The rates of proliferation and developmental capabilities of reticular cells in the hemopoietic tissues of the rat were studied after administering single and multiple injections of tritiated thymidine. Radioautographic analyses were made using both smears and sections of the various hemopoietic tissues. Liquid scintillation counting of tissues was also employed. Sublethal gamma irradiation was administered to animals previously labeled with H3-thymidine in order to evaluate the role of reticular cells during the repopulation of the marrow.
The labeling patterns (per cent label and average grain count) of reticular cells, both phagocytic and non-phagocytic types, are compared with the labeling patterns of blast cells. It is shown that reticular cells are proliferating at a slow rate and do not demonstrate the labeling patterns expected of stem cells for the rapidly proliferating myeloblasts, pronormoblasts, lymphoblasts and megakaryoblasts. The labeling patterns of cells which are generally believed to precede the committed blast cells, the hemo-cytoblasts, are also discussed and are contrasted with the labeling patterns of the reticular cells. During recovery from sublethal irradiation there was no evidence of differentiation or transformation of labeled reticular cells into blast cells of the various blood cell lines. The labeling patterns of blast cells suggest that the great majority are derived from members of their own group and that any proposed precursors of these blasts would need to be relatively few in number, rapidly proliferating and highly radiosensitive.