Light and electron-microscopic observations on the thymic capillaries and postcapillary venules in Rana temporaria L.

The aim of this study was to examine thymic capillaries and capillary venules in a common frog, Rana temporaria, using the light (LM) and transmission electron microscopy (TEM) as well as scanning electron microscopy (SEM) of vascular corrosion casts. The endothelial cells of capillaries and postcapillary venules show in frog thymus mostly flat morphology. However, some postcapillary venules are lined with tall endothelial cells which are characteristic for the thymus-dependent areas of mammalian secondary lymphatic organs (high endothelial venules, HEV). Such endothelial cells occurring in frog thymus are ultrastructurally characterized by increased amount of cytoplasmic organelles, mainly rough endoplasmic reticulum, ribosomes and electron dense lysosome-like granules, as well as by deep intercellular clefts. In HEV-like vessels, lymphocytes were observed to adhere to the endothelial surface, migrate through the endothelium, or lay beneath the endothelial basement membrane in the perivascular space. These findings were corroborated by SEM of corrosion casts which revealed in some postcapillary venules or their segments the presence of irregular casts surfaces with numerous depressions and protrusions reflecting the cobblestone appearance of the tall endothelial lining. Such venules are fed directly by capillaries and begin with abrupt increase in the vessel diameter, accompanied by the appearance of deep endothelial imprints. Our results demonstrate in the frog thymus the presence of specialized, HEV-like segments of postcapillary venules, suggesting that besides its function as the central lymphoid organ, frog thymus may also function as a peripheral one.