Erythroblastic Differentiation of Stem Cells in Hemopoietic Colonies

An early phase of cellular replication precedes the initiation of erythropoiesis in hemopoietic spleen colonies of mice receiving a supralethal split-dose of irradiation, first with the leg shielded and then followed by leg irradiation 3 hr later. Previous studies indicated that this proliferative phase represents the repopulation of a depleted, endogenous stem cell compartment. To label replicating stem cells, tritiated thymidine (³HTdR) was injected intraperitoneally in mice on days 1-4 after irradiation. Touch preparations and autoradiograms of splenic colony cells were examined with light and electron microscopy from 2 to 4 days after irradiation and 5-8 days later, when erythroblasts and other differentiated cells first appeared. Replicating mononuclear cells that were pulse labeled during the early proliferative phase of stem cell renewal resembled medium- to large-sized leptochromatic lymphocytes. Electron microscopic examination of autoradiograms demonstrated a labeled undifferentiated cell with a thin rim of nuclear heterochromatin. Sequential studies showed that the injection of ³HTdR on days 1-4, when only the mononuclear cells were present, resulted in labeled erythroblasts and other differentiated cells on days 5 and 6. These results confirm the presence of an early proliferative phase in endogenous splenic hemopoietic colonies and strongly suggest that mononuclear cells, replicating during this period of self-renewal of the stem cell compartment, transform to erythroblasts and other cells.