Distinct developmental changes in the distribution of calcium, phosphorus and sulphur during fetal growth-plate development

Gradients in the concentrations of free phosphate (Pi) and calcium (Ca) exist in fully developed growth zones oflong bones and ribs, with the highest concentrations closest to the site of mineralization. As high concentrationsof Pi and Ca induce chondrocyte maturation and apoptosis, it has been hypothesized that Ca and Pi drive chondrocytedifferentiation in growth plates. This study aimed to determine whether gradients in the important spectralelements phosphorus (P), Ca and sulphur (S) are already present in early stages of development, or whetherthey gradually develop with maturation of the growth zone. We quantified the concentration profiles of Ca, P, S,chloride and potassium at four different stages of early development of the distal growth plates of the porcinefemurs, using particle-induced X-ray emission and forward- and backward-scattering spectrometry with a nuclearmicroprobe. A Ca concentration gradient towards the mineralized area and a stepwise increase in S was found todevelop slowly with tissue maturation. The increase in S co-localizes with the onset of proliferation. A P gradientwas not detected in the earliest developmental stages. High Ca levels, which may induce chondrocyte maturation,are present near the mineralization front. As total P concentrations do not correspond with former free Pimeasurements, we hypothesize that the increase of free Pi towards the bone-forming site results from enzymaticcleavage of bound phosphate.