Tracing the pathway of compositional changes in bone mineral with age: Preliminary study of bioapatite aging in hypermineralized dolphin's bulla

Background

Studies of mineral compositional effects during bone aging are complicated by the presence of collagen.

Methods

Hypermineralized bullae of Atlantic bottlenose dolphins of < 3 months, 2.5 years, and 20 years underwent micrometer-scale point analysis by Raman spectroscopy and electron microprobe in addition to bulk analysis for carbon.

Results

Bulla central areas have a mineral content of ~ 96 wt.% and 9–10 wt.% carbonate in their bioapatite, which is ~ 2 wt.% more than edge areas. Ca/P atomic ratios (~ 1.8) and concentrations of Mg, S, and other minor/trace elements are almost constant in central areas over time. Maturity brings greater over-all homogeneity in mineral content, stoichiometry, and morphology throughout the central and edge areas of the bullae. During aging, edge areas become less porous, whereas the concentration of organics in the edge is reduced. Enhancement of coupled substitutions of CO₃^2 − for PO₄^3 − and Na for Ca during aging increases carbonate content up to ~ 10 wt.% in the adult bulla.

Conclusions

1) Changes in physical properties during aging did not occur simultaneously with changes in chemical properties of the bone mineral. 2) Compositional changes in bone mineral were minor during the neonatal to sub-adult stage, but significant during later maturity. 3) Na and CO₃ concentrations co-vary in a 1:1 molar proportion during aging. 4) The mineral's crystallinity did not decrease as CO₃ concentration increased during aging.

General significance

Hypermineralized dolphin's bulla, due to extreme depletion in collagen, is an ideal material for investigating mineralogical changes in bioapatite during bone aging.