Biochemical aspects of biomineralization

Current Ornithology

“…The membranes appear to pose negligible resistance to gaseous diffusion during the major portion of incubation except in the initial stages, when the membranes are wet (Section 3.2.1). The membranes serve to contain egg contents during the process of shell formation, to anchor the seeding sites of crystal formation (Krampitz and Witt, 1979), and possibly to present a temporary barrier to larger diameter microorganisms invading the interior of the egg (Board and Fuller, 1974).…”

“…Eggshells contain about 98% crystalline calcite, a structural form of CaC03 (Simons, 1971;Krampitz and Witt, 1979). Minute amounts of aragonite and vaterite, alternative forms of CaC03 , have been detected in shells or shell coverings of a few species (Erben, 1970;Tullett et a1., 1976).…”

“…Minute amounts of aragonite and vaterite, alternative forms of CaC03 , have been detected in shells or shell coverings of a few species (Erben, 1970;Tullett et a1., 1976). Other components of the shell include small concentrations of magnesium and phosphorus (Romanoff and Romanoff, 1949;Board and Love, 1980) and a matrix of organic molecules distributed throughout the crystalline layer (Simkiss and Tyler, 1957;Tyler, 1969;Krampitz and Witt, 1979). The organic matter is principally a protein-mucopolysaccharide complex (Simkiss and Tyler, 1957).…”

“…This process is hypothesized to have some crucial role in the formation of both pores and the shell thickness (Simons, 1971;Tullett, 1978;Board and Love, 1980). As egg contents and membranes pass into the distal end of the isthmus, organic granules, the mamillary cores, are attached to the surface of the outer membrane, possibly by shrinkage and desiccation of colloidal droplets of organic matter secreted by glandular cells (Erben, 1970;Wyburn et a1., 1973). The cores appear to be the site of formation of the first calcite crystals, possibly because calcium is che-lated by the mucopolysaccharide or proteins in the core matrix (Simkiss and Tyler, 1958; Krampitz and Witt, 1979). The initiation of formation of cores is evident as the egg arrives in the pouch of the shell gland or uterus (Wyburn et al, 1973;Erben and Kriesten, 1974).…”

“…It does not occur in the portion of the shell over the air cell nor in infertile eggs (Simkiss, 1975). The decalcifying fluid is thought to be either carbonic or citric acid (Simons, 1971;Krampitz and Witt, 1979). We have no clear idea where it is produced, how it gets to the mammillary cores, and how the calcium and bicarbonate are transported through the shell membranes to the chorioallantois.…”