First Experimental Evidence for Human Dentine Crystal Formation Involving Conversion of Octacalcium Phosphate to Hydroxyapatite

Abstract

Biological apatite-crystal formation is a complex process starting with heterogeneous nucleation of inorganic calcium phosphate on an organic extracellular matrix [Cuisinier et al. (1995), J. Cryst. Growth, 156, 443±453]. Further stages of crystal growth are also controlled by the organic matrix and both nucleation and growth processes are under cellular control [Mann (1993), Nature (London), 367, 499±505]. The ®nal mineral in calci®ed tissue is constituted by poorly crystalline hydroxyapatite (HA) with a low Ca:P ratio, containing foreign ions such as carbonate and¯uoride. This study reports the ®rst observation of octacalcium phosphate (OCP) [Brown (1962), Nature (London), 196, 1048± 1055] in a biological tissue; OCP was found in the central part and HA at the extremities of the same crystal of calcifying dentine. This observation is of key importance in understanding the ®rst nucleation steps of biological mineralization. The presence of OCP in a forming human dentine crystal and the observation in the same tissue of nanometer-sized particles with a HA structure [Houlle  et al. (1997), J. Dent Res. 76, 895±904] clearly proves that two mechanisms, direct nucleation of nonstoichiometric HA crystals and nucleation of OCP, occur simultaneously in same area of mineralization. OCP is found to be a transient phase during the growth of biological crystals. In small crystals, OCP is completely transformed into HA by a hydrolysis reaction (Brown, 1962) and can only be detected in larger crystals because of its slow kinetics of transformation.

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…The principal support for this concept derived from the following: (1) the close structural similarity of OCP and HA (Brown 1962;Brown et al 1962); (2) formation of interlayered single crystals of OCP and HA (pseudomorphs of OCP); (3) the easier precipitation of OCP compared with HA; (4) the apparent plate-or lath-like habit of biological apatites that does not conform to hexagonal symmetry, but looks like a pseudomorph of triclinic OCP; (5) the presence of HPO 4 2-in bone mineral, particularly in newly formed bones (Elliott 2002). Some evidences supporting this idea were found using high-resolution transmission electron microscopy: computer-simulated lattice images of the ''central dark line'' in mineralized tissues revealed that it consisted of OCP (Brown 1966;Bodier-Houllé et al 1998;Rodríguez-Hernández et al 2005). In addition, Raman spectroscopic indications for an OCP precursor phase were found during intra-membranous bone formation (Crane et al 2006).…”

“…It plays an important role in formation of apatitic biominerals in vivo (Suzuki 2010b). A ''central OCP inclusion'' (also known as ''central dark line'') is seen by transmission electron microscopy in many biological apatites and in synthetically precipitated CDHA (Iijima et al 1996;Bodier-Houllé et al 1998;Rodríguez-Hernández et al 2005). Although OCP has not been observed in vascular calcifications, it has been strongly suggested as a precursor phase to biological apatite found in natural and prosthetic heart valves (Tomazic et al 1994;Nancollas and Wu 2000).…”

“…They are followed by ribbon-like crystal formation, which until recently was considered as the first step of biological crystal formation (Cuisinier et al 1993;Houllé et al 1997). These complicated processes, starting with the heterogeneous nucleation of inorganic CaPO 4 on an organic extracellular matrix, are controlled in both tissues by the organic matrix and are under cellular control (Bodier-Houllé et al 1998;Mann 1993). To complicate the process even further, regular and discrete domains of various charges or charge densities on the surface of apatite crystals derived from the maturation stage of enamel development were discovered by a combination of atomic and chemical force microscopy (Kirkham et al 2000).…”

Comparative study of hydroxyapatite and octacalcium phosphate coatings deposited on metallic implants by PLD method

“…OCP, in biological environment, is a precursor of HA and takes part in early stages of tissue calcification process [1][2][3][4]. In the past few years, OCP has drawn special attention as an alternative material to HA.…”

Maturational changes in dentin mineral properties

“…Ionic substitutions, such as the HPO 4 2-, are important in determining the reactivity and physical properties of hydroxyapatite. While there has been some suggestion that either amorphous calcium phosphate or octacalcium phosphate is the initial mineral phase formed in bones and teeth [30][31], recent NMR studies by Jager et al [32] have demonstrated that the presence of acid phosphate on the surface of the hydroxyapatite nano-crystals can explain the observed spectroscopic data without invoking the presence of another mineral phase. Using infrared spectroscopy and X-ray diffraction, increases in HPO 4 2-content have been determined to produce an expansion of the hydroxyapatite a axis at a rate of ∼0.0015=Å/ wt% [33].…”