Coculture with macrophages in reconstituted erythroblastic islands increases proliferation of FVA erythroblasts. (A) Fold increase in total numbers of FVA erythroblasts cocultured with macrophages in reconstituted erythroblastic islands (■) and in control cultures without macrophages (○). Control cultures increased from 5 × 10⁵ erythroblasts/mL at 6 hours to 2.3 × 10⁶ erythroblasts/mL at 44 hours. Cocultures increased from 0.9 × 10⁵ erythroblasts/mL to 1.2 × 10⁶ erythroblasts/mL. (B) Fold increase in FVA erythroblasts of adherent erythroblasts in cocultures (), nonadherent erythroblasts in cocultures (■), and in control cultures (○). Results are the means plus or minus SE from 24 separate experiments; increased accumulations of erythroblasts in cocultures compared with control cultures were significant (*P < .001) at all times after 6 hours.