Characterization of an additional articular cartilage vesicle fraction that generates calcium pyrophosphate dihydrate crystals in vitro.

OBJECTIVE

We previously identified a unique fraction of porcine articular cartilage vesicles, sedimentable at 8 x 10(6) g/min, which generate calcium pyrophosphate dihydrate crystals (CPPD) in vitro. We sought to identify and characterize other fractions of articular cartilage digest, sedimentable at lower g forces, which may also contain mineralizing vesicles.

METHODS

Electron microscopy and alkaline phosphatase and nucleoside triphosphate pyrophosphohydrolase (NTPPPH) assays were used to analyze each fraction. Radiometric mineralization assays, Fourier transform infrared (FTIR) spectroscopy, and compensated polarized light microscopy were used to analyze crystals formed by these fractions.

RESULTS

Vesicles of varying sizes identical to epiphyseal cartilage matrix vesicles were seen in all sedimentable fractions examined, but were the exclusive component of fractions sedimentable at 3 x 10(6) g/min, termed the heavy vesicle fraction (HVF), and at 8 x 10(6) g/min, now termed the light vesicle fraction (LVF). All vesicle containing fractions supported ATP dependent calcium pyrophosphate precipitation. The HVF and LVF precipitated 30 x more calcium than vesicle poor supernatant (p < 0.01) and 1.5-4 x more than cell-free unfractionated digest (p < 0.01). HVF differed from LVF in that it contained 3-4 x higher NTPPPH specific activity (p < 0.05). HVF resembled LVF in that both precipitated crystals consistent with CPPD by FTIR spectroscopy and compensated polarized light microscopy.

CONCLUSION

These data expand our previous estimate of the total number of vesicles available for biologic mineralization and demonstrate heterogeneity of vesicle fractions. They support a key role for vesicles in CPPD crystal formation.