Importance of ov chimerism in the thymus depends on the number of injected T-cell progenitors. (A) BM cells derived from E13 H.B19 animals stained for c-kit and HEMCAM. The FACS profile shows BM gated for lymphoid cells in the forward and side scatter. HEMCAM⁺ lymphoid cells represented 10% of total BM cells and 50% of lymphoid cells. (B) Percentage of ov⁺ donor thymocytes when increasing numbers of total (□), HEMCAM⁺ sorted (▪), and HEMCAM⁺c-kit⁺ sorted (○) E13 BM cells were injected into irradiated chicks. (C) Determination of T-cell precursor frequency in HEMCAM⁺ c-kit⁺ E13 BM population by limiting dilution analysis. Titrated numbers of BM cell populations were injected intrathymically into ov⁻ recipients and the thymus was assayed 2 weeks later for ov⁺ cells by flow cytometry using MoAb 11A9. These data were obtained after injection of 12 cells, 20 cells, 40 cells into 5, 7, and 12 recipients, respectively.