Identification and characterization of three classes of erythroid progenitors in human fetal liver.

Erythroid progenitors from 1 8 fetal livers (7-1 5-wk gestational period) and 1 3 normal adult marrows were grown in methylcellubose cultures containing erythropoietin (Ep) and burst-enhancing factor(s) (BEF). Additional experiments were carried out on 1 fetal, 1 5 neonatal. and 26 adult blood specimens. Three classes of progenitors (primitive or mature erythroid burst-forming unit(s). P-BFU-E. M-BFU-E respectively; erythroid colony-forming unit(s), CFU-E) have been identified in fetal liver. Identification was based on their differential cbonogenic characteristics (i.e., colony morphology and number. time/growth curve, Ep and BEF sensitivity. in vitro 3H-thymidine suicide index). The multistep differentiation of fetal erythroid progenitors is associated with: (1 ) gradual amplification of their pool size, (2) progressive decline of their BEF response. (3) gradual enhancement of their Ep sensitivity. Marked differences are observed, however, between the characteristics