Paraphenylenediamine: a new method for tracing human visual pathways.

Morphologic investigations of the human visual pathways have been limited by the infeasibility of modern neuroanatomical approaches. Although contemporary methods for tracing axon pathways (such as tracer injections and electrophysiology) have elucidated the visual system in experimental animals, these techniques cannot be similarly applied in humans. Thus, the present view of the neuroanatomy of the human visual system is based largely on experimental animal studies, classical simple observations of gross human brains, and clinical inference. We demonstrate use of the stain, paraphenylenediamine (PPD), in conjunction with standard methods of tissue preparation for transmission electron microscopy. Osmium precipitates on degenerating neural processes, resulting in a dark profile when examined by electron microscopy. PPD chelates the osmium in osmium tetroxide-fixed tissue, and thus becomes a light-opaque marker of degenerating processes. This technique allows the identification and tracing of degenerating or degenerated axons. Postmortem studies of six patients, four with documented optic nerve lesions, are presented. The degenerated retinal ganglion nerve fibers are followed with PPD and confirmed with electron microscopy. Previously proposed, primary visual projections are confirmed and new retinofugal pathways are demonstrated in the human brain.