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Lens (anatomy)
Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens.
Schematic diagram of the human eye.
Latin lens crystallina
Gray's subject #226 1019
MeSH Crystalline+lens
Dorlands/Elsevier l_06/12483326

The lens is a transparent, biconvex (lentil-shaped) structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. the Peripheral organs of the Special senses the organs of Taste ( Peripheral gustatory or Medical Subject Headings ( MeSH) is a huge Controlled vocabulary (or metadata system for the purpose of indexing journal articles and books Elsevier, the world's largest Publisher of Medical and Scientific literature, forms part of the Reed Elsevier group A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber. Refraction is the change in direction of a Wave due to a change in its Speed. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 In Geometrical optics, a focus, also called an image point, is the point where Light rays originating from a point on the object converge. The vertebrate retina is a light sensitive part inside the inner layer of the Eye. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina. The focal length of an optical system is a measure of how strongly it converges (focuses or diverges (diffuses Light. In Optics, a real image is a representation of an actual object (source formed by rays of Light passing through the Image. This adjustment of the lens is known as accommodation (see also Accommodation, below). Accommodation is the process by which the:eye increases Optical power to maintain a clear image ( focus) on an object as it draws near the eye It is similar to the focusing of a photographic camera via movement of its lenses. A photographic lens (also known as objective lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with

The lens is also known as the aquula (Latin, a little stream, dim. of aqua, water) or crystalline lens. In humans, the refractive power of the lens in its natural environment is approximately 18 dioptres, roughly one-third of the eye's total power. Optical power ( dioptric power or refractive power) is the degree to which a lens or Mirror converges or diverges light A dioptre, or diopter, is a Unit of measurement of the Optical power of a lens or curved Mirror, which is equal to the reciprocal


Contents

Position, size, and shape

The lens is located in the anterior segment of the eye. The anterior segment is the front third of the Eye that includes the structures in front of the Vitreous humour: the Cornea, iris, Ciliary Anterior to the lens is the iris, which regulates the amount of light entering the eye. The lens is suspended in place by the zonular fibers, which attach to the lens near its equatorial line and connect the lens to the ciliary body. The zonule of Zinn ( Zinn's membrane, ciliary zonule) is a ring of fibrous strands connecting the Ciliary body with the crystalline lens of the The ciliary body is the circumferential tissue inside the Eye composed of the Ciliary muscle and Ciliary processes. Posterior to the lens is the vitreous body. The lens has an ellipsoid, biconvex shape. In the adult, the lens is typically 10 mm in diameter and has an axial length of 4 mm, though it is important to note that the size and shape can change due to accommodation and because the lens continues to grow throughout a person’s lifetime. [1]

Lens Structure and Function

The lens is comprised of three main parts: the lens capsule, the lens epithelium, and the lens fibers. The lens capsule is a component of the Eye. It is a clear membrane-like structure that is quite elastic a quality that keeps it under constant tension The lens capsule forms the outermost layer of the lens and the lens fibers form the bulk of the interior of the lens. The cells of the lens epithelium, located between the lens capsule and the outermost layer of lens fibers, are found only on the anterior side of the lens.

Lens Capsule

The lens capsule is a smooth, transparent basement membrane that completely surrounds the lens. The basement membrane is a structure that supports overlying Epithelial or Endothelial cells. It is synthesized by the lens epithelium and its main components are Type IV collagen and sulfated glycosaminoglycans (GAGs). Type-IV collagen is a type of Collagen found primarily in the Basal lamina. Glycosaminoglycans (GAGs or mucopolysaccharides are long unbranched Polysaccharides consisting of a repeating Disaccharide unit [2] The capsule is very elastic and so causes the lens to assume a more globular shape when not under the tension of the zonular fibers, which connect the lens capsule to the ciliary body. The zonule of Zinn ( Zinn's membrane, ciliary zonule) is a ring of fibrous strands connecting the Ciliary body with the crystalline lens of the The ciliary body is the circumferential tissue inside the Eye composed of the Ciliary muscle and Ciliary processes. The capsule varies from 2-28 microns in thickness, being thickest near the equator and thinnest near the posterior pole. [3]

