DOCSLIB.ORG

Structure of Cone Photoreceptors

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Structure of Cone Photoreceptors Progress in Retinal and Eye Research 28 (2009) 289–302 Contents lists available at ScienceDirect Progress in Retinal and Eye Research journal homepage: www.elsevier.com/locate/prer Structure of cone photoreceptors Debarshi Mustafi a, Andreas H. Engel a,b, Krzysztof Palczewski a,* a Department of Pharmacology, Case Western Reserve University, Cleveland, OH 44106-4965, USA b Center for Cellular Imaging and Nanoanalytics, M.E. Mu¨ller Institute, Biozentrum, WRO-1058, Mattenstrasse 26, CH 4058 Basel, Switzerland abstract Keywords: Although outnumbered more than 20:1 by rod photoreceptors, cone cells in the human retina mediate Cone photoreceptors daylight vision and are critical for visual acuity and color discrimination. A variety of human diseases are Rod photoreceptors characterized by a progressive loss of cone photoreceptors but the low abundance of cones and the Retinoids absence of a macula in non-primate mammalian retinas have made it difficult to investigate cones Retinoid cycle directly. Conventional rodents (laboratory mice and rats) are nocturnal rod-dominated species with few Chromophore Opsins cones in the retina, and studying other animals with cone-rich retinas presents various logistic and Retina technical difficulties. Originating in the early 1900s, past research has begun to provide insights into cone Vision ultrastructure but has yet to afford an overall perspective of cone cell organization. This review Rhodopsin summarizes our past progress and focuses on the recent introduction of special mammalian models Cone pigments (transgenic mice and diurnal rats rich in cones) that together with new investigative techniques such as Enhanced S-cone syndrome atomic force microscopy and cryo-electron tomography promise to reveal a more unified concept of cone Retinitis pigmentosa photoreceptor organization and its role in retinal diseases. Age-related macular degeneration Published by Elsevier Ltd. Contents 1. Introduction . ................................................ 290 1.1. The eye across species . .......................290 1.2. Roles of rods and cones in the retina . .......................290 2. Structural basis of cone phototransduction . ................................... 292 2.1. Cone response to photons . .......................292 2.2. Structural dynamics of cone opsins during phototransduction . .......................293 2.3. Evolution of retinal circuitry . .......................294 2.4. Cone cell evolution . .......................295 3. Cone structure . ................................................ 296 3.1. Cone disc renewal . .. .......................296 3.2. Cone disc morphogenesis . 297 4. Importance of cones in retinal diseases . ........................................... 298 5. New mammalian models to study cone structure . ................................... 299 6. Conclusions and future outlook . ........................................... 300 Acknowledgments . .......................300 References................................................................. ................................ .......................300 Abbreviations: AFM, atomic force microscopy; BE, basal evagination; cGMP, cyclic guanosine monophosphate; COS, cone outer segment(s); Crx, cone-rod homeobox; cryo- ET, cryo-electron tomography; DI, distal invagination; EM, electron microscopy; ERG, electronretinography; ESCS, enhanced S-cone syndrome; GDP, guanosine diphosphate; GFP, green fluorescent protein; GPCR, G protein-coupled receptor; GTP, guanosine triphosphate; IS, inner segment(s); LCA, Leber congenital amaurosis; Myo3B, Myosin 3B; Nr2e3, photoreceptor nuclear receptor transcription factor; Nrl, neural retina leucine zipper; OS, outer segment(s); PDE, phosphodiesterase; PNA, peanut agglutinin; ROS, rod outer segment(s); RPE, retinal pigment epithelium; RPE65, RPE specific protein 65 kDa; TR-b2, thyroid hormone receptor-b2. * Corresponding author at: Department of Pharmacology, School of Medicine, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106-4965, USA. Tel.: 1 216 368 4631; fax: 1 216 368 1300. þ þ E-mail address: [email protected] (K. Palczewski). 