Department of Biomaterials, Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, 14476, Potsdam, Germany.
Adv Sci (Weinh). 2023 Aug;10(22):e2301231. doi: 10.1002/advs.202301231. Epub 2023 May 19.
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, cryogenic focused ion beam-scanning electron microscopy (cryo-FIB/SEM) is used to image forming bone tissue at day 13 of a chick embryo femur. Both cells and matrix in 3D are visualized and observed as 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, 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 can be estimated. This velocity at 0.27 µm s is estimated, which is too large for a diffusion process and rather suggests active transport through the cellular network. It is concluded 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.
在骨骼发育过程中，骨骼的生长和矿化需要运输大量的钙，同时保持非常低的浓度。生物体如何克服这一重大的物流挑战，在很大程度上仍未得到解释。为了揭示这一过程的动态，使用低温聚焦离子束扫描电子显微镜（cryo-FIB/SEM）来观察鸡胚股骨第 13 天正在形成的骨组织。通过低温聚焦离子束扫描电子显微镜（cryo-FIB/SEM），可以观察到三维的细胞和基质，并观察到富含钙的细胞内囊泡结构。通过计算每单位体积中这些囊泡的数量，并根据背散射电子信号测量其钙含量，可以估计这些囊泡在一天内将沉积在胶原组织中的所有钙运输所需的细胞内速度。根据电子背散射信号测量，这个速度估计为 0.27 µm/s，对于扩散过程来说太大了，而更像是通过细胞网络的主动运输。因此得出结论，钙的物流是分层的，基于几种运输机制：首先通过血管利用钙结合蛋白和血流，然后通过成骨细胞和骨细胞的网络进行数十微米的主动运输，最后通过最后一到两个微米进行扩散运输。
