Diversity in photoreceptor composition among different retinal environments.

The retina is a thin neural tissue that lines the inside of the eye, as shown through a cutaway in a schematized eyeball. In humans, the macula is a region of the retina that is very rich in cones, including blue-, green-, and red-sensitive cone types. The cones of the macula are responsible for daytime photopic vision, color discrimination, and visual acuity, thus the macula is an important target for vision restoration. The zebrafish and murine retinas are well-studied examples of cone rich and rod rich respectively, exemplary of a spectrum of retinal environments. Zebrafish have a high and even density of cones across the entire retina, with rods dispersed throughout. Zebrafish cones are arranged in a row mosaic where rows of ultraviolet and blue cones (colored purple and blue) alternate with rows of red and green cones (red and green, shown in double cone morphology). The mouse retina has a high density of rods and low density of cones. Mouse cones express green-sensitive opsin in the dorsal retina, blue-sensitive opsin in the ventral retina, and co-express both in the central retina. Rods are depicted as long grey bodies; cones depicted as stout bodies. In photoreceptor schematics, dorsal is towards the top.

Effects of photoreceptor identity on retinal regeneration and functional vision restoration.

Both cellular regeneration and the kinetics of synaptic plasticity are differentially affected by the identity of the photoreceptor that is lost. Loss of ultraviolet (UV) cones in zebrafish via pharmacogenetic ablation biases the retina to regenerate UV cones, resulting in restoration of their position and abundance. Alternatively when blue cones are ablated, a robust and rapid restoration of visual function occurs through synaptic plasticity. Meanwhile, the remaining photoreceptor types survive, likely through many pro-survival mechanisms. Thus multiple mechanisms act in response to cone loss, and the identity of the cone type lost (or perhaps the type[s] that remain) affects the mechanisms employed for regeneration and restoration of function. Based on these inferences, the need to consider photoreceptor types during regeneration strategies is apparent: regeneration, survival, and re-wiring of cones into the cone-rich human macula obliges use of appropriately cone-dominant model systems. HC: Horizontal cell.