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Experimental Eye Research Mar 2017The crystalline lens plays an important role in the refractive vision of vertebrates by facilitating variable fine focusing of light onto the retina. Loss of lens... (Review)
Review
The crystalline lens plays an important role in the refractive vision of vertebrates by facilitating variable fine focusing of light onto the retina. Loss of lens transparency, or cataract, is a frequently acquired cause of visual impairment in adults and may also present during childhood. Genetic studies have identified mutations in over 30 causative genes for congenital or other early-onset forms of cataract as well as several gene variants associated with age-related cataract. However, the pathogenic mechanisms resulting from genetic determinants of cataract are only just beginning to be understood. Here, we briefly summarize current concepts pointing to differences in the molecular mechanisms underlying congenital and age-related forms of cataract.
Topics: Aging; Animals; Cataract; Crystallins; Humans; Lens, Crystalline; Mutation
PubMed: 27334249
DOI: 10.1016/j.exer.2016.06.011 -
Progress in Molecular Biology and... 2015In order to accomplish its function of transmitting and focusing light, the crystalline lens of the vertebrate eye has evolved a unique cellular structure and protein... (Review)
Review
In order to accomplish its function of transmitting and focusing light, the crystalline lens of the vertebrate eye has evolved a unique cellular structure and protein complement. These distinct adaptations have provided a rich source of scientific discovery ranging from biochemistry and genetics to optics and physics. In addition, because of these adaptations, lens cells persist for the lifetime of an organism, providing an excellent model of the aging process. The chapters dealing with the lens will demonstrate how the different aspects of lens biology and biochemistry combine in this singular refractive organ to accomplish its critical role in the visual system.
Topics: Aging; Animals; Humans; Lens, Crystalline
PubMed: 26310153
DOI: 10.1016/bs.pmbts.2015.04.006 -
Progress in Molecular Biology and... 2015The primary function of the lens resides in its transparency and ability to focus light on the retina. These require both that the lens cells contain high concentrations... (Review)
Review
The primary function of the lens resides in its transparency and ability to focus light on the retina. These require both that the lens cells contain high concentrations of densely packed lens crystallins to maintain a refractive index constant over distances approximating the wavelength of the light to be transmitted, and a specific arrangement of anterior epithelial cells and arcuate fiber cells lacking organelles in the nucleus to avoid blocking transmission of light. Because cells in the lens nucleus have shed their organelles, lens crystallins have to last for the lifetime of the organism, and are specifically adapted to this function. The lens crystallins comprise two major families: the βγ-crystallins are among the most stable proteins known and the α-crystallins, which have a chaperone-like function. Other proteins and metabolic activities of the lens are primarily organized to protect the crystallins from damage over time and to maintain homeostasis of the lens cells. Membrane protein channels maintain osmotic and ionic balance across the lens, while the lens cytoskeleton provides for the specific shape of the lens cells, especially the fiber cells of the nucleus. Perhaps most importantly, a large part of the metabolic activity in the lens is directed toward maintaining a reduced state, which shelters the lens crystallins and other cellular components from damage from UV light and oxidative stress. Finally, the energy requirements of the lens are met largely by glycolysis and the pentose phosphate pathway, perhaps in response to the avascular nature of the lens. Together, all these systems cooperate to maintain lens transparency over time.
Topics: Crystallins; Cytoskeletal Proteins; Gap Junctions; Humans; Lens, Crystalline; Membrane Proteins; Models, Biological
PubMed: 26310155
DOI: 10.1016/bs.pmbts.2015.04.007 -
Progress in Retinal and Eye Research Sep 2017The factors that regulate the size of organs to ensure that they fit within an organism are not well understood. A simple organ, the ocular lens serves as a useful model... (Review)
Review
The factors that regulate the size of organs to ensure that they fit within an organism are not well understood. A simple organ, the ocular lens serves as a useful model with which to tackle this problem. In many systems, considerable variance in the organ growth process is tolerable. This is almost certainly not the case in the lens, which in addition to fitting comfortably within the eyeball, must also be of the correct size and shape to focus light sharply onto the retina. Furthermore, the lens does not perform its optical function in isolation. Its growth, which continues throughout life, must therefore be coordinated with that of other tissues in the optical train. Here, we review the lens growth process in detail, from pioneering clinical investigations in the late nineteenth century to insights gleaned more recently in the course of cell and molecular studies. During embryonic development, the lens forms from an invagination of surface ectoderm. Consequently, the progenitor cell population is located at its surface and differentiated cells are confined to the interior. The interactions that regulate cell fate thus occur within the obligate ellipsoidal geometry of the lens. In this context, mathematical models are particularly appropriate tools with which to examine the growth process. In addition to identifying key growth determinants, such models constitute a framework for integrating cell biological and optical data, helping clarify the relationship between gene expression in the lens and image quality at the retinal plane.
