Specialized structure and metabolic activities of high endothelial venules in rat lymphatic tissues.

Microscopic, histochemical and ultrastructural techniques were used to define characteristics of high endothelial venules (HEV) in rat lymphatic tissues. This endothelium contained acetyl esterase and acid hydrolase activities which were not altered by lymphocyte depletion. No immunoglobulins were detected on luminal surfaces of HEV by fluorescent antibody staining. Only minor structural differences were seen between HEV within lymph nodes and Peyer's patches. At both sites, high endothelial cells were linked together by macular junctional complexes and interlocking basal foot processes. Endothelial cell cytoplasm moulded about surfaces of lymphocytes migrating through the venular wall, and flocculant deposits of basement membrane formed over lymphocytes penetrating the basal lamina. The endothelium was ensheathed by three to five layers of overlapping reticular cell plates and connective tissue. Each plate was linked to the reticular meshwork of the node by collagen bundles and anchoring filaments which inserted into the plate's external limiting membrane. This permitted individual paltes to separate or approximate each other as tissue and intravascular pressure varied, and lymphocytes moved across the sheath by insinuating themselves into gaps between overlapping plates. This composite structure of the HEV wall appeared to facilitate lymphocyte entry into the node and minimized vascular leakge.