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A model of the development ofL/Mcone ratios in the Old World primate retina is presented. It is supposed that during gestation, the cone cycles randomly between states in which it transcribes eitherLorMopsin. The current state determines and increases the probability that it will transcribe the same opsin in future cycles. These assumptions are sufficient to formalize the process as a Markov chain that can be modeled as an urn containing two types of balls,LandM. Drawing one ball results in the increase of its species and the decrease of the other. Over the long run, the urn will become populated with a single type of ball. This state corresponds to the photoreceptor adopting a fixed identity for its lifetime. We investigate the effect of the number of states and the rule that regulates the advantage of transition toward one cone type or another on the relation between fetal and adultL/Mcone ratios. In the range of 100 to 1000 states, small variations of the initialL/Mratio or the transition advantage can each generate large changes in the finalL/Mratio, in qualitative accord with the variation seen in human adult retinas. The time course to attain stableL/Mratios also varies with these parameters. If it is believed that the cycling follows a circadian rhythm, then finalL/Mcone ratios would be expected to stabilize shortly after birth in the human being and the macaque.

PURPOSE. Melanopsin, expressed in a subset of intrinsically photosensitive ganglion cells that project to the suprachiasmatic nucleus (SCN), is involved in the photic entrainment of circadian rhythms and other non–image-forming functions (pupil light reflex, masking, acute heart rate response, and alertness). Melanopsin has recently been shown to be a “bireactive” photopigment that functions as a photosensory opsin using 11-cis retinaldehyde as a chromophore and has intrinsic photoisomerase activity. Melanopsin is widely distributed in the retina of vertebrates and, depending on the species, is expressed in ganglion, amacrine, horizontal, and photoreceptor cells. The present study was conducted to determine the distribution of this opsin in the human retina. METHODS. Human donor eyes were obtained from donors and fixed shortly after death. Immunohistochemistry was used to determine melanopsin expression in the retinas of three donors. The possible coexpression of this photopigment with other opsins was studied by double-labeling immunocytochemistry and confocal analysis. RESULTS. In addition to the expected labeling in ganglion cells of the inner retinal layers, an unexpected finding showed melanopsin-immunopositive label in the outer segments of cones that did not coexpress other known opsins. These melanopsin-expressing cones are extremely sparse (5–25 cones/ mm 2 ; 0.1%– 0.5% of the entire cone population) and are located in the peripheral retina. CONCLUSIONS. The presence of melanopsin in human cones suggests image and non–image-forming roles in visual responses at both the cone input and ganglion cell output stages and their involvement in a broad spectrum of irradiance detection functions in the visual system. (Invest Ophthalmol Vis Sci. 2006;47:1636 –1641) DOI:10.1167/iovs.05-1459