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.

The development of dentin and of enamel share a common starting locus: the dentinoenamel junction (DEJ). In this study the relationship between enamel and dentin crystals has been investigated in order to highlight the guiding or modulating role of the previously mineralized dentin layer during enamel formation. Observations were made with a high-resolution electron microscope and, after digitalization, image-analysis software was used to obtain digital diffractograms of individual crystals. In general no direct epitaxial growth of enamel crystals onto dentin crystals could be demonstrated. The absence of direct contact between the two kinds of crystals and the presence of amorphous areas within enamel particles at the junction with dentin crystals were always noted. Only in a few cases was the relationship between enamel and dentin crystals observed, which suggested a preorganization of the enamel matrix influenced by the dentin surface structure. This could be explained either by the existence of a proteinaceous continuum between enamel and dentin or by the orientation of enamel proteins by dentin crystals.

Ribbon-like crystals, from developing enamel of human fetuses, were studied by high resolution electron microscopy. These crystals were classically described as the first organized mineral formed during amelogenesis. They were characterized by a mean width-to-thickness ratio (W.T-1) of 9.5, and 40% were bent. On lattice images we noted the presence of the central dark line (CDL) associated with white spots. Both structures were found in crystals with a minimum thickness of 8-10 nm. CDLs were localized in the center of the crystals and seemed to be linked to the initial growth process, but their exact structure and function were not fully determined. We were able to study the structure of the ribbon-like crystals with a Scherzer resolution close to 0.2 nm. The good correspondence between experimental and computed images showed that their structure was related to hydroxyapatite (HA). In addition, the presence of ionic substitutions and deficiencies were also compatible with HA. In this study, about 50% of the crystals showed structural defects. Screw dislocations were the most often noted defects and were observed within crystals aligned along five different zone axes. Low- and high-angle boundaries were also detected. Low-angle boundaries, found in the center of the crystals, could thus be related to CDLs and be implicated in the nucleation step of crystal formation, whereas high-angle boundaries could result from the fusion of ribbon-like crystals. Such mechanisms could induce an acceleration of the growth in thickness of the crystal observed during the maturation stage of amelogenesis.

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