Fig. 3. Osteogenesis in pellet cultures. BMSCs were cultured under chondrogenic conditions for 4 weeks and subsequently in mineralization-inducing medium for 1 week. Histological sections of the cultured tissue harvested at 35 days of culture. (a) Toluidine-blue staining. Notice the absence of metachromatic staining (purple) in a peripheral rind of tissue. (b) Alizarin-red-S staining of a comparable section shows that the peripheral rind of non-chondroid tissue has accumulated calcium, which indicates mineralization. (c) Haematoxylin and eosin staining of another section, demonstrating a central core of histologically typical hyaline cartilage (hc) surrounded by a transition zone (tz) where basophilia (blue staining) fades out and an outermost layer of mineralized bone-like tissue (b) containing individual cells encased in lacunar spaces (d). Bar: a,b, 100 µm; c, 50 µm; d, 12.5 µm.