Erythroblastic islands can be reconstituted with splenic macrophages and developmentally synchronized CFU-E/proerythroblasts from the spleens of Friend virus-infected mice. (Left) Photomicrographs of cultures fixed in situ at 6 hours of coculture. (Top panel) Purified, Wright-stained splenic macrophages attached to tissue culture plates. These macrophages did not receive erythroblasts for coculture. The lower 2 panels in the left column show adherent erythroblasts that reconstituted erythroblastic islands at 6 hours of coculture. Arrows point to macrophage nuclei. The remaining columns show photomicrographs of 3,3′-dimethoxybenzidine and hematoxylin-stained cytocentrifuge preparations of control erythroblasts cultured without macrophages (Cont), nonadherent erythroblasts from cocultures with macrophages (NonAdh), and erythroblasts that were adherent to macrophages (Adh) at 20, 32, and 44 hours of culture. At 20 hours, almost all cells are proerythroblasts, which accumulate heme (amber stain) by 32 hours. At 44 hours, control and nonadherent cultures have numerous reticulocytes (enucleated, amber cells) and extruded nuclei (dense, round, purple bodies). Slightly fewer adherent cells have enucleated at this time. Bar represents 10 μm.