Mutable Collagenous Structure or Not? A Comment on the Re-interpretation by del Castillo et al. of the Catch Mechanism in the Sea Urchin Spine Ligament

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.