Morphometric analysis of serotoninergic bipolar cells in the retina and its implications for retinal image processing

The entire population of retinal serotoninergic bipolar cells in the turtle Pseudemys scripta elegans was labeled by immunocytochemical methods. This allowed a systematic analysis to be made of the morphological variabilities among a functionally homogeneous neuronal population. The analyzed morphological characteristics included: (1) size of the Landolt club, (2) size of the soma, (3) lateral extension of the ramification within the outer plexiform layer, (4) course of the axon across the inner nuclear layer, (5) pattern of the axonal ramification within the inner plexiform layer, (6) lateral extension of these ramifications, and (7) density of the cells. Whereas characteristics 1–4 and 7 show a morphological variability strictly related to the location of the bipolar cell with respect to the visual streak, a fovealike structure, characteristics 5 and 6 show no such correlation. The size of the soma increases by a factor of 4 from the visual streak toward the periphery. The area covered by the ramification in the OPL increases from 330 μm2 at the visual streak to 50,000 μm2 at the dorsal and ventral edges of the retina. The coverage factor remains the same throughout the retina, as well as the area covered by the ramifications in the IPL, which is about 2,000 μm2. At the visual streak and at 110μ ventral and 68μ dorsal of the visual streak, the bipolar axons cross the INL perpendicularly. In between the axons take an oblique course leading to an axon‐induced shift between input and output of up to 250 μm. The density of cells has its maximum of about 8,000 cells/mm2 at the visual streak and its minimum of about 500 cells/mm2 at the dorsal and ventral periphery. These morphometric data reveal a distorted projection from the outer plexiform layer onto the inner plexiform layer with an overrepresentation of the visual streak area. This helps to increase the visual acuity of this area and possibly supports motion detection.