1350-9462/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.preteyeres.2009.05.003 290 D. Mustafi et al. / Progress in Retinal and Eye Research 28 (2009) 289–302 1. Introduction M cones, inner segments (IS) of S cones are slightly extended. On average, there are over twice as many L-cones than M-cones in 1.1. The eye across species humans, but Old World primates do exist that exhibit more variation with some actually exhibiting M-cones that outnumber L-cones Vertebrate eyes are based on a common structural plan. Despite (Marc and Sperling, 1977); otherwise these two cone types show differences in embryological development and optical layouts similar spatial distributions and appear to be randomly intermixed. L among invertebrates, visual pigment genes are all descended from and M cones are most concentrated in the fovea where they are the same remote ancestor. Furthermore, genes involved in densely packed in a hexagonal pattern that accounts for the high formation of the vertebrate eye have proved to be homologous with visual acuity capability of the fovea. However, the spatial distribu- those of Drosophila eye (Cook and Zelhof, 2008). Remarkably, all tion of S cones across the retina differs from that of L and M cones in jawed vertebrates possess eyes so closely similar to our own that it is several respects. S cones constitute only about 5% of the total cone virtually indisputable that the last common ancestor of jawed population (Roorda et al., 2001), they are more peripherally located vertebrates (that lived around 430 million year ago) was equipped in the retina and are absent from the center of the human fovea. with an eye fundamentally like ours (Lamb et al., 2008). This strongly Indeed, pigments in the lens and macula selectively reduce the suggests that, despite certain differences, eyes across vertebrates fraction of higher-frequency photons that reach the retina and, have a common origin, with perhaps PAX6 as the universal master hence, the photoreceptors, thereby reducing the need for S-cones. control gene for eye morphogenesis (Gehring, 2002, 2005). Differ- This ‘filtration’ process improves vision in two respects. First, ences among vertebrates, therefore, must reflect adaptation to removal of higher-frequency photons serves to sharpen the image particular environments. Visual capabilities of animals have evolved (due to the refractive properties of water in the interior of the eye). to match aspects of their photic environment, and it is likely that the Second, such ‘filtration’ reduces damage to the retina and photore- primary adaptive selective pressure is the spectral range and ceptors produced by high frequency photons. Because most high intensity of daylight needed to optimize color vision. Other frequency photons are absorbed by the lens, images are produced structural features like size and packing density of photoreceptors mainly by L and M cones with less of a contribution from S-cones, are important for low light settings. Furthermore, structural which are nonetheless important because they contribute to color in differences of cone photoreceptors across species (Table 1) reflect image formation. adaptations of these species to their photic habitat and the demands of visual acuity. At the most basic level, nocturnal animals have the 1.2. Roles of rods and cones in the retina most rod-dominated retinas, whereas diurnal species have more cone-rich retinas. One of the most striking modifications of the The retina is the eye tissue layer that converts light into visual ancestral pattern of four spectral classes of cone opsins is found in signals transmitted to the brain. This process is carried out by two mammals, where only the two spectrally extreme classes are major types of photoreceptors, rods and cones that are distin- present. One explanation for this intermediate class loss relates to guished by their shape, type of photopigment, retinal distribution, the evolution of mammals when reptilian ancestors went through and pattern of synaptic connections (Fig. 1). These properties reflect a prolonged nocturnal phase. It is thought that because genes have the fact that rod and cone systems are specialized for different no long-term storage mechanism, a gene cannot be retained unless it aspects of vision. The rod system has low spatial resolution but is continuously remains functional, although there are exceptions to extremely sensitive to light so it is specialized for sensitivity at the this idea, as supported by the blind