MELANIN AND THE REGULATION OF MAMMALIAN PHOTORECEPTOR TOPOGRAPHY

Melanin, or products directly associated with it, regulates the matura tion of the neural retina because in hypopigmented mammals the central retina fails to develop fully. To determine whether this deficit is r eflected in the distribution of photoreceptors, their topography has b een studied in the retinae of normally reared pigmented and albino fer rets and animals reared under reduced light conditions. In both strain s, the general distribution of rods and cones was similar to that in t he cat, cone density peaking in the central retina and rod density pea king in an annulus around the area centralis. The cone population was organized in the form of an orderly mosaic whose regularity was measur ed at a wide range of retinal eccentricities. No differences were foun d in cone numbers or their mosaic distribution between pigmented and a lbino strains, either at the area centralis or at more peripheral regi ons. In both cases order within the cone mosaic was independent of cel l density or retinal eccentricity. In the albinos there was a signific ant deficit in the number of rods at all retinal locations when compar ed with rod numbers in the pigmented animals. There were no difference s between normally reared and dark-reared animals in this respect eith er within or between the strains. Therefore, the albino gene must have a selective and specific effect on the development of this cell type in the outer retina. Ganglion cells and rods are both affected by the albino gene, while cones are not. Because cones and ganglion cells are generated during the same period and rods are generated later, the al bino gene cannot be acting during a particular developmental time wind ow. Because the cone mosaic was normal in the albinos, in spite of a l arge rod deficit, the factors that regulate the spacing of cones canno t depend in any significant manner upon the later generation and subse quent addition of rods to the outer retina.