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

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Abstract
Young adult rats were sacrificed at 4 periods of a day (10 a.m., 4 p.m., 10 p.m., 4 a.m.). Their thymuses were fixed in Bouin-Hollande, sectioned at 5[mu] and stained with Dominici. Resting and dividing nucleic of the 4 main cell types present (reticular cells, large, medium and small lymphocytes) were counted and their mitotic indices were respectively 1.5, 6.5, 13.1 and 1.5%. These high values indicate that the 4 cell types are continuously renewed. Furthermore, the numbers of cells and mitoses were consistent with the following pattern: On the average, a reticular cell would divide at regular intervals to yield another reticular cell and a large lymphocyte, which would divide in such a way as to produce 4 generations, the last one of which would yield medium lymphocytes. These in turn would pass through 2 short-lived generations to give rise to small lymphocytes, which would also divide and yield a total of 128 mature small lymphocytes.