In this work, we found that bone mineral formation proceeded very rapidly in mice by 1 day of age, where the degree of mineralization, the tissue mineral density, and the mineral crystallinity reached 36%, 51%, and 87% of the adult values, respectively. However, even though significant mineralization had occurred, the elastic modulus of 1‐day‐old bone was only 14% of its adult value, indicating that the intrinsic stiffening of the bone lags considerably behind the initial mineral formation.

Introduction:​ To meet the mechanical challenges during early development, the skeleton requires the rapid accretion of bone quality and bone quantity. Here, we describe early bone development in the mouse skeleton and test the hypothesis that specific compositional properties determine the stiffness of the tissue.

Materials and Methods:​ Tibias of female BALB mice were harvested at eight time‐points (n = 4 each) distributed between 1 and 40 days of age and subjected to morphometric (μCT), chemical (Fourier transform infrared microspectroscopy), and mechanical (nanoindentation) analyses. Tibias of 450‐day‐old mice served as fully mineralized control specimens.

Results:​ Bone growth proceeded very rapidly;​ at 1 day of age, the degree of mineralization (phosphate/protein ratio), the density of mineralized bone (TMD), and mineral crystallinity had reached 36%, 51%, and 87% of the adult (450 days) values, respectively. Spatially, the variability in mineralization across the mid‐diaphysis was very high for the early time‐points and declined over time. In contrast to the notable changes in mineralization, carbonate substitution into the mineral lattice (carbonate/phosphate ratio) and collagen cross‐linking did not show any significant changes over this time period. Even though significant mineralization had occurred, the elastic modulus of 1‐day‐old bone was only 14% of the adult value and increased to 89% (of its adult value) after 40 days. Between samples of different time‐points, significant positive correlations were observed between the elastic modulus and TMD (r² = 0.84), phosphate/protein ratio (r² = 0.59), and crystallinity (r² = 0.23), whereas collagen cross‐linking showed a small but significant negative correlation (r² = 0.15).

Conclusions:​ These data indicate that specific chemical and morphometric properties modulate bone's stiffness during early growth. The intrinsic stiffening of the bone, however, lags considerably behind the initial mineral formation, emphasizing the importance of bone mineral quality for optimizing matrix integrity.