Seewald, A.; Zhong, J.; Sutaria, V.; El Charkawi, I.; Valleriani, A.; Fratzl, P.; Raguin, E.

Bone formation during embryonic development requires the rapid and sustained delivery of large amounts of calcium to mineralizing tissue. In avian embryos, this process coincides with a unique physiological transition in calcium supply, shifting from limited yolk reserves to massive mobilization from the eggshell. To address the question of how calcium transport responds to the increasing mineral demand during this transition, we combine micro computed tomography with three-dimensional cryogenic FIB-SEM imaging at different embryonic stages in the developing quail femur. Over this developmental period, bone mineralized volume increases rapidly through periosteal expansion. We observe that bone forming cells consistently contain numerous membrane-bound carriers loaded with mineral precursors throughout all growth stages. By integrating measurements across length scales, we calculate the intracellular velocities required to sustain the observed mineral deposition in the successive developmental stages. Surprisingly, these velocities remain within a similar range across development, despite the significant difference of bone formation rates, and are consistent with active transport by molecular motors. The increasing mineral demand corresponds to the expansion of mineralizing surfaces, which leads to an increasing number of cells, while the transport capacity of individual cells remains similar. Our work reveals a perfect tuning between the calcium transport capacity and bone growth, even in a situation where the skeletal growth is accelerating in the quail embryo.