FIG. 1.

Magnified (10×) micrographs of alkaline phosphatase (ALP)-stained cell cultures (A, C, D, E) after 1 week in osteoinductive medium and Oil red O–stained cell cultures (B, D, F) after 1 week in adipoinductive medium for all three cell types: MC3T3-E1 osteoblasts (A, B), primary periosteal cells (C, D), and NIH3T3 fibroblasts (E, F). All micrographs are of cell cultures initially seeded at high density; as in all cases, a larger percentage of positive staining was observed (p < 0.01). Panel (D) demonstrates that periosteal cells cultured in adipo-inductive medium for 1 week stain positively for ALP, suggesting that there was a subpopulation within the periosteal culture that expressed ALP in the absence of soluble osteogenic differentiation factors.