THE EXPRESSION OF TYPE X COLLAGEN mRNA BUT NOT ALKALINE PHOSPHATASE mRNA INCREASES AS GROWTH PLATE CHONDROCYTES HYPERTROPHY

Abstract

In order to study the regulation of the process of growth plate chondrocyte hypertrophy, freshly isolated newborn calf rib growth plate chondrocytes were separated using Percoll density gradient centrifugation. Five subpopulations of chondrocytes of different size were obtained. Alkaline phosphatase activity/ugDNA was lowest in the smallest cells and increased with increasing cell size. mRNA isolated from the five subpopulations was analyzed by northern blot using radiolabeled cDNAs encoding the genes for type II and X collagen, alkaline phosphatase and G-6-PD. The type II collagen gene was highly expressed in all subpopulations. The expression of the type X collagen gene was lowest in the smallest cells and relative expression increased with increasing cell size. By contrast, the expression of the alkaline phosphatase gene did not vary with cell size. These data suggest that while type X collagen synthesis in maturing growth plate chondrocytes may be regulated at the gene level, the increase in alkaline phosphatase activity observed in maturing growth plate chondroctyes may be post-transcriptionally regulated.