Fine Structure and Mechanical Properties of the Catch Apparatus of the Sea-Urchin Spine, A Collagenous Connective Tissue with Muscle-Like Holding Capacity

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Abstract
The catch apparatus (CA) of the sea-urchin spine has been known to have a muscle-like holding property, though it is composed mainly of extracellular collagen fibres. An electron microscopic study has been made on the CA of the sea urchin, Anthocidaris crassispina, with special reference to its content of muscle cells and to structural changes of the collagen components on elongation of the CA. The stretch resistance of the CA in a highly extensible state and in a very inextensible state was also measured. Although very thin smooth muscle cells were found scattered among the collagen fibres in the CA, the difference in the passive tension was greater than the estimated stress which could be generated by the muscle cells in the CA by three orders of magnitude. The collagen fibrils remained undeformed but slid along one another during the length change of the CA. The present results suggest that the cohesive force between the collagen fibrils rather than the contractile activity of the muscle cells plays a significant role in determining the mechanical properties of the CA.