Mineral Induction by Matrix from Mineralized Biological Tissues
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MINERAL INDUCTION BY MATRIX FROM MINERALIZED BIOLOGICAL TISSUES Miles A. Crenshaw Dental Research Center, University of North Carolina, Chapel Hill, NC. 27599-7455 ABSTRACT The polymeric matrix of mineralized tissues controls the form and structure of the mineral that is deposited. This matrix has an insoluble fraction which provides a structural framework for the mineralized tissue, and a soluble fraction which is rich in polyanionic macromolecules. One hypothesis envisages mineral being nucleated by an atomic dimensional matching between crystal lattice and anionic spacing in the polyanionic macromolecules. An alternate hypothesis considers that fixed polyanions provide a surface for an adsorbed layer, enriched in lattice ions by ionotropy, to induce mineral formation from the metastable body fluids. We found that soluble matrix polyanions, immobilized by attachment to insoluble substrates, would induce mineral from metastable solutions. The insoluble substrates included natural and synthetic hydrogels not derived from mineralized tissues. Whether the polyanions were prepared from apatitic or CaC03 tissues, the mineral induced was independent of the source and was determined by the composition of the solution. Other immobilized, calcium-binding, polyanionic macromolecules, obtained from non-mineralizing tissues, also induced mineral. These and other data indicate that mineral induction by biological matrices is less specific than implied in the atomic dimensional matching extension of the epitaxial hypothesis.
INTRODUCTION
The organic matrix of mineralized tissue has been assigned several functions including nucleating the mineral, determining the mineral phase deposited and controlling crystallographic orientation and growth. For some time, it was thought that insoluble proteins obtained from a mineralized tissue fulfilled these functions. Collagen from bone and dentin and conchiolin from molluscan shells were the two such proteins Mat. Res. Soc. Symp. Proc. Vol. 218. @1991 Materials Research Society
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that received considerable attention. More recently, collagen and conchiolin have been regarded as the structural frameworks for the respective mineralized tissues, and acidic proteins associated in a specific pattern with these networks are thought to be the functional matrices [1, 2]. Significant progress has been made recently concerning possible roles of acidic macromolecules in mineral deposition in two systems, mammalian dentin and molluscan shell nacre. *The major non-collagenous protein of dentin is a polyanionic phosphoprotein, phosphophoryn [3, 4]. Aspartic and phosphoserine account for 970 of the 1130 residues of the bovine phosphophoryn [5]. Autoradiographic studies show that phosphoprotein is deposited at the mineralization front of dentin while newly synthesized collagen is deposited at the growing predentin border [6]. Subsequent immunohistochemical studies confirmed an absence of phosphophoryn in the predentin and indicated that phosphophoryn is secreted from the odontoblastic process and de
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