Logistics of Bone Mineralization in the Chick Embryo Studied by 3D Cryo FIB-SEM Imaging

Abstract

During skeletal development, bone growth and mineralization require transport of substantial amounts of calcium, while maintaining very low concentration. How an organism overcomes this major logistical challenge remains mostly unexplained. To shed some light on the dynamics of this process, we use cryogenic Focused Ion Beam-Scanning Electron Microscopy (cryo-FIB/SEM) to image forming bone tissue at day 13 of a chick embryo femur. We visualize both cells and matrix in 3D and observe calcium-rich intracellular vesicular structures. Counting the number of these vesicles per unit volume and measuring their calcium content based on the electron back-scattering signal, we are able to estimate the intracellular velocity at which these vesicles need to travel to transport all the calcium required for the mineral deposited in one day within the collagenous tissue. We estimate this velocity at 0.27 m/s, which is too large for a diffusion process and rather suggests active transport through the cellular network. We conclude that calcium logistics is hierarchical and based on several transport mechanisms: first through the vasculature using calcium-binding proteins and the blood flow, then active transport over tens of micrometers through the network of osteoblasts and osteocytes and, finally, diffusive transport over the last one or two microns.