Unusual distribution of ganglion cells in the retina of the three-toed sloth (Bradypus variegatus)

RESUMO

1. The distribution ans size of retinal ganglion cells labelled with horseradish preoxidase (HRP) were studied in flat-mounted retinas of three-toed sloths. 2. Massive injections of HRP solution were made throughout the thalamus and midbrain in anesthetized sloths in order to retrogradely label the retinal ganglion cell population. Twenty to thirty h later the eyes were and the retinas flat-mounted and reacted with phenylenediamine-HCL and H2O2 to label ganglion cells,thus distinguishing then from other cells int he same retinal layer. 3. Ganglion cell density graually increased from about 500 cell/mm2 at the far periphery to a peak of about 1,500 cells/mm2 in an area, termed the area centralis, deep in the inferior temporal retina. The presence of a vertical visual steak was also noted. 4. The area centralis contained a higher frequency of small ganglion cells than the peripheral retina where large cells preominated. 5. /the unusual postion of the area centralis and cisual streak in the retina can be explained by the slot's unique ability to rotate its head 180- while climbing upside-dow along horizontal branches so that the head is right-side-up. If it is assumed that the branch directly above the sloth's head needs to be visualized for accurate claw placement then the branch would be imaged on the inferior temporal retina in an area corresponding to the maximum density region