Rod and Cone Photoreceptor Cells

Mutations in phototransduction proteins are a major cause of retinitis pigmentosa and other photoreceptor degenerations 

Photoreceptor Inner Segment, Soma, and Synaptic Terminal 

---

Rodriguez de Turco EB, Gordon WC, Bazan NG. Rapid and selective uptake, metabohsm and cellular distribution of docosahexaenoic acid among rod and cone photoreceptor cells in the frog retina. J Neurosci 1991 lll(l) 3667-3678. 

The retinal pigment epithelium cells (RPE) of the retina form an epithelial cell layer that takes up retinol from choroid capillaries and stores it as retinyl esters, to be used as substrate for the generation of 11-ds-retinal. In the layer of rod and cone photoreceptor cells adjacent to the RPE, 11-ds-retinal combines covalently with the protein opsin to generate the visual pigment rhodopsin in rods and, similarly, iodopsin in cones. Each rod outer segment is densely packed with some 10 molecules of rhodopsin per cell. The small quantity of vitamin A stored in the retina would be inadequate to maintain vision were it not for the visual cycle, a process in which 11-ds-retinal is regenerated after... 

F. Osakada, H. Ikeda, M. Mandai, T. Wataya, K. Watanabe, N. Yoshimura, A. Akaike, Y. Sasai, and M. Takahashi, Toward the generation of rod and cone photoreceptors from mouse, monkey and human embryonic stem cells, Nat. Biotechnol, 26 (2) 215-224, Feb. 2008. 

Members of the first group of NCS are recoverin and visinin. Recoverin is a 23 kDa myristoylated protein found under normal conditions only in photoreceptor cells (rods and cones). The main function of recoverin is to bind to and inhibit rhodopsin kinase, thereby prolonging the light response. 

Vitamin A is essential throughout life, including foetal development, but perhaps its most well researched role is that in vision where 11 -cis retinaldehyde is the initial part of the photoreceptor complex in rods and cones. Retinoic acid induces differentiation in epithelial cells and deficiency leads to... 

In photoreceptor cells, the rods and cones of the human retina, the retinal is linked to a specific protein termed opsin. The resulting pigment is known as rhodopsin. When a photon of light of the proper wavelength hits a molecule of rhodopsin, two chemical events take place. First, the ll-c -retinal is converted to the all-trans form and, secondly, the all-trani-retinal is released from the rhodopsin ... 

Two types of photoreceptor cell are found in the human retina—rods and cones. Rods are sensitive to low levels of light, while the cones are responsible for color vision at higher light intensities. 

Removal of the eyes in every mammal studied abolishes photoentrainment (Foster 1998). Because the rods and cones were the only known ocular photoreceptors, this led to the assumption that all photoreception can be attributed to these cells. Initial studies on rdjrd mice, which lack rod photoreceptors, and more recent studies on rdjrd cl mice, which lack all functional rods and cones, have provided overwhelming evidence that these classical photoreceptors are not required for photoentrainment (Foster 2002). By extension, the eye must contain at least one additional class of photoreceptor. In addition, studies on rd rd cl mice have shown that the non-rod, non-cone photoreceptors do more than regulate the circadian system. They also contribute to both pupillary constriction and acute alterations in locomotor behaviour, and may be involved in a broad range of physiological and behavioural responses to light (Foster 2002). 

The opsins probably perform a variety of different tasks, but their known roles are as photosensors or photoisomerases (Foster Bellingham 2002). Photosensory opsins such as the rod and cone opsins, P opsin and VA opsin use light to activate a phototransduction cascade that ultimately results in a change in membrane potential of the photoreceptor cell. By contrast, photoisomerases are involved in photopigment regeneration. The best described photoisomerase is... 

---