Australian lungfish Neoceratodus forsteri may be the closest
living relative to the first tetrapods and yet little is known about their
retinal ganglion cells. This study reveals that lungfish possess a
heterogeneous population of ganglion cells distributed in a horizontal
streak across the retinal meridian, which is formed early in development
and maintained through to adult stages. The number and complement of both
ganglion cells and a population of putative amacrine cells within the
ganglion cell layer are examined using retrograde labelling from the optic
nerve and transmission electron-microscopic analysis of axons within the
optic nerve. At least four types of retinal ganglion cells are present and
lie predominantly within a thin ganglion cell layer, although two
subpopulations are identified, one within the inner plexiform and the
other within the inner nuclear layer. A subpopulation of retinal ganglion
cells comprising up to 7% of the total population are significantly larger
(>400 μm2) and are characterized as giant or alpha-like
cells. Up to 44% of cells within the retinal ganglion cell layer represent
a population of presumed amacrine cells. The optic nerve is heavily
fasciculated and the proportion of myelinated axons increases with body
length from 17% in subadults to 74% in adults. Spatial resolving power,
based on ganglion cell spacing, is low (1.6–1.9 cycles
deg−1, n = 2) and does not significantly
increase with growth. This represents the first detailed study of retinal
ganglion cells in sarcopterygian fish, and reveals that, despite variation
amongst animal groups, trends in ganglion cell density distribution and
characteristics of cell types were defined early in vertebrate
evolution.