Biomineralization involves complex processes and interactions between organic and inorganic matters, which are controlled in part by the cells. The objectives of this study were, first, to perform a systematic and ultrastructural investigation of the initial mineral formation during secondary ossification center of mouse femur based on material science and biology viewpoint, and then develop novel biomaterials for mineralization based on the in vivo findings. First, we identified the very initial mineral deposition at postnatal day 5 (P5) at the medial side of femur epiphysis by nanocomputed tomography. Initial minerals were found in the surroundings of hypertrophic chondrocytes. Interestingly, histological and immunohistochemical analyses showed that initial mineralization until P6 was based on chondrocyte activity only, i.e., it occurred in the absence of osteoblasts. Moreover, electron microscopy-based ultrastructural analysis showed that cell-secreted matrix vesicles were absent in the early steps of osteoblast-independent endochondral ossification. Instead, chondrocyte membrane nanofragments were found in the fibrous matrix surrounding the hypertrophic chondrocytes. EDS analysis and electron diffraction study indicated that cell membrane nanofragments were not mineralized material, and could be the nucleation site for the newly formed calcospherites. The phospholipids in the cell membrane nanofragments could be a source of phosphate for subsequent calcium phosphate formation, which initially was amorphous, and later transformed into apatite crystals. Finally, artificial cell nanofragments were synthesized from ATDC5 chondrogenic cells, and in vitro assays showed that these nanofragments could promote mineral formation. Taken together, these results indicated that cell membrane nanofragments were the nucleation site for mineral formation, and could potentially be used as material for manipulation of biomineralization.
Keywords: biomineralization; cell membrane nanofragments; endochondral ossification; matrix vesicles; secondary ossification center.