Origin and Fate of Cells in the Medulla of Rat Thymus. ∗

Abstract

Summary

Counts of resting and dividing cells in the thymic medulla of 10-week-old male rats revealed that reticular cells are more abundant than in cortex, but the 3 types of lymphocytes (large, medium, small) are less abundant than in cortex. However, except for small lymphocytes, each cell type has approximately the same mitotic index and, therefore, approximately the same life span in the 2 regions. 2) In view of the small number of large lymphocytes present, it appears that the evolution of reticular cells into lymphocytes is the exception in medulla, instead of being the rule as in cortex. Mitoses of reticular cells would mainly give rise to other reticular cells. Since these cells frequently show signs of degeneration, it is suggested that their loss through degeneration balances the overproduction indicated by their high mitotic activity. Clumps of reticular cells, many of which are in various stages of degeneration, make up the structures known as Hassall's corpuscles. 3) That the small lymphocyte is the end product of a series of 8 successive generations from large lymphocytes through smaller and smaller cells, seems to hold in medulla as in cortex. However, the presence of many more small lymphocytes in medulla than expected from the number of medium lymphocytes raised the possibility that small lymphocytes immigrate from cortex. Indeed, since diapedesis of small lymphocytes was commonly seen across the walls of the perivascular lymphatic channels and blood vessels in medulla but not in cortex, a migration of cortex-formed small lymphocytes through the medulla must be postulated to allow them to reach the circulation. To a lesser extent, medium lymphocytes (and perhaps even the odd large lymphocyte) may also migrate from cortex to medulla and from there to the circulation. It is concluded that only a small degree of lymphocyte formation occurs in medulla. This region is mainly a passageway allowing the lymphocytes arising in cortex to reach the circulation.