Fig. 5. Immunoreactivity for type I procollagen N-terminal propeptide at 7 (a), 14 (b) and 28 (c) days of culture under chondrogenic conditions. Notice the growing peripheral region of type I collagen production (double arrows). Extracellular immunoreactivity, denoting the initial deposition (id) of collagen, marks the boundary of cartilage. Intracellular labelling (arrows) is restricted to a peripheral rind of tissue where immunoreactivity is lost in the extracellular matrix, consistent with zonally defined matrix maturation events (m). Bar, 50 µm.