Abstract
The in vitro effect of sheep erythropoietin on bone marrow cells was studied in order to elucidate its point of action. It was concluded that erythropoietin in vitro acts primarily or exclusively on stem cells and does not have a measurable effect on differentiated nucleated red blood cells or on reticulocytes. Attempts to isolate and study stem cells by freezing or by long or short term culture were not successful.