Calcificationin vitro of demineralized bone matrix

In vitro models of collagen biomineralization

Over the last several years, significant progress has been made toward understanding the mechanisms involved in the mineralization of hard collagenous tissues, such as bone and dentin. Particularly notable are the identification of transient mineral phases that are precursors to carbonated hydroxyapatite, the identification and characterization of non-collagenous proteins that are involved in controlling mineralization, and significant improvements in our understanding of the structure of collagen. These advances not only represent a paradigm shift in the way collagen mineralization is viewed and understood, but have also brought new challenges to light. In this review, we discuss how recent in vitro models have addressed critical questions regarding the role of the non-collagenous proteins in controlling mineralization, the nature of the interactions between amorphous calcium phosphate and collagen during the early stages of mineralization, and the role of collagen in the mineralization process. We discuss the significance of these findings in expanding our understanding of collagen biomineralization, while addressing some of the limitations that are inherent to in vitro systems.

Evidence for a Serum Factor That Initiates the Re-calcification of Demineralized Bone

The present studies show for the first time that demineralized bone re-calcifies rapidly when incubated at 37 degrees C in rat serum: re-calcification can be demonstrated by Alizarin Red and von Kossa stains, by depletion of serum calcium, and by uptake of calcium and phosphate by bone matrix. Re-calcification is specific for the type I collagen matrix structures that were calcified in the original bone, with no evidence for calcification in periosteum or cartilage. Re-calcification ceases when the amount of calcium and phosphate introduced into the matrix is comparable to that present in the original bone prior to demineralization, and the re-calcified bone is palpably hard. Re-calcified bone mineral is comparable to the original bone mineral in calcium to phosphate ratio and in Fourier transform infrared and x-ray diffraction spectra. The serum activity responsible for re-calcification is sufficiently potent that the addition of only 1.5% serum to Dulbecco's modified Eagle's medium causes bone re-calcification. This putative serum calcification factor has an apparent molecular mass of 55-150 kDa and is inactivated by trypsin or chymotrypsin. The serum calcification factor must act on bone for 12 h before re-calcification can be detected by Alizarin Red or von Kossa staining and before the subsequent growth of calcification will occur in the absence of serum. The speed, matrix-type specificity, and extent of the serum-induced re-calcification of demineralized bone suggest that the serum calcification factor identified in these studies may participate in the normal calcification of bone.

Osteonectin inhibiting de novo formation of apatite in the presence of collagen

The effect of bone matrix protein of osteonectin on de novo formation of apatite was studied in a wide range of calcium phosphate solutions in the presence of collagen. In every solution, from which amorphous calcium phosphate, octacalcium phosphate, or apatite precipitated as a possible initial phase, osteonectin at concentrations less than 1 microM retarded the precipitation, subsequent transformation to apatite, and ripening crystal growth of apatite. Collagen present as either reconstituted or denatured form had no effect on the osteonectin-associated reactions as well as osteonectin-free reactions, and no structural correlation was observed between collagen fibrils and any of the calcium phosphates that appeared in our system. Direct measurement of free calcium levels in the solutions suggested that the reduction in calcium activity due to complexing with osteonectin hardly explained the inhibitory activity of osteonectin in retarding the formation of apatite. Instead, our transmission electron microscopic (TEM) observation strongly suggested that the primary mechanism for osteonectin to inhibit the formation of apatite is to block growth sites of calcium phosphates nucleated. The apatite thus formed in the presence of osteonectin showed less resolved X-ray diffraction patterns, partly because of smaller crystallites as suggested by TEM.

Histological investigation of bone induction by demineralized allogeneic bone matrix: a natural biomaterial for osseous reconstruction

The objective of the study was to delineate the histological sequence of events associated with bone formation induced in extraosseous sites by implants of demineralized allogeneic bone matrix (DABM) in order to provide a more complete basis for the clinical use of the natural biomaterial for osseous reconstruction. The rat was the experimental animal model. Implants were prepared by demineralization of diaphyseal cortical rat bone in 0.6N HCl for 18-24 h. A total of 104 specimens, 4 mm square, were implanted both subcutaneously and intramuscularly in 26 rats for periods of 3 days to 22 weeks. Histological evaluation revealed a sequence of events that at 3 days displayed an inflammatory response to surgical trauma and after 6 weeks exhibited the formation of a mature lamellar ossicle with central marrow which was maintained through 22 weeks. Resorption of the DABM, first evident at 1 week, was accompanied by the appearance of chondroblast in vascular canals and in crevices in the implant. Osteoblastic activity, initially seen at 2 weeks, appeared to be associated with capillary and blood vessel formation around the implant and within enlarged resorptive cavities within the specimens. Calcification occurred in the cartilaginous matrix and osteoid and within the implanted matrix. The osteoinductive process comprises (1) the inflammation/granulation/scar formation processes elicited by implantation of a material, (2) the resorptive response elicited by DABM, and (3) the process of induction of nonosseous mesenchymal cells to differentiate to chondroblasts and osteoblasts. The results of the present study support previous investigations demonstrating the bone-inducing property of DABM and provide further evidence for the potential value of this natural biomaterial for use in osseous reconstruction.

Mechanisms of metal--salt methods in enzyme cytochemistry with special reference to acid phosphatase

This review is concerned with theoretical and experimental aspects of the factors governing the localizing potentialities of cytochemical enzyme reactions that are based on the metal-salt principle, that is, the precipitation of the primary product of the enzymatic reaction by a heavy-metal ion at the enzymatic site. Special attention is given to the lead phosphate precipitation process in acid phosphatase cytochemistry. The various model systems developed for the study of the factors involved in precipitation are described and their advantages and disadvantages discussed. Furthermore, the various cytochemical methods so far used for the demonstration of acid phosphatase activity are critically evaluated in the light of the results obtained with the model systems.