Lens Epithelium

The lens epithelium, located in the anterior portion of the lens between the lens capsule and the lens fibers, is a simple cuboidal epithelium. Simple cuboidal epithelia are Epithelial cells with a cuboidal shape arranged in a single layer [4] The cells of the lens epithelium regulate most of the homeostatic functions of the lens. Homeostasis (from Greek: ὅμος hómos, "equal" and ιστημι istēmi, "to stand" lit [5] As ions, nutrients, and liquid enter the lens from the aqueous humor, Na+/K+ ATPase pumps in the lens epithelial cells pump ions out of the lens to maintain appropriate lens osmolarity and volume, with equatorially positioned lens epithelium cells contributing most to this current. The aqueous humor is a thick watery substance that is between the lens and the cornea Osmolarity is a measure of the osmoles of solute per Liter of solution while the osmolality is a measure of the osmoles of Solute per Kilogram The activity of the Na+/K+ ATPases keeps water and current flowing through the lens from the poles and exiting through the equatorial regions.

The cells of the lens epithelium also serve as the progenitors for new lens fibers.

Lens fibers

The lens fibers form the bulk of the lens. They are long, thin, transparent cells, with diameters typically between 4-7 microns and lengths of up to 12 mm long. [6] The lens fibers stretch lengthwise from the posterior to the anterior poles and are arranged in concentric layers rather like the layers of an onion. These tightly packed layers of lens fibers are referred to as laminae. The lens fibers are linked together via gap junctions and interdigitations of the cells that resemble “ball and socket” forms. A gap junction or nexus is a specialized Intercellular connection between certain animal cell -types

The lens is split into regions depending on the age of the lens fibers of a particular layer. Moving outwards from the central, oldest layer, the lens is split into an embryonic nucleus, the fetal nucleus, the adult nucleus, and the outer cortex. New lens fibers, generated from the lens epithelium, are added to the outer cortex. Mature lens fibers have no organelles or nuclei. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed

Accommodation: changing the power of the lens

An image that is partially in focus, but mostly out of focus in varying degrees.
An image that is partially in focus, but mostly out of focus in varying degrees.

The lens is flexible and its curvature is controlled by ciliary muscles through the zonules. The Ciliary Muscle is a muscle in the eye that controls the eye's accommodation for viewing objects at varying distances The zonule of Zinn ( Zinn's membrane, ciliary zonule) is a ring of fibrous strands connecting the Ciliary body with the crystalline lens of the By changing the curvature of the lens, one can focus the eye on objects at different distances from it. This process is called accommodation. Accommodation is the process by which the:eye increases Optical power to maintain a clear image ( focus) on an object as it draws near the eye At short focal distance the ciliary muscles contract, zonule fibers loosen, and the lens thickens, resulting in a rounder shape and thus high refractive power. Changing focus to an object at a distance requires the stretching of the lens by the ciliary muscles, which flattens the lens and thus increases the focal distance. In Geometrical optics, a focus, also called an image point, is the point where Light rays originating from a point on the object converge.

The refractive index of the lens varies from approximately 1. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium 406 in the central layers down to 1. 386 in less dense cortex of the lens[7]. This index gradient enhances the optical power of the lens. Gradient-index optics is the branch of Optics covering optical effects produced by a gradual variation of the Refractive index of a material Optical power ( dioptric power or refractive power) is the degree to which a lens or Mirror converges or diverges light

Aquatic animals must rely entirely on their lens for both focusing and to provide almost the entire refractive power of the eye as the water-cornea interface does not have a large enough difference in indices of refraction to provide significant refractive power. The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber. As such, lenses in aquatic eyes tend to be much rounder and harder.