Topics: Animals; Cell Differentiation; Humans; Intercellular Signaling Peptides and Proteins; Lens, Crystalline; Signal Transduction
PubMed: 28411123
DOI: 10.1016/j.preteyeres.2017.04.001 -
Clinical & Experimental Optometry Sep 2002Microspherophakia is present when the crystalline lens is small and relatively spherical with increased antero-posterior thickness. Clinical findings for a patient with... (Review)
Review
Microspherophakia is present when the crystalline lens is small and relatively spherical with increased antero-posterior thickness. Clinical findings for a patient with idiopathic bilateral microspherophakia are described. The patient was moderately myopic with slightly reduced visual acuity. The anterior chambers (R: 1.57 and L: 1.37 mm) were shallow compared with normals (3.46 to 3.80 mm) and the crystalline lenses were thicker (R: 4.77 and L: 4.89 mm) than normal (3.3 to 3.96 mm) with steeper than normal anterior (radii of curvature R: 6.2 and L: 6.3 mm) and posterior (R: 6.3 and L: 5.6 mm) surfaces. Microspherophakia may be associated with various syndromes and there is a strong possibility of glaucoma, particularly if the small lens is displaced.
Topics: Anterior Chamber; Child; Family; Female; Humans; Lens, Crystalline; Medical Records; Myopia; Visual Acuity
PubMed: 12366350
DOI: 10.1111/j.1444-0938.2002.tb03085.x -
Philosophical Transactions of the Royal... Apr 2011Cataract is a visible opacity in the lens substance, which, when located on the visual axis, leads to visual loss. Age-related cataract is a cause of blindness on a... (Review)
Review
Cataract is a visible opacity in the lens substance, which, when located on the visual axis, leads to visual loss. Age-related cataract is a cause of blindness on a global scale involving genetic and environmental influences. With ageing, lens proteins undergo non-enzymatic, post-translational modification and the accumulation of fluorescent chromophores, increasing susceptibility to oxidation and cross-linking and increased light-scatter. Because the human lens grows throughout life, the lens core is exposed for a longer period to such influences and the risk of oxidative damage increases in the fourth decade when a barrier to the transport of glutathione forms around the lens nucleus. Consequently, as the lens ages, its transparency falls and the nucleus becomes more rigid, resisting the change in shape necessary for accommodation. This is the basis of presbyopia. In some individuals, the steady accumulation of chromophores and complex, insoluble crystallin aggregates in the lens nucleus leads to the formation of a brown nuclear cataract. The process is homogeneous and the affected lens fibres retain their gross morphology. Cortical opacities are due to changes in membrane permeability and enzyme function and shear-stress damage to lens fibres with continued accommodative effort. Unlike nuclear cataract, progression is intermittent, stepwise and non-uniform.
