Secondary ionization mass spectrometry imaging of dilute stable strontium labeling in dentin and enamel

The labeling of the zones of active mineralization in bone and tooth is usually achieved with calcium-binding fluorescent dyes. However, these compounds are labile and mostly lost during the maturation process of the growing tissue. Here we labeled mouse teeth using injections of dilute strontium (SrCl(2), 500 microg/ml), a trace element that is naturally incorporated in hydroxyapatite, and resolve the subtle induced compositional changes using secondary ion mass spectrometry (SIMS) imaging and analysis. Entire hemi-mandibules sampled at 14 and 28 days after birth are embedded in resin and polished along longitudinal sections. SIMS chemical imaging reveals the double Sr labeling both in dentin and enamel of molar teeth as two stripes with excess Sr concentration parallel to the dentino-enamel junction. In order to quantify the variations of the strontium/calcium ratio (Sr/Ca), two international standards were sintered and measured along with the samples. The concentration of Sr in the two stripes is about 300 microg/g, which corresponds to an enrichment factor of about 1.3-1.4 relative to the natural baseline. These results show that SIMS provides rapid and quantitative imaging of small abundances of stable isotopes in growing mineralized tissues with a sensibility that is at least two orders of magnitude higher than classical X-rays microanalysis.