Migration into pseudo-follicles of draining lymph nodes of medullary small thymocytes injected in the mediastinal cavity.

After an intramediastinal injection of labelled thymic cells, few cells were found in the pseudo-follicles of the draining nodes while the extrafollicular zone of their cortex contained abundant labelled cells (Sainte-Marie and Peng, in press). We proposed that the cells having migrated into the pseudo-follicles were small lymphocytes of the thymic medulla, or medullary small thymocytes, which accounts for about 5% of the thymocyte population. The purpose of the present study was to test the validity of the proposal. Rats received corticosterone injections to destroy, their cortical thymocytes and, thereafter, a dose of 3H-cytidine in order to label the surviving medullary small thymocytes. One hour later, these cells were suspended and injected in the mediastinal cavity of recipients which were killed 3 and 24 hours after the injection. The radio-autographs of the draining, and of the remaining, nodes revealed that most labelled cells, present in the nodes at the 24-hour interval, were situated mostly in the pseudo-follicles. The finding indicates, that, unlike the cortical small thymocytes, the medullary small thymocytes can migrate into pseudo-follicles. This is probably due to the greater motility of the medullary small thymocytes and possibly, to their involvement in the function(s) carried out in the pseudo-follicles.