The effect of PH on fibrillogenesis of collagen in the egg capsule of the dogfish, Scyliorhinus canicula

The collagen of the egg capsule of the dogfish, Scyliorhinus canicula is stored and secreted by the secretory cells of the D-zone of the nidamental gland (Rusaou?n-Innocent, 1990b). The collagen appears to pass through several morphologically distinct textures during storage, secretion and fibril formation which may represent different lyotropic liquid crystalline phases (Knight et al., 1993). In the present communication we report evidence that a fall in hydrogen ion concentration induces fibrillogenesis during the secretion of the dogfish egg capsule. In an attempt to understand the factors involved in collagen assembly, we investigated the effects of subjecting isolated collagen storage granules in vitro to solutions ranging in pH from 2-11 and Na(+), K(+), Ca(++), Mg(++), Zn(++) and Cu(++) ions at concentrations varying from 0.01-0.5 M. From pH 2 to pH 4 most granules appeared completely amorphous; from pH 5 to pH 7 granules showed the following previously reported liquid crystalline textures: isotropic, lamellar, micellar, hexagonal columnar, transversely banded twisted nematic, and unbanded twisted nematic. At pH 8 granules showed both the hexagonal columnar phase (phase IV) and small quantities of the final fibrillar phase together with a previously undescribed texture. The latter texture, which we refer to as phase VII, had a D period (17.5 nm) half that of the lamellar texture (phase II) and the final egg capsule fibrils (phase VI). From pH 9 to pH 11, only the final fibrillar texture (phase VI) together with small quantities of the new texture (phase VII) were present. Na(+), K(+), Ca(++), Mg(++), Zn(++) and Cu(++) ions did not appear to have an observable effect on the phases found in isolated granules at pH 7.0. The role of pH in collagen storage and fibrillogenesis was confirmed by direct estimation of the pH in vivo using vital staining with neutral red, a range of pH indicators applied to unfixed cryostat sections and direct measurements of the pH of the jelly within the egg capsule. The implications of these findings for the mechanism of collagen storage and fibrillogenesis in the dogfish egg capsule and other collagenous systems are discussed.