cavefish (Parry et al., 2003). expense of resolution. Conversely, the cone system has high spatial Among mammals, only primates have evolved trichromatic color resolution but is relatively insensitive to light. Thus, it is specialized vision. The primary mechanism for trichromacy in New World for visual acuity at the expense of sensitivity. primates is through allelic diversity of the L/M cone opsin gene on Basically, the different architectures of their outer segments (OS) the X-chromosome. This single visual pigment gene has multiple represent a major distinctive feature of these two cell types. Rods alleles. Heterozygous females segregate expression of the alleles with their longer OS composed of individualized discs unconnected into separate populations of cones that are trichromatic. Old World to the ciliary plasma membrane contrast starkly with cones. The primates, including humans, have
Load more

Recommended publications
  • How Can Retroreflective Clothing Provide More Safety Through Visibility in a Semi-Dark Urban Environment? a Study Taking Plac
    MASTER’S THESIS How can retroreflective clothing BY VIOLA SCHMITZ provide more safety through visibility in a semi-dark urban Royal Institute of Technology environment? KTH School of Architecture Master’s Program in A study taking place in Scandinavia. Architectural Lighting Design 2018-2019 24.05.2019 AF270X VT19-1 Tutor: Foteini Kyriakidou 0 Index Abstract P. 2 1. Introduction P. 2 2. Background P. 3 2.1. Urban Background P. 4 2.2. Biological background P. 4 2.2.1. Reflexes and reactions P. 4 2.2.2. Types of vision P. 4 2.2.3. Effect of pattern P. 5 recognition 2.2.4. Human field of vision P. 5 3. Analysis P. 6 3.1. Analysis: Retroreflectors P. 6 3.2. Analysis: Existing products P. 7 4. Methodology P. 9 5. Methods P. 10 5.1. Survey: P. 10 Lines defining the human body 5.2. Video Experiment: P. 10 Designs in motion 5.2.1. Analysis: Location P. 10 5.2.2. Video Experiment P. 11 5.2.3. Procedure P. 12 5.3. Experimental survey: P. 12 Size of a human 5.4. Visualization: P. 13 Pattern recognition in surroundings 6. Results P. 14 6.1. Survey: P. 14 Lines defining the human body 6.2. Video Experiment: P. 15 Designs in motion 6.2.1. Analysis: Location P. 15 6.2.2. Video Experiment P. 16 6.2.3. Observation P. 17 6.3. Experimental survey: P. 17 Size of a human 6.4. Visualization: Pattern P. 17 recognition in surroundings 7. Discussion P.
    [Show full text]
  • Blue Cone Monochromacy: Visual Function and Efficacy Outcome Measures for Clinical Trials
    RESEARCH ARTICLE Blue Cone Monochromacy: Visual Function and Efficacy Outcome Measures for Clinical Trials Xunda Luo1☯‡, Artur V. Cideciyan1☯‡*, Alessandro Iannaccone2, Alejandro J. Roman1, Lauren C. Ditta2, Barbara J. Jennings2, Svetlana A. Yatsenko3, Rebecca Sheplock1, Alexander Sumaroka1, Malgorzata Swider1, Sharon B. Schwartz1, Bernd Wissinger4, Susanne Kohl4, Samuel G. Jacobson1* 1 Scheie Eye Institute, Department of Ophthalmology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America, 2 Hamilton Eye Institute, Department of Ophthalmology, University of Tennessee Health Science Center, Memphis, Tennessee, United States of America, 3 Pittsburgh Cytogenetics Laboratory, Center for Medical Genetics and Genomics, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, United States of America, 4 Molecular Genetics Laboratory, Institute for Ophthalmic Research, Centre for Ophthalmology, University of Tuebingen, Tuebingen, Germany ☯ These authors contributed equally to this work. ‡ OPEN ACCESS These authors are joint first authors on this work. * [email protected] (SGJ); [email protected] (AVC) Citation: Luo X, Cideciyan AV, Iannaccone A, Roman AJ, Ditta LC, Jennings BJ, et al. (2015) Blue Cone Monochromacy: Visual Function and Efficacy Abstract Outcome Measures for Clinical Trials. PLoS ONE 10(4): e0125700. doi:10.1371/journal.pone.0125700 Academic Editor: Dror Sharon, Hadassah-Hebrew University Medical Center, ISRAEL Background Blue Cone Monochromacy (BCM) is an X-linked retinopathy caused by mutations in the Received: December 29, 2014 OPN1LW / OPN1MW gene cluster, encoding long (L)- and middle (M)-wavelength sensitive Accepted: March 21, 2015 cone opsins. Recent evidence shows sufficient structural integrity of cone photoreceptors in Published: April 24, 2015 BCM to warrant consideration of a gene therapy approach to the disease.
