Variable tensility in echinoderm collagenous tissues: A review

Abstract

Certain echinoderm collagenous structures can sustain rapid changes in their mechanical properties. In some cases the structure can switch reversibly between stiff and compliant states; in other cases, usually associated with autotomy mechanisms, the structure disintegrates irreversibly. The ultrastructure and microarchitecture of the extracellular elements of such mutable collagenous tissues (MCTs) are unremarkable. The only unusual aspect of their biochemistry so far discovered is a high glycoprotein content. All MCTs are permeated by granule‐containing processes belonging to neurosecretory‐like peri‐karya which may be in synaptic contact with motor neurons. In their maximally stiffened condition, most parallel‐fibred MCTs behave as if their collagen fibrils are strongly crosslinked by ground substance macromolecules. Mechanical and ultrastructural data suggest that their compliant state is achieved through the destabilization of interfibrillar linkages which permits slippage between adjacent collagen fibrils. Interfibrillar cohesion appears to be highly dependent on electrostatic interactions. Physicochemical and morphological data support the hypothesis that variable tensility involves the active control of extracellular pH or Ca‐ion availability. Evidence for the nervous control of variable tensility is fragmentary but accumulatively convincing. The coelomic fluid of a wide range of echinoderms contains factors that can influence the mechanical behaviour of MCTs, but whose physiological significance awaits elucidation.