B220⁺CD19⁻ adherent cells from differentiated LSK cells cultured on OBs in vitro differentiate into IgM⁺ B cells in vivo. LSK cells from CD45.1⁺ mice were cultured on (CD45.2⁺) OB monolayers for 10 days, and the OB-adherent hematopoietic cells (OB-adh) were recovered. B220⁺CD19⁻ pre-pro-B cells were isolated by FACS and transplanted into sublethally irradiated CD45.2 mice. (A) CD45.1⁺ cell contributions to recipient bone marrow, spleen, thymus, and blood after 21 days (means ± SD, n = 5). (B) Costaining with CD45.1 and B-cell differentiation markers, demonstrating the presence of donor-derived CD45.1⁺B220⁺CD19⁺IgM⁺ immature B lymphocytes in BM and spleen (n = 5). (C) Phenotypic characterization of (CD45.1⁺) donor-derived B220⁻CD19⁻ cells in recipient (CD45.2⁺) bone marrow.