IMMUNOHISTOCHEMICAL CHARACTERIZATION OF THE RETINA–OPTIC NERVE–BRAIN VISUAL PATHWAY IN THE AFRICAN TREE SQUIRREL

Diurnal mammals depend on specialized visual circuits for rapid perception and visuomotor coordination, yet most mechanistic knowledge derives from nocturnal rodent models. African tree squirrels (Heliosciurus gambianus and Funisciurus anerythrus) are arboreal, diurnal rodents with exceptional visual acuity and provide a comparative system to examine photopic visual pathway organization. Here, we extend prior histological descriptions by mapping neuronal, synaptic, axonal, myelin, astrocytic, and microglial features along the retina–optic nerve–brain axis. Paraffin-embedded retina, optic nerve, and brain tissues from five wild-caught squirrels were analyzed using immunohistochemistry for NeuN and neuron-specific enolase (neurons/metabolic activity), synaptophysin (synapses), neurofilament and myelin basic protein (axons/myelination), glial fibrillary acidic protein (astrocytes), and ionized calcium-binding adaptor molecule 1 (microglia). The retina showed dense NeuN-positive ganglion cells, prominent synaptophysin labeling within plexiform layers, and stratified neurofilament immunoreactivity consistent with organized inner retinal circuitry. The optic nerve exhibited robust myelin basic protein and neurofilament labeling with extensive GFAP-positive astrocytic support. In central targets, the dorsal lateral geniculate nucleus displayed strong neuronal and synaptic marker expression consistent with a specialized thalamic relay, whereas the rostral colliculus showed intense neuronal and axonal labeling with comparatively weak synaptophysin distribution, suggesting an axon-rich integrative architecture. Together, these findings reveal progressive specialization across successive visual pathway nodes and support African tree squirrels as valuable comparative models for studying diurnal visual processing beyond standard laboratory rodents.