Specific transcellular carbocyanine-labelling of rat retinal microglia during injury-induced neuronal degeneration

The present work employed a new technique for labelling phagocytizing microglia in the axotomized retinal of adult rats. Transection axotomy was performed within the intraorbital segment of the optic nerve, and the fast-transported, vital fluorescent carbocyanine dyes DiI and 4Di-10ASP were deposited at the ocular stump of the nerve in order to retrogradely prelabel the ganglion cells which were destined to die. Optic nerve transection resulted in progressive degradation of ganglion cell axons, perikarya and dendrites within the retina and in release of fluorescent material which was then incorporated into cells identified as microglia but not into other cells of the retina. Incorporation of labelled material into microglia occurred only when the ganglion cells degenerated and not when the non-lesioned ganglion cells were labelled from the superior colliculus. Double-staining of microglia with both dyes helped to compare the pattern of labelling for each dye. After progression of ganglion cell degeneration, microglia displayed a staggered, bilaminated distribution within the ganglion cell layer and within the inner plexiform layer. Fluorescent microglia were not found within the deeper layers of the retina indicating that transneuronal degeneration and subsequent labelling of microglial cells do not occur. The results show that one major function of microglia within the ganglion cell and inner plexiform layers of the lesioned retina is to remove debris produced after degradation of neurons.