Tentative Pattern for Renewal of Lymphocytes in Cortex of the Rat Thymus. ∗

Abstract

Summary

1 Counts of resting and dividing cells in the cortex of the thymus of 10-week-old male rats reveal a high mitotic index of the 4 cell types present: reticular cells, large, medium and small lymphocytes. This observation indicates that each one of the 4 cell types is being renewed. Presumably this renewal yields lymphocytes which leave the thymus and enter the circulation. 2) From the mean numbers of cells and of mitoses and the mean mitotic indices of the four cell types over a 24-hour period, a tentative scheme referred to as “Stem cell renewal theory” is presented to account for the continuous production of lymphocytes in the thymus. This theory (Fig. 4) is that each reticular cell at regular intervals yields large lymphocytes which pass through 4 generations and then produce medium lymphocytes, which in turn pass through 2 short-lived generations and then give rise to small lymphocytes, of which there are also 2 successive generations.