Microvasculature of normal and involuted mouse thymus. Light- and electron-microscopic study.

The spatial arrangement of blood vessels in the thymus of normal and hydrocortisone-injected mice was studied by light and electron microscopy. The thymus is supplied by one thymic artery which branches into arterioles as it enters the parenchyma. These, in turn, feed capillary networks in the cortex and in the medulla. Cortical networks at the periphery of the lobule form loops which return blood to postcapillary venules at the corticomedullary junction and in the medulla. There is no subcapsular venous drainage. The overall distribution of blood vessels in the involuted thymus is essentially the same as in the normal thymus but the pattern becomes irregular and the vessels are tortuous. The endothelium of the postcapillary venules is flat and surrounded by a wide perivascular space containing many lymphocytes. This space is delimited by basal laminae, on the one side by that of the abluminal surface of the venular endothelium, on the other side by that of a thin, sheet-like epithelial layer formed by cytoplasmic processes of reticular cells. The perivascular space is in continuity with the surrounding interstitial space via gaps in its epithelial sheet. It does not form continuous longitudinal channels along the venules, but is interrupted by epithelial trabeculae. There is no obvious difference in structure between the postcapillary venules and their perivascular space of normal and those of involuted thymuses. Lymphocytes are intercalated in the venular endothelium particularly in involuted glands. They are present in the perivascular space and in gaps of the outer epithelial sheet. These findings suggest that the postcapillary venules and the perivascular spaces may function as pathways for the migration of thymic lymphocytes into or out of the blood circulation.