Light passes through the GCL first, continues through the IPL, INL, and OPL, and is absorbed by photoreceptors whose nuclei lie in the ONL ( Figure 1). The layers of the human retina are inverted relative to the path of light. The cell types of the retina are stratified into three layers of cell bodies and two plexiform layers: 1) the outer nuclear layer (ONL) contains the photoreceptors, 2) the outer plexiform layer (OPL) contains the synapses from the photoreceptors onto interneurons, 3) the inner nuclear layer (INL) contains the cell bodies of bipolar, horizontal, and amacrine interneurons, which transmit and process visual information, and Müller glia, which provide structural support and maintain the extracellular environment, 4) the inner plexiform layer (IPL) is the location where interneurons synapse onto the retinal ganglion cells, and 5) the ganglion cell layer (GCL) primarily contains retinal ganglion cells that relay information from the eye to the brain ( Figure 1). Retinal progenitor cells also generate one glial type called Müller glia. The vertebrate eye contains five major neuronal types generated from a common progenitor pool: photoreceptors, bipolar cells, retinal ganglion cells (RGCs), horizontal cells, and amacrine cells. We conclude with a description of how studies of human retinal organoids have advanced our understanding of photoreceptor subtype differentiation and how organoids serve as a promising new model to understand mechanisms of retinal development and disease. We continue with the patterning of photoreceptor subtypes across the retina, with a particular focus on the fovea and foveola, the structures that enable high visual acuity in primates. Next, we discuss how genetic and signaling pathways influence photoreceptor subtype specification. We then focus on photoreceptors, discussing how light sensitivities, cell morphologies, gene expression profiles, and functions define photoreceptor subtypes. We briefly describe the major cell types of the human retina and their roles in vision. The goal of this review is to describe patterning and development of the photoreceptors in the human retina. The three cones signal through interneurons and retinal ganglion cells to the brain to perceive the colors of the visible spectrum. Trichromacy is achieved by three distinct cone photoreceptor subtypes that enable the sensation of blue, green, and red wavelengths of light. Unlike other placental mammals, primates have trichromatic color vision and high visual acuity.
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