Interaction of collagen with hydrophobic protein granules in the egg capsule of the dogfish scyliorhinus canicula

The egg capsule of the dogfish is a composite material containing collagenous fibrils and 2 mum spherical hydrophobic protein granules. The latter appear to owe much of their hydrophobicity to an exceptionally high tyrosine content (approximately 20% of total amino acid residues). The hydrophobic component appears to form as an emulsion in the secretory granules of the D and E zone gland cells of the nidamental gland. Droplets of the hydrophobic material appear to become coated with remarkably regular layers of radially-arranged collagen molecules which form a series of concentric, evenly spaced layers around each hydrophobic granule. Numerous disclinations were seen where the layers around adjacent granules interfered with one another. The layers are thought to represent a lamellar liquid crystalline phase previously described for this collagen (Knight et al., 1993). The fine structural appearance of the concentric layers and evidence for radial arrangement of collagen molecules within them is compatible with the suggestion that the layers are built from a dumbbell-shaped unit approximately 35 nm long with hydrophobic groups concentrated at the ends. This unit may represent a dumbbell-shaped molecule or an oligomer of two or more molecules lying parallel with one another in a head-to-tail arrangement. Such a unit can be readily incorporated into models for the micellar, hexagonal columnar and final fibrillar phases previously described for this collagen (Knight et al., 1993). Evidence from the TEM study of stretched egg capsule wall suggests that there is a mechanical interaction between the hydrophobic granules and the collagen fibrils in the fully formed material. We suggest that the radial, concentric layered arrangement of collagen molecules is established by hydrophobic interactions within the liquid crystalline material and locked into place by oxidative covalent cross-linking to give a 3-dimensional cross-linked meshwork of collagen fibrils and hydrophobic granules. The latter arrangement helps to account for the high tensilestrength and toughness of this material.