Molecular orientations in an extruded collagenous composite, the marginal rib of the egg capsule of the dogfish Scyliorhinus canicula ; a novel lyotropic liquid crystalline arrangement and its origin in the spinnerets

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Abstract
The egg case of the dogfish, Scyliorhinus canicula is a composite material largely constructed from collagen fibrils. It is formed as a drawn extrusion from transverse rows of spinnerets within the lining of the nidamental gland. In the L 2 layer, which forms over 90% of the thickness of the marginal rib, each spinneret extrudes a flattened ribbon which runs the length of the rib and measures approximately 150 x 8.5 μm in cross section. The structure of these ribbons and the orientation of collagen molecules and fibrils within them has been investigated in a correlative study using: low angle x-ray diffraction; bright field microscopy of peeled preparations; transmission (TEM) and scanning (SEM) electron microscopy; confocal and quantitative polarizing microscopy. The way in which the molecular orientations are defined within the spinneret has been followed by SEM of fixed material from actively secreting nidamental glands. The extruded ribbon showed a predominantly biaxial fibril orientation in low angle x-ray diffraction patterns recorded with the beam passing horizontally through the marginal rib. This x-ray pattern is derived from a remarkably regular parabolic arrangement of fibrils superficially resembling that seen in biological twisted nematic liquid crystals. However, evidence is presented here that the arrangement in the marginal rib is novel, apparently arising from authentically curved fibres showing a splayed- or bent- rather than twisted-nematic construction. Evidence is also presented that the spinnerets are able to control molecular orientations in a nematic liquid crystal to produce this and the other arrangements seen in the egg case.