Investigations into the in Vitro Sensitivity to Erythropoietin of Haematopoietic Tissue from Ex‐hypoxic, Erythrocytotic Mice

If 'sensitivity' is defined as the ratio of haem synthesis in erythropoietin (Ep)-stimulated cultures to that in cultures without added Ep, then haematopoietic tissue of ex-hypoxic mice is less sensitive to Ep in vitro than similar tissue from normal donors. This appears to be due primarily to the loss of erythropoietin responsive cells from the marrow and spleen. In addition, saline extracts of mature erythrocytes inhibited erythropoiesis in normal bone marrow cultures in a manner directly related to the 'haematocrits' used during their preparation. These data support recent suggestions that the preference of erythrocytotic mice over normal animals for Ep assay probably can be attributed to their low baseline level of erythropoiesis rather than an increased sensitivity to Ep.