(A) Morphology of undifferentiated AS periosteal cells. (B and C) Osteogenic differentiation of periosteal cells after 21 days of culture in osteogenic medium. Secretion of a calcified extracellular matrix was clearly observed. (D) Control cells after 21 days of osteogenic induction without signals of osteogenic differentiation (mineralized matrix are absent). Differentiation was accessed by von Kossa staining.