    [Show full text]
  • A Real-Time Retinomorphic Simulator Using a Conductance-Based Discrete Neuronal Network
    A Real-Time Retinomorphic Simulator Using a Conductance-Based Discrete Neuronal Network Seungbum Baek1;5, Jason K. Eshraghian2;5, Wesley Thio2, Yulia Sandamirskaya3, Herbert H.C. Iu 4 and Wei D. Lu2 1College of Electrical and Computer Engineering, Chungbuk National University, Cheongju 362763, South Korea 2School of Electrical, Electronic and Computer Engineering, University of Michigan, Ann Arbor, MI 48109 USA 3Institute of Neuroinformatics Neuroscience Center Zurich University and ETH Zurich, Switzerland 4School of Electrical, Electronic and Computer Engineering, University of Western Australia, Crawley, WA 6009, Australia 5These authors contributed equally to this manuscript. Abstract—We present an optimized conductance-based retina [2], and fabricating high-performance image sensors that are microcircuit simulator which transforms light stimuli into a on par with the specifications of the retina in terms of power series of graded and spiking action potentials through photo dissipation, dynamic range, and resolution [3]–[7]. At present, transduction. We use discrete retinal neuron blocks based on a collation of single-compartment models and morphologically most bio-inspired image processors trade-off the ability to pass realistic formulations, and successfully achieve a biologically low-frequency content and low spatial resolution, in favor of real-time simulator. This is done by optimizing the numerical practicality. methods employed to solve the system of over 270 nonlinear Beyond hardware, a similar distinction exists between bi-
    [Show full text]
  • Birdshot Chorioretinopathy
    Ocular Inflammation Service, Oxford Eye Hospital Birdshot Chorioretinopathy Information for patients What is birdshot chorioretinopathy? Birdshot chorioretinopathy (or retinochoroidopathy), normally shortened to ‘birdshot’, is a rare, potentially blinding, posterior uveitis. This is chronic inflammation of the choroid, which also tends to affect the retina and retinal vessels. It affects both eyes. In the picture below, you can see the position of the vitreous, retina and uvea (iris, ciliary body, pars planar and choroid), which has three sections; anterior, intermediate and posterior. The dotted area represents the uvea. Choroid Retina Vitreous Cornea Macula Lens Fovea Iris Optic nerve Ciliary body anterior intermediate posterior page 2 Birdshot chorioretinopathy is characterised by inflammation of the vitreous (clear jelly in the eye) which causes orange, yellow or cream coloured oval shaped spots at the back of your eye on your retina. These affect the macula (an area near the centre of the retina used for detailed vision) and can cause vision loss. The reason this disease is called ‘birdshot’ is because these spots look like the pattern seen when you fire birdshot pellets from a shotgun. What causes birdshot? It is believed to be due to an autoimmune disease. An autoimmune disease is an illness that occurs when the body tissues are attacked by its own immune system, which causes chronic inflammation. It is most likely to develop in people aged between 45 and 55, although it can also occur in much younger and older people. Birdshot is a relatively new disease. It was first discovered in 1949 and only given the title ‘birdshot’ in 1980.
