Hemopoietic progenitors, stimulating factors, and hemoglobin switching.

Studies with murine hematopoietic cells have characterized three separate molecular species acting at some point on the erythropoietic lineage (GM-CSF, BPA, Epo). These studies have been performed mainly with adult tissues, and during the embryonic phase of murine development only macrophage colony-stimulating factor and an activity capable of stimulating CFU-E can be detected in yolk sac fluid. Multipotential or erythroid-committed progenitor cells (BFU-E) obtained from 9-day murine embryos produce only adult hemoglobins when stimulated with an adult source of BPA. Studies with human cord blood erythroid progenitor cells have failed to detect a human equivalent of murine BPA. Clone transfer experiments suggest that erythropoietin alone is required to stimulate human cord blood BFU-E. Experiments of similar design are required to determine the role of various hemopoietic regulatory factors in adult human erythropoiesis.