Cellular and Chemical Events During Enamel Maturation

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.

Amelogenins are the principal proteins of the extracellular matrix of developing dental enamel and are postulated to function in the processes of biomineralization of the developing tooth although the molecular mechanisms concerned are poorly understood. Recent imaging studies, employing dynamic light scattering, atomic force, and transmission electron microscopy (TEM) have shown that a recombinant amelogenin (M(r) approximately 20,000 Da) spontaneously forms supramolecular quasi-spherical aggregates ("nanospheres") of 15-20 nm in diameter. By comparison with in vitro experiments employing the recombinant amelogenin we show that the nanospheres appear as electron-lucent structures when treated with conventional electron microscopy contrast reagents (phosphotungstate or uranyl acetate) and we speculate that this property derives from the hydrophobic nature of the amelogenin protein. Employing TEM preparations of developing enamel from mouse, bovine, and hamster we demonstrate that the amelogenin nanospheres occur as beaded rows of electron-lucent structures aligned with, and separating, the enamel mineral crystallites. We postulate that the amelogenin monomers self-assemble to form nanospheres which function to space the initial crystallites, control crystal habit, inhibit intercrystalline fusions, and, through the apposition of their surfaces, create anionic channels which facilitate ion transport within the mineralizing matrix.