    [Show full text]
  • The Genetic Horizon Improving Clinical Sensitivity in Difficult-To-Sequence Genes for Rare Hereditary Disorders
    White paper The Genetic Horizon Improving clinical sensitivity in difficult-to-sequence genes for rare hereditary disorders Meeting clinical needs using custom solutions From Bench to Bedside: The Genetic Horizon Ongoing challenges The long-awaited promise of tailored treatments for individual patients based on their genetic Clinically relevant but highly homologous and · Masked regions in the reference data sets contain makeup is beginning to materialize. Next generation sequencing, along with artificial intelligence, repetitive regions within important genes are an uncertain or ambiguous variant calls that are the source ongoing challenge to analyze: of false positives, false negatives and other genotype have facilitated the rapid, accurate analysis and interpretation of genomic data for disease causing calling errors (e.g., calling homozygous variant at a variants which may be treatable through emerging gene therapy technology. · Certain genomic regions are difficult to analyze due to heterozygous site)1. complex sequence variations therein The number of clinical trials and drug development involves delivering a functional copy of the SMN1 gene in a Blueprint Genetics remains committed to resolving pipelines for rare disease are quickly growing and, one-time intravenous infusion (www.avexis.com). · Not only are there variants in these regions but there difficult-to-sequence regions that are hard to validate, encouragingly, the first approved treatments have been are types of variants that we cannot detect in the clinical interpret and confirm, by developing custom solutions. In successful. As the field of precision medicine is still in its The power of gene therapies is in the specificity of the diagnostic laboratory with current technologies (Table 1) this paper, we share our strategies and what is next in our infancy, increased awareness about the clinical utility of treatments: understanding the genetic mechanism of R&D pipeline.
    [Show full text]
  • Recognition of Retroreflective Road Signs During Night Driving
    Recognition of retroreflective road signs during night driving J. Berzinsh1, M.Ozolinsh1, P.Cikmacs1, and K. Pesudovs2 1Department of the Optometry and Vision Science, University of Latvia Riga, Latvia 2Department of the Optometry, Bradford University Bradford, West Yorkshire, UK Temporal waveforms of the illumination at the driver eyes position were determined in various night traffic and weather conditions (ideal weather and correct aligned lights as compared with dirty lamps and raindrops on the windscreen). The statistics of retinal illumination were analysed, and a computer controlled technique was developed to simulate similar changes of eye illumination. The participant fixated on retroreflective optical stimuli at a distance of 5 m. The participant was then subjected to dazzle, and recovery from the glare took place. The background illumination was in the mesopic range. Experiments showed that at background illumination 0.1 Lx no dazzling took place in case of correctly installed clean headlights. The participant was dazzled if the high beam lamps were incorrectly aligned or cycloplegia was used for pupil dilation. The dazzle time depended on the background illumination level and could increase to three seconds for the illumination changes corresponding to the equivalent speed of vehicles 50 km/h. Introduction Vision plays a significant role in safe driving. Standards for vision must be met for a driver to hold a licence. These standards prescribe the vision quality in normal situations with sufficient illumination levels. Driving at night is a more difficult task (Charman, 1996; Priez et al., 1998), more accidents per vehicle happen on roads at night (Federal Office of Road Safety, 1996).
    [Show full text]
  • Intermixing the OPN1LW and OPN1MW Genes Disrupts the Exonic Splicing Code Causing an Array of Vision Disorders
    G C A T T A C G G C A T genes Review Intermixing the OPN1LW and OPN1MW Genes Disrupts the Exonic Splicing Code Causing an Array of Vision Disorders Maureen Neitz * and Jay Neitz Department of Ophthalmology and Vision Science Center, University of Washington, Seattle, WA 98109, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-206-543-7998 Abstract: Light absorption by photopigment molecules expressed in the photoreceptors in the retina is the first step in seeing. Two types of photoreceptors in the human retina are responsible for image formation: rods, and cones. Except at very low light levels when rods are active, all vision is based on cones. Cones mediate high acuity vision and color vision. Furthermore, they are critically important in the visual feedback mechanism that regulates refractive development of the eye during childhood. The human retina contains a mosaic of three cone types, short-wavelength (S), long-wavelength (L), and middle-wavelength (M) sensitive; however, the vast majority (~94%) are L and M cones. The OPN1LW and OPN1MW genes, located on the X-chromosome at Xq28, encode the protein component of the light-sensitive photopigments expressed in the L and M cones. Diverse haplotypes of exon 3 of the OPN1LW and OPN1MW genes arose thru unequal recombination mechanisms that have intermixed the genes. A subset of the haplotypes causes exon 3- skipping during pre-messenger RNA splicing and are associated with vision disorders. Here, we review the mechanism by which splicing defects in these genes cause vision disorders. Citation: Neitz, M.; Neitz, J.
