Biosynthesis of matrix vesicles in epiphyseal cartilage

Abstract

The in vivo metabolism of 32P orthophosphate into phospholipids of chondroxyte, matrix vesicle, and membrane fractions of chicken epiphyseal cartilage has been studied. Incorporation of radioactive phosphate into the total phospholipid fraction of matrix vesicles was rapid, the labeling of phosphatidylcholine and lysophosphatidylserine being even more rapid in matrix vesicles than in chondrocytes. These findings indicate that matrix vesicles are formed by a rapid, metabolically active process, and are not remnants of dead cells, as had previously been postulated by some workers. The rate of incorporation of 32P orthophosphate into phosphatidylserine and sphingomyelin of matrix vesicles was significantly slower than that of phosphatidylcholine and certain other vesicle phospholipids. These findings are paradoxical because, compared with chondrocytes, matrix vesicles were enriched in phosphatidylserine and sphingomyelin and depleted in phosphatidylcholine. These results indicate that in vesicle formation the rates of degradation of the various phospholipid classes must be markedly different: phosphatidylcholine must be degraded much more rapidly than either phosphatidylserine or sphingomyelin. Support for this comes from previous data which revealed that substantial phospholipase activity is present in epiphyseal cartilage, especially in the zone of hypertrophy where matrix vesicle formation appears to be particularly active.

Matrix compartments in the growth plate of the proximal tibia of rats

“…They transverse the PG envelope and terminate in the territorial matrix. They may play a role in the release of collagen precursors and other matrix components and in the segregation of matrix vesicles (Wuthier et al, 1977;Schenk et al, 1982). The ITM is thought to facilitate diffusion in the longitudinal direction, especially from the nutritive vessels along its epiphyseal surface toward the metaphysis.…”

Matrix vesicles: structure, composition, formation and function in calcification

“…In vivo studies on the incorporation of 32 Pi into phospholipids of chondrocytes and MV have revealed that MV formation is a dynamic process in which labeling of MV PC and PE -which tend to be depleted -was nearly as rapid as that of the cellular lipids. In contrast, labeling of PS and SM -which are enriched in MV -was significantly slower (398). This paradox suggests that MV formation involves processes in which the plasma membrane becomes modified by action of phospholipases that selectively degrade PC and PE, while retaining PS and SM.…”

Scanning Electron Microscopy of Cartilage

“…Membranous vesicles also occur in fibrocartilage, but these are linked with cell disintegration (Ghadially et al, 1978b) rather than the metabolically active process postulated for epiphyseal cartilage (Wuthier et al, 1977). We do not think they have been isolated from elastic and fibrocartilage or examined in association with the chondrocytes by SEM.…”