The number of reticulocytes (A) and reticulocyte profile sizes (B) at various times during the in vitro differentiation of control and folate-deficient proerythroblasts. Cytocentrifuge preparations were made of control proerythroblasts cultured in control medium (▪), control proerythroblasts cultured in folate-deficient medium (▴), folate-deficient proerythroblasts cultured in control medium (□), or folate-deficient cells in folate-deficient medium (▵). Reticulocytes were counted in a blinded manner at 100× magnification. Reticulocyte sizes were determined by a computerized calculation of light microscopic images. For each culture, determination of reticulocyte numbers was made by counting consecutive microscopic fields until 300 nuclei (free or intracellular) were counted. For each culture, the determination of reticulocyte size was made from 30 consecutive reticulocytes. All data are the means ± 1 SEM of three separate cultures. *P < .05 when data from folate-deficient cells cultured in folate-deficient medium are compared with all other treatments.