Avian eggshells are natural composite bioceramics containing organic and inorganic phases. The occurrence and role of particular collagens and proteoglycans in the eggshell and their pattern of secretion by the oviduct is discussed. While type X collagen, the main constituent of the shell membranes, functions to inhibit their mineralisation, mammillan, a mammillary keratan sulphate proteoglycan, is involved in nucleation of the first calcite crystals of the shell and ovoglycan, a unique dermatan sulphate proteoglycan, is involved in the growth of the crystalline palisade. Eggshell biomineralisation is effected by specialised oviduct cell populations in a defined topographically and temporally regulated process.
