In the hope of clarifying the sequence of cells in thymus, mitotic figures were classified into four types and counted in the cortex and medulla of thymus of young adult rats. The four types were designated as reticular cell mitoses (characterized by an unstained cytoplasm with a very large mitotic figure), and lymphocyte mitoses (characterized by a basophilic cytoplasm with a mitotic figure, which may be large, 5.6-8.0 µ, medium sized, 4.5-5.6 µ, or small, 3.5-4.5 µ). The relative mitotic counts of the four cell types in thymic cortex were 3, 46, 172 and 240 respectively.
Analysis of these counts, assuming equal mitotic duration and orderly sequence of the cells led to two sets of conclusions. (1) Large, light cells, the divisions of which are referred to as reticular cell mitoses, would produce large lymphocytes. These cells would proliferate to give rise to medium lymphocytes, which in turn would proliferate to give rise to small lymphocytes. (2) There would be 2-4, most probably 4, successive generations of large lymphocytes, followed by 2 successive generations of medium lymphocytes, which would in turn be followed by 1 generation of mitosable small lymphocytes, probably giving rise to another generation whose cells would not divide, at least in thymic cortex. A model of lymphocytopoiesis for the cortex of thymus is proposed on the basis of these conclusions.