FIBROUS MINI-COLLAGENS IN HYDRA NEMATOCYSTS

Nematocysts (cnidocysts) are exocytotic organelles found in all cnidar ians. Here, atomic force microscopy and field emission scanning electr on microscopy reveal the structure of the nematocyst capsule wall. The outer wall consists of globular proteins of unknown function. The inn er wall consists of bundles of collagen-like fibrils having a spacing of 50 to 100 nanometers and cross-striations at intervals of 32 nanome ters. The fibrils consist of polymers of ''mini-collagens,'' which are abundant in the nematocysts of Hydra. The distinct pattern of mini-co llagen fibers in the inner wall can provide the tensile strength neces sary to withstand the high osmotic pressure (15 megapascals) in the ca psules.