Lymphatic valves of the rat pancreas.

The detailed structure of pancreatic lymphatic valves in rats was examined in an attempt to identify ultrastructural features that could be correlated with the ability of these delicate structures to withstand retrograde flow. Sprague-Dawley rats were perfusion-fixed and the pancreas processed for light and electron microscopy. Lymphatic vessels were identified by their typical appearance coupled with the presence of valves within their lumen. These valves were consistently formed of cuspid leaflets joined to the lymphatic wall at the bases and sides enclosing valvular pockets or sinuses between cusp and wall. Each cusp or leaflet consisted of two simple squamous endothelial layers separated by a connective tissue core and thus appeared, at first sight, as a simple infolding of the lymphatic vessel lining with its underlying connective tissue. However, certain differences were seen. Frequently the free margins of the cusps, instead of being smooth as might be expected, exhibited endothelial extensions or processes which were arranged in such a way that they could interdigitate with similar extensions on the opposing cusps and thus aid in closure of the valves. A striking difference between the endothelial lining of the vessel and that of the cusp was the presence of a distinct and almost continuous basal lamina underlying the endothelial cells which lined the surface of the cusp facing the valve pocket. The opposing surface of the cusp, which faced the central lumen was similar to the typical lining of lymphatics in showing little or no basal lamina.(ABSTRACT TRUNCATED AT 250 WORDS)