Fine structural detection of calcium ions by photoconversion

We propose a tool for a rapid high-resolution detection of calcium ions which can be used in parallel with other techniques. We have applied a new approach by photo-oxidation of diaminobenzidine in presence of the emission of an excited fluorochrome specific for calcium detection. This method combines the selectivity of available fluorophores to the high spatial resolution offered by transmission electron microscopy to detect fluorescing molecules even when present in low amounts in membrane- bounded organelles. We show in this paper that Mag-Fura 2 photoconversion via diaminobenzidine oxidation is an efficient way for localizing Ca2+ ions at electron microscopy (EM) level, is easily carried out and reproducible, and can be obtained on a good amount of cells, since the exposure in our conditions is not limited to the direct irradiation of the sample via an objective but obtained with a germicide lamp. The end product is sufficiently electron dense to be detected clearly when present in sufficient amount within a membrane boundary.