Crystallins and Transparency

Crystallins are water-soluble proteins that comprise over 90% of the protein within the lens. In Biology, a crystallin is a water-soluble structural Protein found in the lens of the Eye, accounting for the transparency of the structure Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl [8] The three main crystallin types found in the eye are α-, β-, and γ-crystallins. In Biology, a crystallin is a water-soluble structural Protein found in the lens of the Eye, accounting for the transparency of the structure Crystallins tend to form soluble, high-molecular weight aggregates that pack tightly in lens fibers, thus increasing the index of refraction of the lens while maintaining its transparency. In Biology, a crystallin is a water-soluble structural Protein found in the lens of the Eye, accounting for the transparency of the structure β and γ crystallins are found primarily in the lens, while subunits of α -crystallin have been isolated from other parts of the eye and the body. α-crystallin proteins belong to a larger superfamily of molecular chaperone proteins, and so it is believed that the crystallin proteins were evolutionarily recruited from chaperone proteins for optical purposes. This article is about the protein For other uses see Chaperone, a disambiguation page This article is about the protein For other uses see Chaperone, a disambiguation page [9] The chaperone functions of α -crystallin may also help maintain the lens proteins, which must last a human for his/her entire lifetime. [10]

Another important factor in maintaining the transparency of the lens is the absence of light-scattering organelles such as the nucleus, endoplasmic reticulum, and mitochondria within the mature lens fibers. In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. Lens fibers also have a very extensive cytoskeleton that maintains the precise shape and packing of the lens fibers; disruptions/mutations in certain cytoskeletal elements can lead to the loss of transparency. cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. [11]

Development and Growth

Development of the human lens begins at the 4 mm embryonic stage. Human development is the process of Growing to maturity In biological terms this entails growth from a one-celled Zygote to an adult Human being Unlike the rest of the eye, which is derived mostly from the neural ectoderm, the lens is derived from the surface ectoderm. Neuroectoderm (or neural ectoderm) is the term for Ectoderm which receives inhibitory signals from proteins such as noggin, which leads to the development The surface ectoderm (or external ectoderm forms the following structures Skin (only epidermisas dermis is derived from mesoderm (along with The first stage of lens differentiation takes place when the optic vesicle, which is formed from outpocketings in the neural ectoderm, comes in proximity to the surface ectoderm. The Eyes begin to develop as a pair of Diverticula from the lateral aspects of the Forebrain. The optic vesicle induces nearby surface ectoderm to form the lens placode. The Lens placode is a thickened portion of ectoderm which serves as the precursor to the lens. At the 4 mm stage, the lens placode is a single monolayer of columnar cells. Columnar epithelia are epithelial cells whose heights are at least four times their width

As development progresses, the lens placode begins to deepen and invaginate. The Lens placode is a thickened portion of ectoderm which serves as the precursor to the lens. As the placode continues to deepen, the opening to the surface ectoderm constricts and the lens cells forms a structure known as the lens vesicle. The surface ectoderm (or external ectoderm forms the following structures Skin (only epidermisas dermis is derived from mesoderm (along with By the 10 mm stage, the lens vesicle has completely separated from the surface ectoderm. The surface ectoderm (or external ectoderm forms the following structures Skin (only epidermisas dermis is derived from mesoderm (along with

After the 10mm stage, signals from the developing neural retina induces the cells closest to the posterior end of the lens vesicle begin to elongate toward the anterior end of the vesicle. The vertebrate retina is a light sensitive part inside the inner layer of the Eye. [12] These signals also induce the synthesis of crystallins. In Biology, a crystallin is a water-soluble structural Protein found in the lens of the Eye, accounting for the transparency of the structure [13] These elongating cells eventually fill in the lumen of the vesicle to form the primary fibers, which become the embryonic nucleus in the mature lens. The cells of the anterior portion of the lens vesicle give rise to the lens epithelium.

