Structural features associated with movement and 'catch' of sea-urchin spines

Spine catch ligaments of a sea urchin Arbacia punctulata were extended under constant load. Ligaments from an undisturbed animal may show any extension rate from zero (catch state) to rapid extension to failure. Replacing the preparation bath with CA(2)- and Mg(2+)-free sea water reversibly abolishes the catch state. The fine structure of the outermuscle layer and inner ligament cone associated with the spine base is described. The unstriated paramyosin muscles bear thin flanges and form compact interlocking rows. Subsurface cisternae are associated with the plasma membrane. The muscles are innervated by glia-free axons ending in bulbous terminals containing lucent synaptic vesicles. The ligament comprises cylindrical bundles of collagen fibrils: one or more minute muscle fibers (paramyosin) lie parallel with and closely adjoining each bundle. The mean diameter of these muscles is 0.3 micrometers and they occupy 2-3% of the ligament's cross-sectional area. Axons containing electron-opaque secretory droplets accompany the muscles between the collagen bundles: the cell bodies of these neurones generally lie on the outer surface of the ligament. When an urchin points a spine, the ligament on the side of the contracting spine muscle shortens but does not buckle. A function of the intraligamental muscles is to effect this non-buckled shortening. The catch mechanism (which reside entirely within the ligament) may be due either to the intraligamental muscles and/or to a locked polymer mechanism in which matrix molecules between collagen fibrils are reversibly cross-linked by divalent cations.