Stage E2 BM proerythroblasts selectively and productively interact with primary BM stromal cells. (A-B) Stage E1 and E2 cells were purified from SP34-ex cultures and assessed for interactions with primary BMSCs. When cultured in parallel (at equal numbers, and cells per milliliter) with BMSCs, E2 proerythroblasts bound BMSCs at ∼ 250% increased levels over E1 cells (Axiovent 200, 160× overall, 1.6 NA, Vectashield, Axiocam MR, Illustrator 10). (C) E2 proerythroblast-BMSC interactions can also promote maturation to DRAQ5⁻ reticulocytes and apparent erythrocytes. On continued coculture (2-3 days), primary Lin⁻ BMSCs supported reticulocyte and apparent red cell formation as assessed by DRAQ5 staining (via flow cytometry) and apparent morphology (∼ 5-μm-diameter cells with apparent (bi)concave shape). (D) Expression of adhesion factors Itgb3 and Icam-4 peaks in stage E2 BM proerythroblasts. Expression of cell surface adhesion factors was assessed initially via analyses of triplicate transcriptome profiles for purified BM E1, E2, and E3 cell populations. For the integrins and Icams illustrated, relative expression levels also were assayed via quantitative RT-PCR (with beta-actin as an internal control).