Cytologic Features and Cellular Migration in the Cortex and Medulla of Thymus in the Young Adult Rat

The cells of the cortex and medulla of thymus and their mitoses were described in 10-week old (200 Gm.) male rats.

The four main cell types found in cortex (reticular cells, large, medium and small lymphocytes were related to each other by transitional cell types. The presence of numerous mitoses of the four cell types indicates rapid cell production. Since the thymus of 10-week old rats is not growing and, therefore, each cell population must be in a steady state, the mitotic production of new cells of a given type must be balanced by transformation into cells of another type or by emigration out of the cortex. Evidence is presented in support of the transformation of reticular cells into large lymphocytes; and of these into lesser and lesser sized lymphocytes. As for small lymphocytes, the evidence indicates that they migrate form cortex to medulla.

The medulla contains numerous small lymphocytes, some reticular cells, and rare large and medium lymphocytes. The lymphocytes of the medulla (presumed to have migrated from the cortex) often show nuclear processes which are attributed to ameboid motion.

The medulla of the rat thymus contains many blood vessels, most of which are enclosed within "perivascular channels." Diapedesis of lymphocytes, chiefly small ones, is frequently seen across the walls of both the perivascular channels and the blood vessels themselves. Furthermore, higher counts of lymphocytes in venous than in arterial blood of thymus indicate that these cells directly enter the blood circulation.

In conclusion, cells of the lymphocytic series are produced by mitosis in the cortex of the thymus. The evidence indicates that lymphocytes arising in this region migrate into the medulla. Thence, these cells pass into perivascular channels and into the enclosed blood vessels to reach the circulation.