    [Show full text]
  • 17-2021 CAMI Pilot Vision Brochure
    Visual Scanning with regular eye examinations and post surgically with phoria results. A pilot who has such a condition could progress considered for medical certification through special issuance with Some images used from The Federal Aviation Administration. monofocal lenses when they meet vision standards without to seeing double (tropia) should they be exposed to hypoxia or a satisfactory adaption period, complete evaluation by an eye Helicopter Flying Handbook. Oklahoma City, Ok: US Department The probability of spotting a potential collision threat complications. Multifocal lenses require a brief waiting certain medications. specialist, satisfactory visual acuity corrected to 20/20 or better by of Transportation; 2012; 13-1. Publication FAA-H-8083. Available increases with the time spent looking outside, but certain period. The visual effects of cataracts can be successfully lenses of no greater power than ±3.5 diopters spherical equivalent, at: https://www.faa.gov/regulations_policies/handbooks_manuals/ techniques may be used to increase the effectiveness of treated with a 90% improvement in visual function for most One prism diopter of hyperphoria, six prism diopters of and by passing an FAA medical flight test (MFT). aviation/helicopter_flying_handbook/. Accessed September 28, 2017. the scan time. Effective scanning is accomplished with a patients. Regardless of vision correction to 20/20, cataracts esophoria, and six prism diopters of exophoria represent series of short, regularly-spaced eye movements that bring pose a significant risk to flight safety. FAA phoria (deviation of the eye) standards that may not be A Word about Contact Lenses successive areas of the sky into the central visual field. Each exceeded.
    [Show full text]
  • Reprogramming of Adult Rod Photoreceptors Prevents Retinal Degeneration
    Reprogramming of adult rod photoreceptors prevents retinal degeneration Cynthia L. Montanaa, Alexander V. Kolesnikovb, Susan Q. Shena, Connie A. Myersa, Vladimir J. Kefalovb, and Joseph C. Corboa,1 Departments of aPathology and Immunology and bOphthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, MO 63110 Edited by Jeremy Nathans, Johns Hopkins University, Baltimore, MD, and approved December 19, 2012 (received for review August 20, 2012) A prime goal of regenerative medicine is to direct cell fates in become cones (17, 18). We reasoned that acute inactivation of Nrl a therapeutically useful manner. Retinitis pigmentosa is one of the in adult rods might result in direct conversion of these cells into most common degenerative diseases of the eye and is associated cones. Furthermore, a recent study demonstrated that retinas in with early rod photoreceptor death followed by secondary cone which Nrl had been knocked out during development showed long- degeneration. We hypothesized that converting adult rods into term survival of cone photoreceptors and preservation of the outer cones, via knockdown of the rod photoreceptor determinant Nrl, nuclear layer, after a transient initial phase of cell loss (19). This could make the cells resistant to the effects of mutations in rod- observation suggests that direct conversion of adult rods into cones specific genes, thereby preventing secondary cone loss. To test this could also lead to long-term survival of the transdifferentiated idea, we engineered a tamoxifen-inducible allele of Nrl to acutely cells. To test this idea, we used a tamoxifen-inducible allele of Nrl inactivate the gene in adult rods.
