Circulating erythroid progenitors in normal and anemic rabbits.

The position of circulating blood erythroid progenitors in the erythroid maturation sequence was determined in rabbits. The erythroid progenitors in blood appeared to be the same as the earliest class of erythroid progenitors detectable in bone marrow on the basis of time course studies of colony formation by blood and marrow cells in vitro. This result was confirmed by the ability of circulating erythroid progenitors to respond to the early erythroid regulator burst-promoting activity with an increase in colony number and 59Fe incorporation into heme. Induction of anemia with phenylhydrazine resulted in large increases in early erythroid progenitor frequency in blood and an influx of more mature progenitors (intermediate BFU-E and CFU-E). Circulating erythroid progenitors in rabbits may be an interesting model for the study of human disorders of erythropoiesis.