Localization of Tc-99m MDP in epiphyseal growth plates of rats.

The distribution and localization of Tc-99m methylene diphosphonate (Tc-MDP) in the epiphyseal growth plates of the rat were elucidated by contact and microautoradiography. The uptake of the tracer was found to be especially high in the calcified cartilage bars at the end of the vascular loops. In addition to areas of mineralization, increased uptake was found in the Howship's lacunae on the resorbing surfaces. This labeling corresponded with the fluorescence of tetracycline, which labeled both forming and resorbing surfaces, when given with short labeling interval. Distribution of Tc-MDP did not coincide with new production of collagen, as judged by H-3 proline labeling; nor was the uptake localized within cells with high alkaline phosphatase activity. The affinity of the tracer for the mineral phase was confirmed by decalcification of in vivo labeled sections with EDTA, which showed loss of radioactivity in contrast to sections incubated in water. By chromatography the activity in the decalcification medium could not be distinguished from that of Tc-MDP.