Freeze-fracture studies of matrix vesicle calcification in epiphyseal growth plate

Abstract

Freeze-fractured, unfixed and glutaraldehyde fixed 6–10 week rat tibial growth plates were studied. Fixed and unfixed specimens appeared similar in ultrastructure. Chondrocyte organelles were examined from the reserve to the calcifying zones. The plasma membrane of intact cells showed variable numbers of microvilli, most numerous in the upper growth plate, with occasional vesicles budding from their tips. Vesicles also appeared to bud from the plasma membrane of cells without microvilli. Matrix vesicles were identified in a typical distribution, appearing as clusters which were concentrated in the longitudinal septal matrix. The latter shows preferential calcification. Both plasma membranes and matrix vesicle membranes showed membrane associated particles (MAPs) at their membrane fracture faces, and matrix vesicle MAPs appeared slightly larger. Within the calcifying zone, the debris of degenerating chondrocytes produced intralacunar vesicles which varied considerably in size but usually were larger than matrix vesicles. Collagen and proteoglycan granules were evident throughout the matrix of the growth plate. The fractured surface of mineralized matrix appeared corrugated and contained a few matrix vesicles which could be identified by their MAPs. These studies using unfixed tissue confirm earlier observations by conventional techniques indicating a role for matrix vesicles In mineralization.