The continuously forming fin bony rays of zebrafish represent a simple bone model system in which mineralization is temporally and spatially resolved. The mineralized collagen fibrils of the fin bones are identical in structure to those found in all known bone materials. We study the continuous mineralization process within the tissue by using synchrotron microbeam x-ray diffraction and small-angle scattering, combined with cryo-scanning electron microscopy. The former provides information on the mineral phase and the mineral particles size and shape, whereas the latter allows high-resolution imaging of native hydrated tissues. The integration of the two techniques demonstrates that new mineral is delivered and deposited as packages of amorphous calcium phosphate nanospheres, which transform into platelets of crystalline apatite within the collagen matrix.one mineral formation has been extensively investigated both at the cellular level and at the level of the mineral and the macromolecules. Key questions, as yet unanswered, are how mineral is delivered to the crystallization site and whether this first-formed mineral is the same as in mature bone. The possibility that calcium phosphate precursor phases are delivered and first deposited in bone has been debated for decades (1-3). In vitro precipitation of calcium phosphates from solution often progresses through the deposition of amorphous calcium phosphate (ACP) as the first-formed mineral phase (4). ACP transforms into octacalcium phosphate (OCP), which then undergoes hydrolysis to form carbonated hydroxyapatite (HAP), the mineral phase of mature bone (4). Crane et al. (5) used Raman microspectroscopy to show that in the forming cranial suture of a mouse, an OCPlike phase was deposited prior to the formation of the mature mineral. Mahamid et al. (6) showed that in the continuously forming fin rays of a zebrafish, the newly formed bone contains large amounts of ACP. Beniash et al. (7) reported the presence of ACP in the forming enamel of mouse incisors. Interestingly, recent experiments successfully reproducing oriented intrafibrillar collagen mineralization in vitro utilized anionic polypeptides (polyaspartate) for transient stabilization of amorphous mineral as a precursor for the mature HAP crystallites (3, 8). All the above studies are consistent with ACP being a precursor phase in bone formation, but direct observation of the process in situ is still absent.The transient precursor phase strategy has been adopted by many invertebrate mineralizing groups (reviewed in ref. 9). Many of these taxa first deposit amorphous calcium carbonate, which subsequently transforms into crystalline calcite or aragonite.