Regulation of red blood cell production by erythropoietin: normal mouse marrow in vitro

The response of immediate erythroid precursors to timed exposures of erythropoietin (Ep) were investigated by examining the formation of murine erythroid colonies in vitro. At various times after the initiation of normal marrow cultures with Ep, enough anti-erythropoietin serum was added to neutralize the actions of the hormone for the rest of the 48 h culture period in plasma clots. Results show that a significant number of erythroid precursors develop to a mature colony after very short Ep exposures (as little as 18 min), but they account for only about 13% of the total colonies generated when Ep is active for 48 h. Only if Ep is active for more than 6 h do additional colonies form, in direct proportion to the length of the Ep exposure. Maximum erythroid colony production requires prolonged exposure to Ep, and the longer Ep is active, the larger the percentage of erythroid colonies that reach 17 cells or more. The mechanism(s) behind these two effects appear intimately connected. These results suggest the existence of two populations of Ep-responsive cells in the immediate erythroid precursor compartment of mice: (1) A CFUE pool, which is on the verge of commitment into the recognizable erythron, and requires very short Ep exposures for this entry; and (2) a "pre-CFUE" stage which feeds into the CFUE pool in the presence of Ep, and which produces colonies only after longer Ep exposures. It is proposed that the pre-CFUE response to Ep involves 1-2 self-replications before entering the CFUE population.