Who builds the bone and who stays living inside it

Bone tissue forms in two stages

Anyone who looks at bone tissue under a microscope finds two components that form one after another: first a scaffold of protein, then the mineral portion that deposits within it. Between the two steps lies not a second, but a span of days. The two elements in this issue occur at different points in that sequence.

Magnesium appears early, namely at the cell that secretes the scaffold. Calcium appears late, namely where the finished scaffold takes up mineral. This separation in time is the common thread running through the following sections.

Everything described here is editorial factual text drawn from bone-biology and structural-biology literature. Only the two boxed lines further below are citable in the sense of the regulation; each belongs to exactly one element.

The cell walls itself into its own product

On the surface of a growing section of tissue sits a closed layer of cube-shaped cells, the osteoblasts. They release protein outward — mainly long, triple-stranded twisted molecules that assemble into fibers outside the cell. Most of these cells eventually stop working and flatten out.

A smaller portion takes a different path: these cells are gradually enclosed by the material they themselves release. What remains are small cavities from which fine extensions reach out in all directions and connect with neighboring cells. In this way, the builder becomes an inhabitant that never leaves the finished tissue. The extensions of two such cells don't end freely but meet one another — a continuous network inside a substance that, viewed from outside, appears lifeless.

Osteoid: the layer where no mineral has settled yet

Freshly deposited material is soft. It consists of roughly ninety percent of a single type of protein, type I collagen, along with smaller amounts of other proteins and sugar chains. In the literature, this unmineralized layer is called osteoid; at active sites it typically measures eight to twelve micrometers.

The layer doesn't stay soft permanently, but it does for a while. Published measurement series show that about ten to fifteen days pass between the deposition of a layer and the start of mineral uptake. After that, the boundary where mineral moves in advances through the deposited material at roughly one micrometer per day. Only behind this boundary is the substance hard. Calcium is the most common cation taken up in the process.

A binding site that stays empty without magnesium

For a cell to deposit fibers, it must adhere to them. To do this, it carries grippers made of two subunits in its outer membrane, known in the literature as integrins; the one responsible for collagen is referred to as alpha-2 beta-1. At the outer end of this gripper lies a shallow pocket formed by a few amino acid residues.

In this pocket sits a single doubly charged metal ion, usually magnesium. It's not incidental: the collagen isn't grasped directly by the protein, but through this ion, which forms a shared coordination with a side chain of the fiber molecule. Without an occupied pocket, the gripper doesn't close. The ion itself isn't converted in the process — it doesn't change its charge and leaves the site unchanged. Its contribution lies in geometry, not in a reaction.

The entry the European register holds for calcium applies to the tissue discussed in both sections. It reads, unchanged: "Calcium contributes to the maintenance of normal bones." Under Regulation (EU) No. 432/2012. Separately, the same annex holds the entry for the second element in this issue: "Magnesium contributes to the maintenance of normal bones." Under Regulation (EU) No. 432/2012. The two lines stem from two separately conducted evaluation procedures and are not merged here into a single joint statement.