DiI labeling combined with conventional immunocytochemical techniques for correlated light and electron microscopic studies.

In order to obtain a detailed understanding of the chemical identity of callosal neurons and of their synaptic targets during development of the rat, a technique was developed combining anterograde and retrograde transport of the carbocyanine dye, DiI, previously applied in living or fixed tissue with conventional immunocytochemistry for peptides. It is reported here that photoconversion of the fluorescent DiI label to a stable diaminobenzidine reaction product is fully compatible with the application of the most widely used immunocytochemical techniques peroxidase-antiperoxidase (PAP) or avidin-biotin (ABC) on the same tissue section, for correlated light and electron microscopic studies. Advantages of this double-labeling procedure over previously described techniques which permit concurrent visualization of projection systems and chemically defined neuronal elements are discussed.