Vascular architecture of thymus and lymph nodes, blood vessels, transmural passage of lymphocytes, and cell-interactions.

Postcapillary or high-endothelial venules were preferentially distributed at the cortico-medullary junction of the thymus. In the lymph nodes these venules were located in he paracortical area between cortex and medulla. The venules in the thymus were surrounded by the so-called perivascular spaces, while those in the lymph nodes were not clearly surrounded by this space. The walls of the thymus venules were fenestrated. The reticular sheets surrounding the perivascular spaces were also fenestrated. These fenestrations facilitated the transmural passage of thymocytes through the vessels. In the lymph nodes where the venules had few fenestrations, lymphocytes in transmural passages were usually suspended between the high- endothelial cells. The macrophages were preferentially located in reticular meshes of the paracortical areas, and most of these cells were in contact with lymphocytes. This might indicate that the paracortical areas are the main interaction sites of lymph node lymphocytes and macrophages. Observations on the casted samples are also reported.