The plasma membrane and matrix vesicles of mouse growth plate chondrocytes during differentiation as revealed in freeze‐fracture replicas

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Abstract
The epiphyseal cartilage in mouse tibia and fibula was investigated with the freeze‐fracture method. Cytodifferentiation of growth plate chondrocytes was found to be marked by changes in both cell membrane and extracellular matrix vesicle membranes. Exocytosis and endocytosis were observed in all zones of differentiation, with endocytosis being predominant in the reserve and proliferative zones and exocytosis occurring with greatest frequency during hypertrophy. Intramembraneous particles (IMPs) on the plasma membrane were distributed evenly on the reserve and proliferative cell membranes, whereas in the hypertrophic zone IMPs tended to be distributed asymmetrically. Several types of matrix vesicles were identifiable on the basis of IMP distribution: IMP‐free, IMP‐aggregated, and IMP‐random. The distribution pattern of IMPs on vesicles varied with differentiation of the chondrocytes. For proliferative and prehypertrophic cells, most matrix vesicles belonged to the IMP‐random category. IMP‐aggregated and IMP‐free matrix vesicles became increasingly frequent in the later stages of differentiation, particularly in the late hypertrophic stage. IMPs were observed more frequently on the convex protoplasmic fracture face of matrix vesicles than on the concave exoplasmic fracture face, as was also observed for the plasma membrane. Matrix vesicles formation appears to occur by budding from chondrocyte projections and bulges at the smooth surfaces of the cells and from cell disintegration. Crystals of mineral were apparent in cross‐fractured matrix vesicles of the calcifying zone, but not in the other zones.