These radioautographic studies using parabiotic rats and partial marrow shielding showed that cells responsible for recovery of irradiated bone marrow had their origin in the shielded marrow. Three morphologically distinct cell types appeared in the blood of these parabionts, mature granulocytes, small lymphocytes and monocytoid cells. The monocytoid was the major cell type which crossed from the shielded to nonshielded marrow, and the observations suggested that it is this cell which served as a stem cell for both the erythrocytic and granulocytic cell lines.
Labeled erythroblasts and myeloblasts were observed in the recovering marrow, and the labeling intensity of these cells indicated that they were the second or third division products of labeled immigrant cells.
The effect of marrow shielding upon the recovery of lymphopoiesis in spleen, thymus, lymph nodes and bone marrow is also discussed.