Localization of serotoninlike‐immunoreactive amacrine cells in the larval tiger salamander retina

Light microscopic immunocytochemistry was used to study the populations of serotoninlike‐immunoreactive cells in the larval tiger salamander retina. Of 1,135 serotonin‐immunostained cells observed in transverse cryosections, 87% were identified as amacrine cells, whereas 13% were tentatively designated as displaced amacrine cells. The somas of the vast majority of serotonin‐amacrine cells were situated in the innermost cell row of the inner nuclear layer. Only a few serotonin‐immunostained amacrine cell somas were observed in the second row of cells from the inner nuclear layer. Serotonin‐immunoreactive processes generally appeared as a diffuse plexus distributed evenly throughout all levels of the inner plexiform layer. As determined in whole‐mount preparations, serotonin‐amacrine cells were divisible into two populations on the basis of the diameters of their somas. Large cells (45%) ranged from 16 to 19 μm in diameter with the vast majority measuring 17 to 18 μm. Smaller and sometimes less intensely stained cells ranged from 14 to 16 μm in diameter with the large majority measuring 15 μm. The diameters of serotonin‐displaced amacrine cells ranged from 19 to 22 μm with the large majority measuring 20 μm in diameter. An examination of whole‐mount retinas revealed that serotonin‐immunoreactive amacrine and displaced amacrine cells were distributed throughout the center and the periphery of the retina. The density of serotonin‐amacrine cells (large and small combined) was calculated to be 173 ± 4.5 (mean ± standard error) cells per mm 2 .