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

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.

Electron microscopy of calcification during high-density suspension culture of chondrocytes

“…Budding from the cell surface [31,32] and cellular degeneration/disintegration [33] are supported by electronmicroscopical studies. Budding and cellular disintegration are the most commonly observed phenomena in the genesis of matrix vesicles of the growth cartilage.…”

Biological Calcification

“…By now it is generally accepted that most cartilage matrix vesicles are formed by budding from the cell membrane or the tip of cell processes, as shown by transmission (Bonucci 1970a(Bonucci , b, 1978Cecil and Anderson 1978;Dickson 1981a, b) and scanning (Ornoy and Langer 1978;Ornoy et al 1979) electron microscopy and by freeze-fracture studies (Cecil and Anderson 1978;Akisaka and Gay 1985a). The same formation mechanism has been shown in bone (Ornoy et al 1980) and dentin (Almuddaris and Dougherty 1979;Katchburian and Severs 1982).…”

Matrix vesicles: Are they anchored exosomes?

“…[13] Early electron microscopic studies of cartilage confirm that vesicles are formed by polarized budding from cell protrusions of the chondrocyte membrane. [19–21] In osteoblast-like cells in culture, Thouverey et al[23] noted that matrix vesicles were formed at the apical membrane. While the location of the bleb has been described, conditions that facilitate blebbing are poorly understood.…”

“…While most studies support the view that matrix vesicles are formed through membrane blebbing, Akisaka et al[19] reported that some vesicles appeared to pass through the intact plasma membrane and, related to this observation, Thyberg et al[24] regarded vesicles as extrusions of lysosomal dense bodies. Adding to this literature discussion, Xiao et al[25] examined osteoblast vesicle biogenesis and reported that budding of aggregate clusters from polarized regions of the plasma membrane resulted in the concerted release of matrix vesicles from the cell surface.…”