The visual system of the Florida garfish, Lepisosteus platyrhincus (Ginglymodi). III. Retinal ganglion cells.

Abstract

The vitreal vascularization, optic nerve fiber layer and the retinal ganglion cells are examined in the Florida garfish, one of the two surviving genera of the ginglymode group of actinopterygian fishes. In the absence of a choroidal gland, a well developed system of hyaloid blood vessels overlie the inner retina. The optic nerve fiber and ganglion cell layers are integrated to form a series of alternating columns or fascicles emanating from an elongated optic nerve head. Horseradish peroxidase or cobaltous lysine infusion from the optic nerve reveals seven ganglion cell classes based on soma size and position, dendritic field size, retinal coverage and terminal stratification. Two discrete populations of giant ganglion cells with large somata and large dendritic fields stratify either within the sclerad region (type I cells) and/or within the middle region (type 2 cells) of the inner plexiform layer. Both giant ganglion cell populations (1% of total) form evenly distributed arrays with the dendritic arbor of each type covering the retina completely. Between seven and nine type 2 cells overlap the dendritic field of a single type 1 cell. Type 3 cells are small and located in the inner nuclear layer with a multistratified dendritic field terminating throughout the inner plexiform layer. Topographic analysis of the population of type 3 cells (4% of total) reveals a temporal area centralis (6.25 x 10(2) cells per mm2) and a ventral horizontal streak (6.25 x 10(2) cells per mm2). The large population of cells within the ganglion cell layer are classified as types 4, 5 and 6 based on soma size, dendritic field size and dendritic stratification and collectively these cells match the distribution of ganglion cells within the inner nuclear layer with 5.3 x 10(3) cells per mm2 in the temporal area centralis and 8.6 x 10(3) cells per mm2 in the horizontal streak. Type 7 cells possess small somata and send branches into both the inner and outer plexiform layers and have been termed biplexiform ganglion cells. Phylogenetic comparisons of several morphological features of the garfish retina reveals the polarity, conservation and the evolution of a number of visual characters.