Chondroitin sulphate and heparan sulphate sulphation motifs and their proteoglycans are involved in articular cartilage formation during human foetal knee joint development

Novel sulphation motifs within the glycosaminoglycan chain structure of chondroitin sulphate (CS) containing proteoglycans (PGs) are associated with sites of growth, differentiation and repair in many biological systems and there is compelling evidence that they function as molecular recognition sites that are involved in the binding, sequestration or presentation of soluble signalling molecules (e.g. morphogens, growth factors and cytokines). Here, using monoclonal antibodies 3B3(-), 4C3 and 7D4, we examine the distribution of native CS sulphation motifs within the developing connective tissues of the human foetal knee joint, both during and after joint cavitation. We show that the CS motifs have broad, overlapping distributions within the differentiating connective tissues before the joint has fully cavitated; however, after cavitation, they all localise very specifically to the presumptive articular cartilage tissue. Comparisons with the labelling patterns of heparan sulphate (HS), HS-PGs (perlecan, syndecan-4 and glypican-6) and FGF-2, molecules with known signalling roles in development, indicate that these also become localised to the future articular cartilage tissue after joint cavitation. Furthermore, they display interesting, overlapping distributions with the CS motifs, reflective of early tissue zonation. The overlapping expression patterns of these molecules at this site suggests they are involved, or co-participate, in early morphogenetic events underlying articular cartilage formation; thus having potential clinical relevance to mechanisms involved in its repair/regeneration. We propose that these CS sulphation motifs are involved in modulating the signalling gradients responsible for the cellular behaviours (proliferation, differentiation, matrix turnover) that shape the zonal tissue architecture present in mature articular cartilage.