CHANGES IN MACROMOLECULAR ORGANIZATION IN COLLAGEN ASSEMBLIES DURING SECRETION IN THE NIDAMENTAL GLAND AND FORMATION OF THE EGG CAPSULE WALL IN THE DOGFISH SCYLIORHINUS-CANICULA

The thickest layer (L2) of the egg capsule wall of the dogfish, Scyliorhinus canicula, is constructed largely from highly ordered collagen fibrils (Knight & Hunt 1976). This collagen is stored and secreted by the nidamental gland and passes through an extraordinary series of ordered phases, many of which have well defined liquid crystalline structure. We have examined the changes in macromolecular packing of the collagen as it moves from the cisternae of the endoplasmic reticulum to secretory granules, is secreted and then formed into the egg capsule wall. Within the endoplasmic reticulum cisternae the collagen appears anisotropic but becomes assembled into a smectic A or lamellar phase in the Golgi cisternae. This phase persists in early secretory granules, where it is found in conjunction with a micellar phase. As these granules mature, the collagen passes through a cholesteric mesophase before adopting a columnar hexagonal arrangement. On merocrine secretion the granules' contents revert rapidly to the smectic A-lamellar and micellar phases. As it passes along the nidamental gland tubules, the collagen is first converted into a second distinct micellar phase before assembling into the final fibrils that constitute the egg capsule. These phase transitions give powerful insights into the way in which the macromolecular arrangement of collagen molecules can be modulated and are discussed in the context of a range of other related structural transitions in collagens.