Ultrastructural and biochemical observations on proteoglycans and collagen in the mutable connective tissue of the feather star Antedon bifida (Echinodermata, Crinoidea).

Mutable connective tissue, unique to echinoderms, changes its mechanical behaviour within seconds of nervous stimulation. The molecular mechanism of this phenomenon is not understood. In this study proteoglycans and collagen of the brachial ligaments connecting neighbouring ossicles of the arms of the feather star Antedon bifida have been investigated by biochemistry, light and electron microscopy and the critical electrolyte concentration (CEC) technique using the dye Cupromeronic Blue (CB). The ligaments consist mainly of parallel cross-striated collagen fibrils, 82 +/- 12 nm in diameter, with a characteristic banding pattern and a D-period of 52.8 +/- 3.2 nm. Some fibrils were disaggregated into bundles of 10-11 nm protofibrils, lying between the normal fibrils. Proteoglycans occur at the surface of the fibrils with 2 binding sites (each with a different CEC) per D-period and also inside the fibrils. The surface proteoglycans are more highly sulphated (i.e. their CECs are > 1.3 M) than the intrafibrillar proteoglycans (CEC < 0.9 M). The glycosaminoglycans consist of a highly sulphated chondroitin sulphate, possibly with fucose residues. The results are consistent with the theory that disaggregation of the fibrils into protofibrils and reaggregation might be a mechanism of mutability, without excluding the possibility that fibrils may slide alongside each other during movements in the viscous phase of the ligament.