LIGHT AND ELECTRON-MICROSCOPIC OBSERVATIONS ON THE THYMIC CAPILLARIESAND 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 tr ansmission electron microscopy (TEM) as well as scanning electron micr oscopy (SEM) of vascular corrosion casts. The endothelial cells of cap illaries and postcapillary venules show in frog thymus mostly flat mor phology. However, some postcapillary venules are lined with tall endot helial cells which are characteristic for the thymus-dependent areas o f mammalian secondary lymphatic organs (high endothelial venules, HEV) . Such endothelial cells occurring in frog thymus are ultrastructurall y characterized by increased amount of cytoplasmic organelles, mainly rough endoplasmic reticulum, ribosomes and electron dense lysosome-lik e granules, as well as by deep intercellular clefts. In HEV-like vesse ls, lymphocytes were observed to adhere to the endothelial surface, mi grate 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 cast surfaces with numero us depressions and protrusions reflecting the cobblestone appearance o f the tall endothelial lining. Such venules are fed directly by capill aries and begin with abrupt increase in the vessel diameter, accompani ed by the appearance of deep endothelial imprints. Our results demonst rate 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.