Fig. 4. Electron microscopic analysis of cartilage and bone tissues formed in pellet culture. The culture was harvested at 35 days (28 days of culture under chondrogenic conditions and 7 days of culture in mineralization-inducing medium). (a) Azur II /methylene blue stained semithin section demonstrating the relative position of the electron-microscopy fields imaged in b-e. Mineralized tissue is on the left and hyaline cartilage on the right. (b) Bone-like matrix containing thick, periodically banded collagen fibrils and areas of mineral deposition (arrows) (undecalcified section, original magnification 28500x). (c) Transition zone containing alternate thick, periodically banded, and thin, non-banded collagen fibrils. Arrow indicates mineral clusters. No proteoglycan granules are seen (undecalcified section, original magnification 28500x). (d) Mineralizing cartilage. Arrows indicate mineral clusters. The remaining matrix has typical features of hyaline cartilage (thin, randomly oriented collagen fibrils and proteoglycan granules) (decalcified section, original magnification 11500x). (e) Hyaline cartilage (decalcified section, original magnification 11500x). Bar: a, 25 µm; b,c, 6 µm; d,e, 2.3 µm.