Low-dose cytarabine decreases division and promotes differentiation of O-2A/OPCs. Cells grown at clonal density were exposed 1 day after plating to low-dose cytarabine (0.01 μM for 24 h), a dosage that killed less than 5% of O-2A/OPCs in mass culture (Figure 9). The number of undifferentiated O-2A/OPCs and differentiated cells (oligodendrocytes) was determined in each individual clone from a total of 100 clones in each condition by morphological examination and by immunostaining with A2B5 and anti-GalC antibodies (to label O-2A/OPCs and oligodendrocytes, respectively), as in Figure 4. (a) Composition of progenitors and oligodendrocytes in a representative experiment of control cultures analyzed 6 days after plating optic nerve-derived O-2A/OPCs cells at clonal density. (b) In parallel cytarabine (Ara-C)-treated cultures analyzed 6 days after plating at clonal density (5 days after the start of cytarabine exposure), there was a marked increase in the representation of small clones consisting wholly of oligodendrocytes, a reduction in the representation of large clones, and a general shift of clone size towards smaller values. Experiments were performed in triplicate in at least two independent experiments.