A discrete helicoid of collagenous fibrils: the cuticle of deep-sea hydrothermal vent worms (Riftia pachyptila).

The fibrillar organization of the collagenous cuticle of the hydrothermal vent worm Riftia pachyptila is described. Fibrils in the posterior part of the cuticle are organized in a classical orthogonal plywood consisting of successive layers of fibrils: in this case, fibrils are oriented in only two directions which are orthogonal, as for pogonophoran. Our new data on the plume of Riftia pachyptila show a new type of fibrillar arrangement of the cuticle: 1) three sets of fibrils are arranged in an hexagonal pattern; 2) fibrils in successive plies are rotated by 60 degrees, and the organization of the fibrillar network is interpreted as a discrete helicoid when compared to continuously twisted plywoods; 3) a fourth set of fibrils crosses the hexagonally arranged plies and is oriented perpendicular to the surface of the body. X-ray diffraction studies of the cuticular fibrils reveal a triple helix which is characteristic of collagen molecules. Results obtained by differential scanning calorimetry (DSC) show that the denaturation temperature of the molecule is 54.7 degrees C for Riftia; whereas it is 58.9 degrees C for type I collagen measured under the same conditions. We discuss the origin of this plywood with respect to biomechanical constraints, self assembly processes, and compartmentation of the extracellular space. The involvement of the cell membrane in the fibrillogenesis of collagen is also discussed.