Light and electron microscopic studies of the paracortical post-capillary high-endothelial venules

The light and electron microscopic structure of the high-endothelial postcapillary venules of lymph nodes were studied in the mouse, rat, guinea pig, and rabbit. The venules were most frequently found in the mouse and rat. In all species, they reached their highest degree of differentiation in the paracortical area. The morphology in the light microscope was identical in all four species. The venules in the rat and mouse were surrounded by a cuff of concentrically-arranged lymphocytes, which was rarely seen in the guinea pig and rabbit. The ultrastructure of the high-endothelial cells in all four species was very complex; a prominent Golgi apparatus was present which often occupied large parts of the cytoplasm. Coated and uncoated vesicles originating in the Golgi apparatus often permeated the cytoplasm. These vesicles emptied their contents into the extracellular space after fusion with the plasma membranes. Numerous lymphocytes traversed vessel walls. During their passage, they were always located between, not inside the high-endothelial cells.