Vascular Permeability in the Thymus of the Mouse

In this study, thymic blood vessels in adult mice were examined particularly in relation to the permeability of their walls. The pattern of distribution of the thymic vessels indicated that the cortex is supplied, for the most part, with capillaries, and the medulla with venules.Cortical capillaries and medullary venules are usually encircled by epithelial reticular cells, but the investment is not completely continuous. Endothelial cells of cortical capillaries and medullary venules contain numerous micropinocytotic vesicles. In addition, larger vacuoles are also seen in the endothelial cells of the venules. After intravenous injection of ferritin, these micropinocytotic vesicles and vacuoles in the endothelial cells of the thymic vessels are labeled by ferritin. Ferritin particles also appear in the perivascular spaces of the thymic vessels. The vascular permeability in the thymus is different between cortical capillaries under the capsule, capillaries in the inner cortex and venules in the medulla. The injected ferritin in only limited amounts also invades the thymic parenchyma especially in the outermost cortex and the medulla. The permeability of ferritin across the walls of thymic blood vessels was compared with that in the splenic lymphatic tissue, mesenteric lymph node and pancreas. The results obtained were discussed with special reference to the vascular barrier mechanism in the thymus.