Additional secondary fibers are derived from lens epithelial cells located toward the equatorial region of the lens. These cells lengthen anteriorly and posteriorly to encircle the primary fibers. The new fibers grow longer than those of the primary layer, but as the lens gets larger, the ends of the newer fibers cannot reach the posterior or anterior poles of the lens. The lens fibers that do not reach the poles form tight, interdigitating seams with neighboring fibers. These seams are readily visible and are termed sutures. The suture patterns become more complex as more layers of lens fibers are added to the outer portion of the lens.

The lens continues to grow after birth, with the new secondary fibers being added as outer layers. New lens fibers are generated from the equatorial cells of the lens epithelium, in a region referred to as the germinative zone. The lens epithelial cells elongate, lose contact with the capsule and epithelium, synthesize crystallin, and then finally lose their organelles as they become mature lens fibers. In Biology, a crystallin is a water-soluble structural Protein found in the lens of the Eye, accounting for the transparency of the structure In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed [14] From development through early adulthood, the addition of secondary lens fibers results in the lens growing more ellipsoid in shape; after about age 20, however, the lens grows rounder with time. [15]

Nourishment

The lens is metabolically active and requires nourishment in order to maintain its growth and transparency. Compared to other tissues in the eye, however, the lens has considerably low energy demands. [16]

By nine weeks into human development, the lens is surrounded and nourished by a net of vessels, the tunica vasculosa lentis, which is derived from the hyaloid artery. The tunica vasculosa lentis is an extensive capillary network spreading over the posterior and lateral surfaces of the lens of the Eye. The hyaloid artery is a branch of the Ophthalmic artery, which is itself a branch of the Internal carotid artery. [17] Beginning in the fourth month of development, the hyaloid artery and its related vasculature begin to atrophy and completely disappear by birth. The hyaloid artery is a branch of the Ophthalmic artery, which is itself a branch of the Internal carotid artery. [18] In the postnatal eye, Cloquet’s canal marks the former location of the hyaloid artery. The hyaloid artery is a branch of the Ophthalmic artery, which is itself a branch of the Internal carotid artery.

After regression of the hyaloid artery, the lens receives all its nourishment from the aqueous humor. The hyaloid artery is a branch of the Ophthalmic artery, which is itself a branch of the Internal carotid artery. The aqueous humor is a thick watery substance that is between the lens and the cornea Nutrients diffuse in and waste diffuses out through a constant flow of fluid from the anterior/posterior poles of the lens and out of the equatorial regions, a dynamic that is maintained by the Na+/K+ ATPase pumps located in the equatorially positioned cells of the lens epithelium. [19]

Glucose is the primary energy source for the lens. Glucose (Glc a Monosaccharide (or simple Sugar) also known as grape sugar, is an important Carbohydrate in Biology. As mature lens fibers do not have mitochondria, approximately 80% of the glucose is metabolized via anaerobic respiration. In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. See also Fermentation (biochemistry Anaerobic respiration (anaerobiosis refers to the Oxidation of molecules in the absence of Oxygen to produce [20] The remaining fraction of glucose is shunted primarily down the pentose phosphate pathway. The pentose phosphate pathway (also called Phosphogluconate Pathway or HexoseMonophosphate Shunt shunt is a process that serves to generate NADPH and the synthesis of pentose [21] The lack of aerobic respiration means that the lens consumes very little oxygen as well. Cellular respiration is the set of the metabolic reactions and processes that take place in Organisms cells to convert biochemical energy from [22]

Diseases and Disorders


Additional images

Cataract in Human Eye- Magnified view seen on examination with a slit lamp

MRI scan of human eye showing lens.

Interior of anterior chamber of eye.

The crystalline lens, hardened and divided.

Section through the margin of the lens, showing the transition of the epithelium into the lens fibers.

