Regularity of Ganglion Cell Distributions in the Dolphin Retina Increases During Postnatal Ontogeny

We report here studies of the regularity of the distribution of ganglion cells in the high-resolution zones of the retina of a neonatal and an adult bottlenose dolphin Tursiops truncatus . Two indicators were used as regularity criteria: the statistical distribution of the distances from each cell to the nearest neighboring cell (nearest neighbor distance, NND distribution) and the density of surrounding cells around each cell as a function of distance (spatial autocorrelogram, SACG). NND distributions for retinal cells in both neonatal and adult animals differed from the distribution for a random array of points, which is taken as an indication that the arrangement of cell is regular. Distributions were similar in neonatal and adult animals. SACG for the retinas of both neonatal and adult animals differed from the autocorrelogram for a random array of points in that each cell was surrounded by a “well,” the zone in which other cells were sparse or absent. This differed from the SACG of a random array of points, and hence another indication that the arrangement of cells is regular. The radius of the zone lacking neighboring cells was greater in the adult animal than the neonate, indicating that cells were more regularly arranged in the adult than the neonate. The absence of differences in NND distributions in neonatal and adult animals is understandable on the basis that this indicator is less informative than SACG. It is suggested that development of the spatial organization of the retina in dolphins is not complete at birth, but continues during postnatal ontogeny. This development consists of continuing ordering of the relative positions of ganglion cells.