Ultrastructural Features of the Early Proliferating Cells Involved in Hemopoietic Colony Formation

After flash labeling with tritiated thymidine for autoradiography, a cohort of early proliferating autologous hemopoietic cells was examined in endocolonized mouse spleens with light and electron microscopy. During the first three to five days postirradiation, proliferating cells within colonies resembled medium to large immature lymphocytes. A thin rim of nuclear chromatin on an irregular inner nuclear membrane characterized the early cells. After day 5, erythroblasts were readily detected. Although unirradiated spleens contained myeloid and erythroid precursors, cells resembling lymphocytes were most frequently seen. Spleens of mice, receiving lethal total-body irradiation four days previously, contained very few cells synthesizing DNA and no erythroblasts. Macrophages ingesting nuclear material could be seen with light and electron microscopy.