Rapid-frozen and freeze-substitution fixed epiphyseal cartilages of mouse tibiae and fibulae were studied. Ultrastructural features of chondrocytes throughout the tissue, from the reserve to the calcifying zone, were well preserved. In particular, various membrane systems in the cytoplasm were wellretained and their membranous components appeared to have a tri-laminar structure with a smooth contour. The electron density of cytoplasmic matrix, various organelles, and extracellular matrix was higher in frozen materials than in conventionally fixed ones. Cartilaginous matrix contained flocculent materials instead of matrix granules in the interfibrillar and penicellular matrices. These materials may represent proteoglycan aggregates. The present study confirmed that the initial crystal deposition occurred within the matrix vesicles. The membrane of the matrix vesicle was in direct contact with flocculent materials. Some matrix vesicles appeared to be derived from the plasma membranes of chondrocytes without cellular spines. Within the hypertrophlc and calcifying zones, cell debris derived from disintegrated cells seemed to be closely related to the production of matrix vesicles. Pleomorphism in cytological characteristics of the chondrocytes in hypertrophic and calcifying zones was observed. In all zones of the epiphyseal plate, the plasma membrane of chondrocytes was always in direct contact with either the surrounding territorial matrix containing collagen fibrils or the pericellular, finely textured matrix without collagen fibrils.
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Akisaka et al. (1983) studied this question.