The effect of adenosine triphosphate, magnesium chloride and phospholipids on crystal formation in the demineralized shell-repair membrane of the snail, Helix pomatia L. An in vitro study

The effect of adenosine triphosphate (ATP), magnesium chloride (MgCl2) and phospholipids on the calcium-binding activity and crystal formation within the decalcified shell-repair membrane of the snail, Helix pomatia, was studied in vitro. The application of ATP produced a characteristic dual effect on calcification: (1) It strongly inhibited the formation of inorganic calcium carbonate (CaCO3) crystals. (2) It stimulated the development of organic crystalline bodies and induced deposition of amorphous calcium carbonate. The demineralized shell-repair membranes became white and rigid after incubation for 7 days in the medium containing 1.0 mM ATP. The inhibitory effect of Mg2+ on CaCO3 crystal formation was diminished by reduction of the concentration of MgCl2 in the incubation solution. Thus, after incubation for only 24h, 1.0 mM MgCl2 promoted the formation of birefringent CaCO3 crystals within the repair membranes. The principal effect of phospholipids on the demineralized shell-repair membrane was stimulatory, but after application of phospholipids to the medium, the formation of crystals proceeded slowly. The very large, composite crystals that were formed within the repair membranes showed strong birefringence. In all cases the development of the crystals and the organic crystalline bodies occurred in close vicinity to the amoebocytes. The role of ATP, MgCl2 and phospholipids in the recalcification of shell-repair membrane is discussed.

Precipitation of calcium phosphates under conditions of double diffusion in collagen and gels of gelatin and agar

One-dimensional double diffusion was applied to determine critical concentrations at which the precipitation of calcium phosphates occurs in reconstituted connective tissue collagen and agar gels at 37 degrees C and in gelatin gels at 25 degrees C. Experiments were performed in the presence of unbuffered 0.15 mol dm-3 NaCl, or 0.15 mol dm-3 NaCl-veronal adjusted to pH 7.4. It was found that critical concentrations of precipitation of both precipitating components, CaCl2 and phosphate buffer (pH 7.4), were equimolar and independent of the ratios of initial concentrations of the components. Critical concentrations of precipitation were not affected by the concentrations and kinds of gels used. The first-formed precipitates showed amorphous structure by X-ray diffraction analyses. Infrared (IR) spectra of the precipitates indicated CaHPO4 . H2O to be their predominant species. The molar Ca/P ratio obtained by chemical analyses was 1.08. This precipitate transformed in time into octacalcium phosphate. In all experiments, two very thin membranes of precipitate were formed in the gel column at the onset of precipitation simultaneously on both sides of the actual disc of precipitate. IR spectra and chemical analyses showed that both membranes were identical to the actual precipitation discs.

Remineralization of dentin in vitro

The remineralization of completely demineralized bovine dentin was studied in vitro by monitoring the velocities of fall of small particles through calcifying solutions. The density of a particle of dentin may be found from its velocity of fall through a fluid using Stokes' law. The minimum concentration product of calcium and acid phosphate ions of the solution in which remineralization would take place was 3.6 (mM)2 in the presence of 22 mM bicarbonate, pH 7.35, and ionic strength 0.1. This just above the solubility product of brushite (CaHPO4-2H2O). Incubation of decalcified dentin in a phosphoprotein removed from dentin during demineralization, or in phosvitin, had no effect on remineralization. The rates of remineralization and of the fraction remineralized were inversely proportional to particle size. This inverse correlation may be due to deposition of mineral in a surface layer of constant depth irrespective of particle size. The fraction of a particle remineralized was greatly increased by the use of highly supersaturated calcifying solutions or by the incorporation of fluoride into the solutions. The empirical reaction order of remineralization for both calcium and phosphate ions was found to be unity, which is, within the error limits, equal to the order of growth of seed crystals of hydroxyapatite in calcifying solutions of the same composition.

Effect of matrix bound phosphate and fluoride on mineralization of dentin

Completely demineralized bovine dentin mineralized to a high degree, but only after extraction of about half of its organic phosphate. Initiation of mineral formation depended on the concentrations of Ca2 plus, Pi, HCO3 minus and H plus ions in the incubation solution. After nucleation, mineral accretion by the dentin continued in solutions that did not, themselves, initiate mineralization. The nucleation phase, but not the subsequent accretion of mineral, was sensitive to small changes in temperature. Inclusion of 0.05 mM NaF in the solution markedly reduced the amount of mineral deposited during the nucleation phase but apparently enhanced subsequent mineralization.

The effect of D-penicillamine of the skeletal development of rat foetuses

Rats received 20, 50 or 100 mg/animal D-penicillamine i.p. twice daily on days 15, 16, 17, 18, and 19 of gestation, i.e. a total dose of 200, 500 resp. 1000 mg/animal. At all dosages the number of fetal resorptions did not increase significantly. Weight of the 20 day old embryos as well as length of the long bones in the extremities in the 100 mg-group showed a significant decrease. Numerous skeletal alterations could be observed in the 1000 mg-group such as absence, deformations or incomplete mineralisation of bones. Light microscopic examinations revealed an inhibition of the ossification as well as a decrease of number and size of the trabecula and of the thickness of the perichondrial bone sheath. A swelling of the collagenous fibrils can be demonstrated with the electron microscope. The first apatite crystals aggregate in collagen-free areas. The fusion of these aggregates to homogenously mineralized trabecula is inhibitied. In contrast to bones from untreated embryos, mineralized areas show varying content of collagen and apatide crystals. A regular spatial relationship between apatite crystals and collagenous fibrils does not develop. These findings show that even after the so-called "critical period" malformations can be pproduced by substances which disturb synthesis and maturation of the mesenchymal intercellular substance.