Displaced retinal ganglion cells project to the accessory optic system in the chameleon (Chamaeleo calyptratus)

Abstract

Retinal ganglion cells were successfully labelled in the chameleon by retrograde axonal transport of dextran amines that were applied to the nucleus of the basal optic root (nBOR) in an in vitro preparation. Labelled ganglion cells were restricted to the contralateral eye. Many cells were completely stained including their dendritic trees. With few exceptions, all cells had displaced somata that were located at the inner margin of the inner nuclear layer. The labelled ganglion cells had two to six primary dendrites that branched frequently and formed large unistratified dendritic trees within sublamina 1 of the inner plexiform layer. There was extensive overlap of the dendritic trees of neighbouring cells leading to an estimated coverage factor of 2–4. The dendritic field areas varied in size according to the retinal position of the cells and were highest in the central retina around the fovea with a maximum of 0.14 mm² and reached a second maximum at the retinal margin with values of 0.08–0.1 mm². The smallest dendritic areas (0.04–0.06 mm²) were measured midway between the fovea and retinal margin. The size of the soma area was not correlated to the dendritic field size and increased from 100 to 150 μm² near the fovea to 150–300 μm² at the retinal margin. There was no evidence for a retinotopic organisation of ganglion cell fibres within the nBOR. All cells were of uniform morphology that was identical to the type of nBOR-projecting displaced ganglion cell (DGC) described previously for the bird retina. Similar to birds, the labelled DGCs were the only source of retinal projection to the nBOR. A small fraction of cells had orthotopic somata located in the ganglion cell layer but were otherwise identical to the labelled DGCs. The similarity of chameleon nBOR-projecting ganglion cells to those described in avian retinas mirrors the close phylogenetic relationship of birds and lizards.