connective channel, which are important components of the drainage unit. The peritoneal or abdominal contents drain indiscriminately first into the lymphatic vessel lumen and then into the valved precollectors by means of the intraendothelial channel, a dynamic entity organized by and according to the requirements of the endothelial wall. The valves block back flow during the rhythmic contractions of the diaphragm. Three-dimensional models and serialized thin slices of the lymphatic vessels and drainage unit components, have not confirmed the 'lymphatic stomata' hypothesis or direct continuity between the abdominal cavity and the lumen of the lymphatic vessels.
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