Topics: Aging; Cataract; Crystallins; Free Radical Scavengers; Humans; Lens, Crystalline; Models, Biological; Optical Phenomena; Oxidative Stress; Presbyopia; Protein Processing, Post-Translational
PubMed: 21402586
DOI: 10.1098/rstb.2010.0300 -
Experimental Eye Research Aug 2021Immune cells, both tissue resident immune cells and those immune cells recruited in response to wounding or degenerative conditions, are essential to both the... (Review)
Review
Immune cells, both tissue resident immune cells and those immune cells recruited in response to wounding or degenerative conditions, are essential to both the maintenance and restoration of homeostasis in most tissues. These cells are typically provided to tissues by their closely associated vasculatures. However, the lens, like many of the tissues in the eye, are considered immune privileged sites because they have no associated vasculature. Such absence of immune cells was thought to protect the lens from inflammatory responses that would bring with them the danger of causing vision impairing opacities. However, it has now been shown, as occurs in other immune privileged sites in the eye, that novel pathways exist by which immune cells come to associate with the lens to protect it, maintain its homeostasis, and function in its regenerative repair. Here we review the discoveries that have revealed there are both innate and adaptive immune system responses to lens, and that, like most other tissues, the lens harbors a population of resident immune cells, which are the sentinels of danger or injury to a tissue. While resident and recruited immune cells are essential elements of lens homeostasis and repair, they also become the agents of disease, particularly as progenitors of pro-fibrogenic myofibroblasts. There still remains much to learn about the function of lens-associated immune cells in protection, repair and disease, the knowledge of which will provide new tools for maintaining the core functions of the lens in the visual system.
Topics: Animals; Epithelial Cells; Eye Injuries; Fibrosis; Humans; Immunity, Cellular; Lens, Crystalline; Wound Healing
PubMed: 34126081
DOI: 10.1016/j.exer.2021.108664 -
Experimental Eye Research Mar 2017Fiber cells of the ocular lens are arranged in a series of concentric shells. New growth shells are added continuously to the lens surface and, as a consequence, the... (Review)
Review
Fiber cells of the ocular lens are arranged in a series of concentric shells. New growth shells are added continuously to the lens surface and, as a consequence, the preexisting shells are buried. To focus light, the refractive index of the lens cytoplasm must exceed that of the surrounding aqueous and vitreous humors, and to that end, lens cells synthesize high concentrations of soluble proteins, the crystallins. To correct for spherical aberration, it is necessary that the crystallin concentration varies from shell-to-shell, such that cellular protein content is greatest in the center of the lens. The radial variation in protein content underlies the critical gradient index (GRIN) structure of the lens. Only the outermost shells of lens fibers contain the cellular machinery necessary for protein synthesis. It is likely, therefore, that the GRIN (which spans the synthetically inactive, organelle-free zone of the lens) does not result from increased levels of protein synthesis in the core of the lens but is instead generated through loss of volume by inner fiber cells. Because volume is lost primarily in the form of cell water, the residual proteins in the central lens fibers can be concentrated to levels of >500 mg/ml. In this short review, we describe the process of fiber cell compaction, its relationship to lens growth and GRIN formation, and offer some thoughts on the likely nature of the underlying mechanism.
Topics: Accommodation, Ocular; Animals; Cell Shape; Crystallins; Humans; Lens, Crystalline; Refraction, Ocular
PubMed: 26992780
DOI: 10.1016/j.exer.2016.03.009 -
The British Journal of Ophthalmology May 1959
Topics: Eye Abnormalities; Humans; Lens, Crystalline
PubMed: 13651569
DOI: 10.1136/bjo.43.5.314 -
Experimental Eye Research Mar 2017The eye lens is a transparent and avascular organ in the front of the eye that is responsible for focusing light onto the retina in order to transmit a clear image. A... (Review)
Review
The eye lens is a transparent and avascular organ in the front of the eye that is responsible for focusing light onto the retina in order to transmit a clear image. A monolayer of epithelial cells covers the anterior hemisphere of the lens, and the bulk of the lens is made up of elongated and differentiated fiber cells. Lens fiber cells are very long and thin cells that are supported by sophisticated cytoskeletal networks, including actin filaments at cell junctions and the spectrin-actin network of the membrane skeleton. In this review, we highlight the proteins that regulate diverse actin filament networks in the lens and discuss how these actin cytoskeletal structures assemble and function in epithelial and fiber cells. We then discuss methods that have been used to study actin in the lens and unanswered questions that can be addressed with novel techniques.
Topics: Actin Cytoskeleton; Animals; Cell Differentiation; Epithelial Cells; Humans; Lens, Crystalline; Microfilament Proteins
PubMed: 26971460
DOI: 10.1016/j.exer.2016.03.005