Search

It is now widely recognized that one of the characteristic features of the phylum Echinodermata is the presence of collagenous tissues that can undergo rapid, nervously mediated changes in mechanical properties (1,2). Whilst examples of such mutable collagenous tissues have been demonstrated in the five major echinoderm classes (3,4), research over the past 30 years has focused on a certain few of them that are easily accessible and amenable to experimental investigation. The capsular ligament ("catch apparatus"--CA) of the echinoid spine-test joint is one of these well-studied collagenous structures. Because the CA can undergo reversible changes in passive stifness, it was believed to be a catch muscle by von Uexkull (5). Subsequently it was shown to consist predominantly of collagenous connective tissue with only a scattering of small muscle fibers. Analysis of its mechanical properties and physiological responses led most workers to the conclusion that the CA is a posture-locking device whose functioning depends almost entirely on the variable tensility of its extracellular connective tissue (6-11). However, de1 Castillo et al. (12) have presented a model in which its catch properties are attributed only to the contractile cells and in which connective tissue mutability plays no role. The aims of this contribution are to evaluate the new model, to show that it cannot explain the behavior of the CA, and to demonstrate that the extracellular matrix must be the primary source of the catch properties of the CA.

Collagen was extracted by pepsin-digestion of the spine ligaments of the sea-urchin Eucidaris tribuloides . The collagen resembled vertebrate type I collagen in its α-chain composition and solubility characteristics. Its amino acid composition had both differences and similarities when compared with vertebrate type I and other echinoderm collagens. The collagen formed SLS-crystallites and native-banded fibrils under appropriate conditions.

The similar proximal and distal attachments to the stereom of primary spine ligament in the echinoid Eucidaris tribuloides are described, from thin sections and SEM studies on frozen and fractured spine articulations and ligaments from decalcified material. The orthogonal structure of the general stereom is modified on the attachment zones where bundles of collagen cylinders enter approximately hexagonally arranged channels. Straps of collagen extend in parallel series between adjacent bundles via regularly placed ports and collagen loops rather than non-striated 'tendons' pass over skeletal trabeculae. The regular pattern of collagen straps is most evident on the proximal and distal attachment zones. Mechanical features of the non-adhesive mode of attachment are considered, together with similarities and differences between insertion of muscle cells and mutable collagenous tissue (ligament) in echinoderms.