Retinal projections and retinal ganglion cell distribution patterns in a sturgeon (Acipenser transmontanus), a non-teleost actinopterygian fish

Retinal projections in a sturgeon were studied by injecting biocytin or HRP into the optic nerve. The tar-get areas are the preoptic area, thalamus, a rea pretectalis, nucleus of posterior commissure, optic tectum, and nuclei of the accessory optic tract. Furthermore, a few labeled fibers and termina ls were found in a ventrolateral ai ea of the caudal telencephalon. All ret inal projections are bilateral, although contralateral projections were mor e heavily labeled. Retrogradely labeled neurons were found in the ventral t halamus bilaterally. Retinal projections in sturgeons are similar to those of other non-teleost actinopterygians and chondrichthyans (sharks), in term s of the targets and extent of bilateral projections. Distribution patterns of ganglion cells in the retina were examined in Nissl-stained retinal who le mount preparations. The highest density areas were found in the temporal and nasal retinas, and a dense band of ganglion cells was observed along t he horizontal axis between the nasal and temporal areas of highest density. The density of ganglion cells in the dorsal retina is the lowest. The tota l number of ganglion cells was estimated to be about 5 x 10(4) in a retina. The existence of a low density area in the dorsal retina suggests reduced visual acuity in the ventral visual field.