MOLECULAR-STRUCTURE AND FUNCTIONAL-MORPHOLOGY OF ECHINODERM COLLAGEN FIBRILS

The collagenous tissues of echinoderms, which have the unique capacity to rapidly and reversibly alter their mechanical properties, resemble the collagenous tissues of other phyla in consisting of collagen fibr ils in a nonfibrillar matrix. Knowledge of the composition and structu re of their collagen fibrils and interfibrillar matrix is thus importa nt for an understanding of the physiology of these tissues. In this re port it is shown that the collagen molecules from the fibrils of the s pine ligament of a seaurchin and the deep dermis of a sea-cucumber are the same length as those from vertebrate fibrils and that they assemb le into fibrils with the same repeat period and gap/overlap ratio as d o those of vertebrate fibrils. The distributions of charged residues i n echinoderm and vertebrate molecules are somewhat different, giving r ise to segment-long-spacing crystallites and fibrils with different ba nding patterns. Compared to the vertebrate pattern, the banding patter n of echinoderm fibrils is characterized by greatly increased stain in tensity in the c(3) band and greatly reduced stain intensity in the a( 3) and b(2) bands. The fibrils are spindle-shaped, possessing no const ant-diameter region throughout their length. The shape of the fibrils is mechanically advantageous for their reinforcing role in a discontin uous fiber-composite material.