Kinetics of in vitro differentiation of erythroid progenitor cells. (A) Aliquots of the cultures shown in Fig 4 were evaluated for the proportion of proerythroblasts (proerbl), polychromatic and orthochromatic erythroblasts (pchrom and ochrom erbl, respectively), and erythrocytes (ery). (B) Cell size profile of the same cells as shown in (A) demonstrate reduction in cell size during differentiation: undifferentiated cells, white; differentiated cells at 24, 48, and 72 hours, grey, dark grey, and black, respectively.