Avian eggshell structure and osteopontin.

The avian eggshell primarily consists of calcium carbonate mineral ( calcite) and matrix proteins. Here we review matrix-mineral relationships in the eggshell at the ultrastructural level using scanning and transmission electron microscopy, and describe the distribution of osteopontin (OPN) as determined by colloidal gold immunolabeling for OPN. A rich protein network integrated within the calcitic structure of the eggshell shows variable, region-specific organization that included layered fibrous planar sheets of matrix, thin filamentous threads, thin film-like surface coatings, vesicular structures and isolated proteins residing on cleaved {104} crystallographic faces of the eggshell calcite. Except for the vesicular structures, these matrix structures all immunolabeled strongly for OPN. Given the potent mineralizationinhibiting function of OPN, we discuss how this protein might regulate eggshell growth rate and inhibit calcification at specific compartmental boundaries to provide eggshell form. Copyright (C) 2008 S. Karger AG, Basel