Evidence of photoreceptor migration during early foveal development: a quantitative analysis of human fetal retinae.

We have analyzed aspects of photoreceptor topography in wholemounts of human fetal retinae in the age range 13-24 weeks of gestation. Fetal retinae were stained with cresyl violet and the sizes and packing densities of rods and cones analyzed in the conventional manner. Cones and rods were present within a differentiating region, free of mitotic figures and approximately centered on the putative fovea, represented by the foveal cone mosaic. At 13 weeks of gestation the foveal cone mosaic was clearly differentiated, cone nuclei reaching a packing density of 14,200 per mm2; a small number of rods were present in the immediately adjacent region. The packing densities of both rods and cones in these regions gradually increased and the area of the foveal cone mosaic gradually decreased throughout the age range sampled, although individual variations were evident. By 24 weeks of gestation, cone density was approximately 38,000 per mm2 in the foveal cone mosaic. The maximum rod density observed was 59,200 per mm2 in the region surrounding the foveal cone mosaic in a specimen of 20-21 weeks of gestation. In all specimens, maximum cone density occurred within the foveal cone mosaic and gradually declined towards the periphery of the differentiating region; a pronounced inverse relationship between cone soma diameter and packing density was also observed. The evidence strongly suggests that both rods and cones migrate centripetally, that is towards the center of the developing fovea, from early in development, possibly from the time that they first differentiate. The implications of these findings for foveal development are discussed.