Proposal for the effect of chondrocyte volume on the mineralization rate

Mineralization of the cartilage matrix in embryonic long bones and growth plates is preceded by hypertrophy of chondrocytes. We hypothesize that the swollen hypertrophic cells exert pressure on the matrix, and that this pressure plays a role in the cartilage mineralization process. For this study, we asked the following questions. First, does the ratio of cell volume to matrix volume (CV/MV) increase from the proliferation to the hypertrophic zone in embryonic long bones? Second, is there a correlation between cell-volume increase and the mineralization rate in embryonic and postnatal long bones? The CV/MV ratios in the proliferation and hypertrophic zones in embryonic mouse metatarsals at 17 days of gestational age were determined using morphometric analyses. Confocal laser scanning microscopy was used to determine chondrocyte volumes. Cell volumes in the proliferation and hypertrophic zones of embryonic mouse metatarsals at 17 days of gestational age were compared to the ones in the metatarsal growth plates of nine-day-old mice. The mineralization rate was determined using photographs at 24-hour intervals. The CV/MV increased significantly from the proliferation to the hypertrophic zone, from 1.30 ± 0.15 (mean ± standard deviation) to 1.80 ± 0.18. The relative increase in cell volume from the proliferation to the hypertrophic zone was 1.6 for embryonic cells, i.e. from 370 ± 101 mm³, and 2.8 for postnatal cells, i.e. from 280 ± 41 mm³ to 786 ± 155³ (p < 0.05). The mineralization rate was 295 ± 47 mm/24 hours and 382 ± 149 mm/24 hours for embryonic and postnatal metatarsals, respectively (p < 0.05). The finding that chondrocyte volume increase is accompanied by a higher mineralization rate supports the hypothesis that cell hypertrophy plays an important role during the mineralization process.