Layer of the neural retina that contains the photoreceptor cells. Photoreceptor cells span the outer nuclear layer as well as the outer and inner segment layers. These light sensing cells of the retina, the cone and rod photoreceptors, are located in the apical-most layer of the retina, and display a stereotypical subcellular organization with their nuclei basal to the inner segment (cell body or soma) and outer segment, which abuts the retinal pigmented epithelium (RPE) and is full of membrane invaginations packed with light-sensitive cell-specific opsins. All photoreceptors have opsin-containing outersegments, which are replaced on a daily basis. The older membrane and protein debris is shed from the distal tip of the outersegment and phagocytosed by the adjacent RPE cells. Because zebrafish are diurnal, their retinae contains a large number of bright-light sensitive cone subtypes in addition to dim-light sensitive rod photoreceptors, all of which are organized into regular mosaic patterns that can best be visualized in tangential sections. Below is more detailed information about the distinct types of photoreceptors found in zebrafish.
Cones are required for bright light vision and can be subdivided into four classes based on opsin expression and morphology. The paired long double cones express both red and green opsin, the long single cones express blue opsin, and the short single cones express uv-sensitive opsin. Antibodies generated against each of the 4 cone opsins or rhodopsin label the outer segments of individual cones or rods, respectively; in situ hybridization of the opsin transcripts label the inner segments. Several transgenic lines, which use various opsin promoters to drive expression of fluorophores (see below), reveal the location and morphology of specific photoreceptor subtypes.
Rods are utilized mainly for dim-light vision, and their cell bodies are located vitreal to the cone nuclei, and in the light-adapted retina, the thin rod inner and outer segments project beyond the cones, interdigitating between the apical microvilli of the adjacent retinal pigmented epithelia (RPE)l cells.