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

The egg case of the dogfish, Scyliorhinus canicula is a composite mate rial largely constructed from collagen fibrils. It is formed as a draw n extrusion from transverse rows of spinnerets within the lining of th e nidamental gland. In the L(2) layer, which forms over 90% of the thi ckness of the marginal rib, each spinneret extrudes a flattened ribbon which runs the length of the rib and measures approximately 150 x 8.5 mu m in cross section. The structure of these ribbons and the orienta tion of collagen molecules and fibrils within them has been investigat ed in a correlative study using: low angle x-ray diffraction; bright f ield microscopy of peeled preparations; transmission (TEM) and scannin g (SEM) electron microscopy; confocal and quantitative polarizing micr oscopy. The way in which the molecular orientations are defined within the spinneret has been followed by SEM of fixed material from activel y secreting nidamental glands. The extruded ribbon showed a predominan tly 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 arr angement of fibrils superficially resembling that seen in biological t wisted nematic liquid crystals. However, evidence is presented here th at the arrangement in the marginal rib is novel, apparently arising fr om authentically curved fibres showing a splayed- or bent- rather than twisted-nematic construction. Evidence is also presented that the spi nnerets are able to control molecular orientations in a nematic liquid crystal to produce this and the other arrangements seen in the egg ca se.