The type X collagen is a short chain collagen associated with calcific cartilage and/or the expression of the hypertrophic chondrocyte

Articular cartilage is an avascular tissue whose chief structural characteristic is a high matrix-to-cell volume ratio. The matrix comprises predominantly type II collagen, other collagen types and high molecular weight aggregating proteoglycans and a variety of non-collagenous proteins. The tissue is biochemically heterogeneous, a heterogeneity which seems to be paralleled by morphologically distinct types of chondrocytes (Stockwell, 1979). Surface cells tend to be discoid in shape; they synthesize less proteoglycan than deeper located cells (Bayliss et al., 1983) which are rounded in shape. In addition, surface cells preferentially do not synthesise keratan sulphate whilst deeper zone chondrocytes do. Other significant quantitative and qualitative differences in proteoglycans also exist through the depth of the tissue (Bayliss et al., 1983; Zanetti et al., 1985). More striking are the differences apparent at the tide mark, a band of calcified cartilage which integrates into the bone of the sub-chondral plate. Cells in this region are often alkaline phosphatase positive and are associated with type X collagen (Schmid et al., 1990; Gannon et al., 1991). The tide mark retains a constant position within the tissue after growth has ceased. However, during pathological change such as in osteoarthritis, the tide mark often advances towards the surface and an increase in the amount of type X collagen is observed (Walker et al., 1991). Whilst a role for type X collagen in the calcification process has been sought, data have been inconclusive. It is important to try and elucidate mechanisms which maintain the biochemical heterogeneity within the normal articular tissue in order to understand what happens during pathogenesis. Most of the reports in the literature discuss type X collagen synthesis in relation to chick chondrocytes (Kielty et al., 1985; Pacifici et al., 1991), but rather less is known about human chondrocytes. Partly this is because of lack of suitable probes which recognise human type X collagen and also the relatively small amounts of type X collagen produced by mammalian chondrocytes (Marriot et al., 1991). In a previous report (Archer et al., 1990) we found that surface located chondrocytes initiated keratan sulphate synthesis in suspension culture. In order to determine whether other components which are differentially synthesized through the tissue can also be synthesized de novo, we have analysed the synthesis of alkaline phosphatase and type X collagen by surface and deep zone chondrocytes in suspension culture.