Biologic activity of HAPO in vitro in a human system. (A) Effect of various concentrations of rhHAPO on the growth of CFU-Mix. Human fetal marrow nonadherent cells were cultured at 1 × 10⁵/35-mm dish in methylcellulose containing growth medium with a cocktail of cytokines as described in Table 1, with or without the indicated concentrations of rhHAPO for 14 days. All cultures were performed in quadruplicate and scored with the use of an inverted microscope. Data represent mean ± SD. (B) Morphology of CFU-Mix colony in semisolid culture. The colonies stimulated by HAPO plus CC (ii) were bigger in size than those stimulated with HAPO alone (i). Original magnification × 40. (C) Morphology of human fetal bone marrow MNCs cultured in a liquid system with or without rhHAPO (100 ng/mL). Human fetal bone marrow cells were cultured without rhHAPO (i) and with rhHAPO (ii) for 4 weeks. Original magnification × 200 (D) Effect of rhHAPO on human fetal bone marrow CD34⁺ cells in liquid culture. Fluorescence-activated cell sorter (FACS) analysis shows an increase of the percentage of CD34⁺ and CD45⁺ cells in the presence of rhHAPO.