The morphology and distribution of normally placed and displaced A
horizontal cells were studied in the retina of a diurnal hystricomorph
rodent, the agouti Dasyprocta aguti. Cells were labeled with
anti-calbindin immunocytochemistry. Dendritic-field size reaches a minimum
in the visual streak, of about 9000 μm2, and increases
toward the retinal periphery both in the dorsal and ventral regions. There
is a dorsoventral asymmetry, with dorsal cells being larger than ventral
cells at equal distances from the streak. The peak value for cell density
of 281 ± 28 cells/mm2 occurs in the center of the
visual streak, decreasing toward the dorsal and ventral retinal periphery,
paralleling the increase in dendritic-field size. Along the visual streak,
the decline in cell density is less pronounced, remaining between
100–200 cells/mm2 in the temporal and nasal
periphery. Displaced horizontal cells are rare and occur in the retinal
periphery. They tend to be smaller than normally placed horizontal cells
in the ventral region, whilst no systematic difference was observed
between the two cell groups in the dorsal region. Mosaic regularity was
studied using nearest-neighbor analysis and the Ripley function. When
mosaic regularity was determined removing the displaced horizontal cells,
there was a slight increase in the conformity ratio, but the bivariate
Ripley function indicated some repulsive dependence between the two
mosaics. Both results were near the level of significance. A similar
analysis performed in the capybara retina, a closely related hystricomorph
rodent bearing a higher density of displaced horizontal cells than found
in the agouti, suggested spatial independence between the two mosaics,
normally placed versus displaced horizontal cells.