References

  1. ^ John Forrester, Andrew Dick, Paul McMenamin, William Lee (1996). The Eye: Basic Sciences in Practice. London: W. B. Saunders Company Ltd. p. 28 ISBN 0-7020-1790-6
  2. ^ The Eye: Basic Sciences in Practice, p. 28, ISBN 0-7020-1790-6
  3. ^ The Eye: Basic Sciences in Practice, p. 28, ISBN 0-7020-1790-6
  4. ^ The Eye: Basic Sciences in Practice, p. 28, ISBN 0-7020-1790-6
  5. ^ O. Candia (2004). Electrolyte and fluid transport across corneal, conjunctival and lens epithelia. Experimental Eye Research 78 (3): 527-535.
  6. ^ The Eye: Basic Sciences in Practice, p. 28, ISBN 0-7020-1790-6
  7. ^ Hecht, Eugene. Optics, 2nd ed. (1987), Addison Wesley, ISBN 0-201-11609-X. p. 178.
  8. ^ W. Hoehenwarter, J. Klose and P. R. Jungblut (2006). Eye lens proteomics. Amino Acids 30(4): 369-389.
  9. ^ U. Andley (2006). Crystallins in the eye: function and pathology. Progress in Retinal and Eye Research 26 (1): 78-98.
  10. ^ U. Andley (2006). Crystallins in the eye: function and pathology. Progress in Retinal and Eye Research 26 (1): 78-98.
  11. ^ H Bloemendal, W. de Jong, R Jaenicke, NH Lubsen, C Slingsby and A Tardieu (2004). Aging and vision: structure, stability, and function of lens crystallins. Progress in Biophysics and Molecular Biology 86 (3): 407-485.
  12. ^ The Eye: Basic Sciences in Practice, p. 102, ISBN 0-7020-1790-6
  13. ^ The Eye: Basic Sciences in Practice, p. 102, ISBN 0-7020-1790-6
  14. ^ U. Andley (2006). Crystallins in the eye: function and pathology. Progress in Retinal and Eye Research 26 (1): 78-98.
  15. ^ The Eye: Basic Sciences in Practice, p. 28, ISBN 0-7020-1790-6
  16. ^ Whikehart, David R. (2003). Biochemistry of the Eye, 2nd ed. 2003. Philadelphia: Butterworth Heinemann, p. 107-8 ISBN 0-7506-7152-1
  17. ^ The Eye: Basic Sciences in Practice, p. 102, ISBN 0-7020-1790-6
  18. ^ The Eye: Basic Sciences in Practice, p. 104, ISBN 0-7020-1790-6
  19. ^ O. Candia (2004). Electrolyte and fluid transport across corneal, conjunctival and lens epithelia. Experimental Eye Research 78 (3): 527-535.
  20. ^ Biochemistry of the Eye, 2nd ed, p. 107-8, ISBN 0-7506-7152-1
  21. ^ Biochemistry of the Eye, 2nd ed, p. 107-8, ISBN 0-7506-7152-1
  22. ^ Biochemistry of the Eye, 2nd ed, p. 107-8, ISBN 0-7506-7152-1

See also

External links

An intraocular lens (IOL is an implanted lens in the Eye, usually replacing the existing crystalline lens because it has been clouded over by a Cataract The iris consists of Pigmented Fibrovascular tissue known as a stroma. The lens capsule is a component of the Eye. It is a clear membrane-like structure that is quite elastic a quality that keeps it under constant tension Melatonin is a naturally occurring Hormone found in most animals including humans and some other living organisms including Algae. Phacoemulsification refers to modern Cataract surgery in which the Eye 's internal lens is emulsified with an ultrasonic handpiece and aspirated In Psychology, visual perception is the ability to interpret information from Visible light reaching the Eyes The resulting Perception is also The zonule of Zinn ( Zinn's membrane, ciliary zonule) is a ring of fibrous strands connecting the Ciliary body with the crystalline lens of the For similarly-named academic institutions see Education in Boston MA.
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