Bone without minerals and its secondary mineralization in Atlantic salmon (Salmo salar): the recovery from phosphorus deficiency.

Calcium and phosphorus (P) are the main bone minerals, and P deficiency can cause hypomineralized bones (osteomalacia) and malformations. This study used a P-deficient salmon model to falsify three hypotheses. First, an extended period of dietary P deficiency does not cause pathologies other than osteomalacia. Second, secondary mineralization of non-mineralized bone is possible. Third, secondary mineralization can restore the bones' mineral composition and mechanical properties. For 7 weeks, post-smolt Atlantic salmon (Salmo salar) received diets with regular P content (RP) or with a 50% lowered P content (LP). For additional 9 weeks, RP animals continued on the regular diet (RP-RP). LP animals continued on the LP diet (LP-LP), on a regular P diet (LP-RP) or on a high P diet (LP-HP). After 16 weeks, animals in all groups maintained a non-deformed vertebral column. LP-LP animals continued bone formation albeit without mineralization. Nine weeks of RP diet largely restored the mineral content and mechanical properties of vertebral bodies. Mineralization resumed deep inside the bone and away from osteoblasts. The history of P deficiency was traceable in LP-RP and LP-HP animals as a ring of low-mineralized bone in the vertebral body endplates, but no tissue alterations occurred that foreshadow vertebral body compression or fusion. Large quantities of non-mineralized salmon bone have the capacity to re-mineralize. If 16 weeks of P deficiency as a single factor is not causal for typical vertebral body malformations, other factors remain to be identified. This example of functional bone without minerals may explain why some teleost species can afford to have an extremely low mineralized skeleton.