The retinal ganglion cell layer and optic nerve in a marsupial, the honey possum (Tarsipes rostratus).

We examined the retina and optic nerve of one of the smallest mammals, the honey possum (Tarsipes rostratus), a marsupial. Ganglion and displaced amacrine cells were distinguished by morphological criteria in retinal wholemounts stained with cresyl violet. There was a mean of 90,000 ganglion cells which were distributed as an area centralis in the mid-temporal retina with a peak density of 9,000/1 mm2 falling in a concentric shallow gradient to 2,000/1 mm2 at the periphery. The unimodal and positively skewed spectra of soma diameters gave no indication of ganglion cell classes. Displaced amacrine cells numbered only 15,000, being outnumbered 6:1 by ganglion cells. This value represents by far the lowest percentage of displaced amacrine cells yet reported for a mammalian retina. The displaced amacrine population was evenly distributed. Counts of optic axons from electron micrographs were within 1-5% of the ganglion cell estimates. In the retrobulbar portion of the nerve, a substantial proportion of axons (26-43%) were unmyelinated, and many of these were concentrated within the lateral quadrant. This specialisation may represent the outflow of the area centralis. At the level of the foramen, the percentage of unmyelinated axons fell to 4-6%, and axons were more uniform in their distribution. Axon diameters were lower for the unmyelinated than the myelinated populations at both levels. As for ganglion cells, however, the spectra of axon diameters were unimodal and positively skewed for both unmyelinated and myelinated axons.