THE TWISTED COLLAGEN NETWORK OF THE BOX-FISH SCUTES

The present article describes the three-dimensional arrangement of col lagen fibrils in dermal plates of different species of Ostraciidae. Th ese dermal plates or 'scales' are transformed scales, which have a pol ygonal shape and form a rigid tiling. They are natural composites, ass ociating a fibrous network with a mineral deposit lying at two differe nt levels of the scute, the 'ceiling' and the 'floor', plus a set of s imilarly mineralized walls joining the two levels. The three-dimension al structure of the collagen ne::work can be compared to that of 'plyw ood', fibrils align parallel within superposed layers of uniform thick ness, and their direction changes from layer to layer. In the dermal p late, two types of plywood have been evidenced; (1) one lying between the two mineralized plates, where the orientation of fibrils rotates c ontinuously, and (2) one under the lower plate, with thick layers of f ibrils, each showing a constant orientation, but abrupt angular change s are observed at the transition from one layer to the following one. In oblique sections, both types of plywood reveal large series of arce d patterns, testifying to a twisted arrangement of collagen fibrils, a nalogous to the arrangement of molecules or polymers in cholesteric li quid crystals. The network is reinforced by some collagen fibrils runn ing unidirectionally and almost normally to the lamellate structure. M oreover in the overall organization of the scute, these plywood system s form a set of nested boxes. This original architecture is compared t o the arrangement of the collagenous network previously described in m ost fish scales and in other extracellular matrices.