    [Show full text]
  • Progressive Cone and Cone-Rod Dystrophies
    Br J Ophthalmol: first published as 10.1136/bjophthalmol-2018-313278 on 24 January 2019. Downloaded from Review Progressive cone and cone-rod dystrophies: clinical features, molecular genetics and prospects for therapy Jasdeep S Gill,1 Michalis Georgiou,1,2 Angelos Kalitzeos,1,2 Anthony T Moore,1,3 Michel Michaelides1,2 ► Additional material is ABSTRact proteins involved in photoreceptor structure, or the published online only. To view Progressive cone and cone-rod dystrophies are a clinically phototransduction cascade. please visit the journal online (http:// dx. doi. org/ 10. 1136/ and genetically heterogeneous group of inherited bjophthalmol- 2018- 313278). retinal diseases characterised by cone photoreceptor PHOTORECEPTION AND THE degeneration, which may be followed by subsequent 1 PHOTOTRANSDUCTION CASCADE UCL Institute of rod photoreceptor loss. These disorders typically present Rod photoreceptors contain rhodopsin phot- Ophthalmology, University with progressive loss of central vision, colour vision College London, London, UK opigment, whereas cone photoreceptors contain 2Moorfields Eye Hospital NHS disturbance and photophobia. Considerable progress one of three types of opsin: S-cone, M-cone or Foundation Trust, London, UK has been made in elucidating the molecular genetics L-cone opsin. Disease-causing sequence variants 3 Ophthalmology Department, and genotype–phenotype correlations associated with in the genes encoding the latter two cone opsins University of California San these dystrophies, with mutations in at least 30 genes
    [Show full text]
  • Difference Between Rod and Cone Cells Key Difference
    Difference Between Rod and Cone Cells www.differencebetween.com Key Difference - Rod vs Cone Cells The photoreceptors are cells in the retina of the eye which respond to the light. The distinguishing feature of these cells is the presence of tightly packed membrane that contains the photopigment known as rhodopsin or related molecules. The photopigments have a similar structure. All photopigments consist of a protein called opsin and a small attached molecule known as a chromophore. The chromophore absorbs the portion of light by a mechanism that involves the change in its configuration. The tight packing in the membranes of these photoreceptors is highly valuable in order to achieve high photopigment density. This allows the large portion of light photons which reach the photoreceptors to be absorbed. In vertebrates, the retina consists of two photoreceptors (rod and cone cells) which are bearing photopigment at their outer region. This particular region is composed of a large number of pancake-like disks. In rod cells, the disks are closed, but in the cone cells, the disks are partially open to the surrounding fluids. In invertebrates, the photoreceptors structure is very different. The photopigment was born in a regularly arranged structure called as microvilli, finger-like projections with about diameter of 0.1µm. This photoreceptor structure in invertebrates is known as rhabdom. The photopigments are less densely packed in the rhabdom than in vertebrates’ disks. The key difference between rod and cone cells is that the rod cells are responsible for vision at the low light levels (scotopic vision) while the cone cells are active at higher light levels (photopic vision).
    [Show full text]
  • Miiller Cells Are a Preferred Substrate for in Vitro Neurite Extension by Rod Photoreceptor Cells
    The Journal of Neuroscience, October 1991, 1 l(10): 2985-2994 Miiller Cells Are a Preferred Substrate for in vitro Neurite Extension by Rod Photoreceptor Cells lvar J. Kljavin’ and Thomas A. Reh2 ‘Neuroscience Research Group, Faculty of Medicine, Lion’s Sight Center, University of Calgary, Calgary, Alberta, Canada T2N lN4 and ‘Department of Biological Structure, University of Washington, Seattle, Washington 98195 To define the factors important in photoreceptor cell mor- (Silver and Sidman, 1980; Krayanek and Goldberg, 1981). Ad- phogenesis, we have examined the ability of rods to extend ditional studies have begun to characterize the moleculesthat neurites in vitro. Retinas from neonatal rats were dissociated are responsiblefor regulating the growth of ganglion cell axons and plated onto substrate-bound extracellular matrix (ECM) during development, and it appearsthat many of the ECM and components or cell monolayers. When rods, identified with cell adhesion molecules involved in axon outgrowth are con- monoclonal antibodies to opsin, were in contact exclusively centrated on the neuroepithelial cell end feet along these long with purified ECM (e.g., laminin, fibronectin, type I collagen, tracts (Silver and Rutishauser, 1984; Halfter et al., 1988). or Matrigel), neurite outgrowth was extremely limited. By However, during the normal development of the retina, as contrast, rods extended long neurites on Miiller cells. Retinal well as other areas of the CNS, most neurons do not extend or brain astrocytes, endothelial cells, 3T3 fibroblasts, or oth- axons into long pathways, but rather, their axons terminate on er retinal neurons were less supportive of rod process out- neighboring cells. In the retina, for example, the axons of the growth.
    [Show full text]