Scanning electron microscope studies of the rat mesenteric lymph node with special reference to high-endothelial venules and hitherto unknown lymphatic labyrinth.

Mesenteric lymph nodes of the rat were studied by scanning electron microscopy after arterial perfusion fixation. Observation was made mainly of the high endothelial venules (HEVs), inner cortex and medulla. The HEVs were distributed not only in the inner cortex, but also in the interfollicular areas in the outer cortex, extending to closely beneath the marginal sinus. Various surface structures of the HEV endothelial cells were recorded, from quite smooth to densely granular. Special attention was paid to endothelial cells with cytoplasmic ridges concentrated at the center of the cell apex. Lymphocytes were numerously attached to the luminal surface of the HEV. Many lymphocytes presumably under and after emigration through the HEV wall could be recognized. Hitherto unknown labyrinths densely filled with lymphocytes were demonstrated in the inner cortex. These tubular and saccular spaces lined by attenuated cells originated from beneath the germinal centers and from around the HEVs to pour into the medullary sinuses. The lymphatic labyrinths were differentiated from the sinuses by their lack of reticular cell trabeculae and by their containing almost exclusively small lymphocytes. It was proposed that lymphocytes in the blood in HEVs massively emigrate to the labyrinth, and via the medullary sinuses, enter the efferent lymphatics of the lymph node. In the medullary sinuses, some lymphocytes in association with a macrophage were found to extend wing-like processes, probably reflecting their activation. Large free cells extending laminar projections, but smooth in surface otherwise, were occasionally found. Their possible identity with the veiled cells (KELLY et al., 1978) was considered.