Adherence to macrophages in coculture increases proliferation of erythroblasts by decreasing G₀/G₁ transit time. (A) Percentages of nonapoptotic erythroblasts in the S + G₂/M phases of cell cycle and (B) percentages of nonapoptotic erythroblasts in the G₀/G₁ phases of cell cycle; adherent erythroblasts (■), nonadherent erythroblasts (♦), control erythroblasts (○). Results are the means plus or minus SE of 3 separate experiments; increased adherent erythroblasts in S + G₂/M and decreased adherent erythroblasts in G₀/G₁ compared with controls and nonadherent erythroblasts were significant at all times (*P < .007). (C) BrdU pulse-chase labeling for determination of cell cycle phases. From 6 hours to 7 hours of incubation, cocultured and control erythroblasts (EBs) were pulse-labeled with 25 μg/mL of BrdU, washed with ECM, and “chased” with normal medium. Green dots indicate BrdU-labeled erythroblasts; black dots, unlabeled erythroblasts. Incorporated BrdU was detected by fluorescein isothiocyanate-conjugated antibodies and total DNA content detected by propidium iodide staining (2N and 4N DNA content are shown on x-axes). In the experiment shown, 63% of adherent erythroblasts and 49% of control erythroblasts labeled with BrdU. The box designated R5 was constructed to demarcate the BrdU-labeled erythroblasts in G₀/G₁ phase of cell cycle as described in the text.