Evidence for a hydroxyapatite precursor in regenerating cyprinid scales

Collagen matrices, mineralized with calcium phosphates in a hydroxyapatite phase, are generally found in skeletal tissues. However, mechanisms of biomineral formation and regulation are still poorly understood. Elasmoid scales are part of the dermal skeleton and regenerate quickly when damaged or lost. This makes the scale a convenient target to study mineral compostion and formation. The X-ray diffractogram of the mineral layer of carp scales only show speaks corresponding with hydroxyapatite. Energy-dispersive X-ray spectroscopy (EDX) during scanning electron microscopy identifies the elements calcium and phosphorus, which are restricted to the external layer of the scale. In contrast with various other species, calcium phosphate crystals are not present in the elasmodine layer. The clean boundary between calcified and uncalcified matrix suggests that subsequental mineralization of the elasmodine layer is inhibited. Quantification of the calcium and phosphorus content allows definition of a calcium/phosphorus ratio, which is indicative ofthe crystalline phase of the minerals. Through accurate measurements of total calcium and phosphorus (by ICP-MS) in zebrafish scales, different ratios were found for newly formed (regenerating) scales compared to ontogenetic or completely regenerated scales. This shows that the mineral is not at first deposited as hydroxyapatite, but in a precursor phase. Although the nature of the phase remains to be established, this first analysis shows that cyprinids scales can easily be used to gain insights in the dynamics of biomineralization.