Histological and Ultrastructural Study of the «Cortico-medullary Perivenular Lymphoid Sheaths» of the Mouse Thymus

Abstract

The present study is concerned with morphological investigations on some special vascular structures of the mouse thymus pertinent to the problem of «cellular traffic» in this organ. Thymuses from 20 normal mice of C3Hf/Gs and C57BL strains, ranging in age from 30 to 50 days, were examined by both light and electron microscopy. At the cortico-medullary junction wide post-capillary venules are present, running parallel to the border between cortex and medulla. They receive at right angles narrow capillaries coming especially from the cortex. The wall of these venules is formed of flattened endothelium, its basement membrane and epithelial reticular cells arranged as adventitial cells. The basement membrane of the endothelium is more or less regularly split in two layers (the inner «vascular» layer and the outer «parenchymal» layer) thus delineating a perivascular space containing one or more rows of lymphoid cells. This space, which has on the whole the character of a cylindrical perivascular channel, accompanies the cortico-medullary venules, although not necessarily along their entire course, and it is recognizable even around the cortical capillaries draining into them. The two layers of the basement membrane are lined on the inner side by the laminar cytoplasm of epithelial reticular cells, often interconnected by desmosomes. The ultrastructural characteristics of the perivascular space do not support the hypothesis of its lymphatic nature, as maintained by other authors. Lymphocytes were found in the process of migrating through either the «parenchymal» or «vascular» layer of the basement membrane. Although the direction of migration cannot be determined by static pictures, some morphological data seem to suggest that lymphocytes, at the level of the cortico-medullary venules, move from the thymic parenchyma towards the perivascular space and from there into the circulating blood stream. It is suggested that the perivascular apparatus be termed «cortico-medullary perivenular lymphoid sheath».