The avian eggshell extracellular matrix as a model for biomineralization

The avian eggshell is a complex, extracellularly assembled structure which contains both mineralized and non-mineralized regions, The composition of the hen eggshell organic matrix was examined by immunohistochemistry with antibodies to different extracellular matrix molecules, Type I collagen is found in the shell membranes, but only after treatment of the tissue sections with pepsin, When incomplete eggshells are removed from the oviduct and immunostained, type I collagen can be detected in the shell membranes without pepsin treatment. The shell membranes, which are non-mineralized, also contain type X collagen, and this immunostaining does not require pepsin treatment. The occurrence of type X collagen in the shell membranes is surprising, since this collagen has not been found in any tissue other than hypertrophic cartilage, Immunostaining for various glycosaminoglycans shows the presence of keratan sulfate and dermatan sulfate, Several different antibodies to keratan sulfate stain different regions of the eggshell; one keratan sulfate epitope is prominent in the calcium reserve assemblies, Dermatan sulfate staining is very intense in the palisade region, Demineralized matrix from the palisade region was extracted with guanidine and fractionated by ion exchange chromatography, A similar to 200-kDa dermatan sulfate proteoglycan is found in these extracts, along with a number of protein components, This preparation was tested for its ability to affect calcium carbonate crystal formation in vitro, Pieces of demineralized shell membranes were used as a substrate for crystal formation and various amounts of the palisade matrix dermatan sulfate proteoglycan preparation were added to the solution from which the crystals were formed, This material causes a concentration-dependent change in crystal morphology to one in which the crystals are smaller and more rounded, which more closely approximates the crystals normally observed in eggshells, These results suggest that the dermatan sulfate proteoglycans may be important in modulating crystal morphology in the hen eggshell and correlate with mineralization-modulating biomolecules from other calcified tissue, which are generally anionic.