Postembedding colloidal-gold immunocytochemistry of noncollagenous extracellular matrix proteins in mineralized tissues.

Immunocytochemistry is a powerful tool for investigating protein secretion, extracellular matrix assembly, and cell-matrix and matrix-matrix/mineral relationships. When applied to the tissues of bones (bone and calcified cartilage) and teeth (dentin, cementum, and enamel), where calcium phosphate-containing extracellular matrices are the predominant structural component related to their weight-bearing and masticatory roles, respectively, data from immunocytochemical studies have been prominent in advancing our understanding of mineralized tissue modeling and remodeling. The present review on the application of postembedding, colloidal-gold immunocytochemistry to mineralized tissues focuses on the advantages of this approach and relates them to conceptual, theoretical, and experimental data currently available discussing matrix-mineral interactions and extracellular matrix formation and turnover in these tissues. More specifically, data are summarized regarding the distribution and role of noncollagenous proteins in different mineralized tissues, particularly in the context of how they interface with mineral, and how this relationship might be affected by the various tissue-processing steps and immunocytochemical strategies commonly implemented to examine the distribution and function of tissue proteins. Furthermore, a technical discussion is presented that outlines several different possibilities for epitope exposure in mineralized tissues during preparation of thin sections for transmission electron microscopy. Cell biological concepts of protein secretion by cells of the mineralized tissues, and subsequent extracellular matrix assembly and organization, are illustrated by examples of high-resolution, colloidal-gold immunolabeling for osteopontin, bone sialoprotein, and osteocalcin in the collagen-based mineralized tissues and for enamel protein (amelogenin) in enamel.