Analysis of bone nodule formation. (A) Photograph of day 16 cultures stained by Von Kossa to show mineralization. All subpopulations exhibit mineralization when grown on plastic dishes (P) but S and L⁻ cells exhibit larger numbers of bone nodules. Mineralization is observed only for S and L⁻ cells when grown on collagen-coated (C) dishes. (B) Bone nodule number and size (μm) determined after Von Kossa staining of the day 16 cultures grown on plastic dishes (mean ± SEM).