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A transparent biconvex structure located in a capsule behind the pupil through which light travels to the retina.
Crystalline transparent structure that, along with the cornea, refracts light to be focused on the retina.
Its function is to focus light onto the retina, and absorb dangerous wavelengths of light.
Can change shape and thereby alter the focal distance of the eye so that it can focus on objects at various distances.
Change in the shape of the lens is refrred to as accommodation.
It allows a sharp object of interest to be formed on the retina
Initially clear and over time becomes yellow from light absorption and oxidative stress.
The lens is flatter anteriorly and its refractive power is approximately 18 dioptres, roughly one-third of the eye’s total power.
Is part of the anterior segment of the eye, and anterior to the lens is the iris, which regulates the amount of light entering into the eye.
It is suspended in place by the suspensory ligament of the lens, a ring of fibrous tissue that attaches to the lens at its equator and connects it to the ciliary body.
Posterior to it is the vitreous body, which, along with the aqueous humor on the anterior surface, bathes the lens.
The anterior surface is less curved than the posterior surface.
The lens is typically about 10 mm in diameter and has an axial length of about 4 mm.
The size and shape can change due to accommodation.
The lens grows throughout life.
Has three main parts: the lens capsule, the lens epithelium, and the lens fibers.
The lens capsule forms the outermost layer of the lens and it has a smooth, transparent basement membrane that completely surrounds the lens.
The capsule is elastic and is composed of collagen which is synthesized by the lens epithelium.
The lens capsule’s main components are Type IV collagen and sulfated glycosaminoglycans.
The elasticity of the lens capsule causes it 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 lens capsule varies from 2-28 micrometres in thickness.
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.
The lens epithelium has cuboidal epithelium, which regulates most of the functions of the lens.
Ions, nutrients, and liquid enter the lens from the aqueous humor.
The presence of Na+/K+ ATPase pumps in the lens epithelial cells maintain appropriate lens osmolarity and volume.
Cells of the lens epithelium are progenitors for new lens fibers.
The bulk of the lens is comprised of lens fibers which are transparent thin cells with diameters between 4-7 µm in length up to 12 mm long, and stretching from the posterior to anterior poles arranged in tightly packed concentric layers.
The lens fibers are linked together with junctions in interdigitations, in layers refer to as laminae.
The lens is split into regions depending on the age, moving outwards from the central oldest layer, the embryonic nucleus, the fetal nucleus, the adult nucleus, and the outer cortex.
The newest lens fibers, generated from the lens epithelium, are added to the outer cortex.
The lens is flexible and its curvature is controlled by ciliary muscles.
Ciliary muscles through the zonules change the curvature of the lens, allowing the eye to focus.
The process of focusing is called accommodation and at short focal distance the ciliary muscle contracts, zonule fibers loosen, and the lens thickens, resulting in a rounder shape and thus high refractive power.
When the eye focuses on a more distant object the ciliary muscle relax and zonule tension increases resulting in flattening of the lens and an increase in the focal distance.
The refractive index enhances the optical power of the lens from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens.
Water soluble proteins, crystallins, comprise 90% of lens protein.
Crystallins α-, β-, and γ, are soluble, high molecular weight tightly packed in lens fibers and increase index of refraction while maintaining transparency.
Lens transparency is maintained because of the absence of light-scattering organelles in the mature lens fibers, such as nucleus, endoplasmic reticulum, and mitochondria.
Lens fibers maintain precise shape and packing preserving transparency.
Derived from the surface ectoderm beginning at the 4 mm embryonic stage, while the remainder of the eye mostly originates from the neural ectoderm.
Continues to grow after birth, with the new secondary fibers being added as outer layers.
New lens fibers are generated from the cells of the lens epithelium in the germinative zone.
Epithelial cells of the lens elongate, lose contact with the capsule, synthesize crystallin, and then lose their nuclei and become mature lens fibers.
Addition of secondary lens fibers results in the lens growing to a more ellipsoid shape, until about age 20 when the lens grows rounder.
To maintain growth and transparency the lens requires nutrition for its metabolic activities
During development, the lens is surrounded and nourished by a rete of vessels, the tunica vasculosa lentis, which is derived from the hyaloid artery.
The hyaloid artery and its vasculature atrophy and disappear by birth.
Cloquet’s canal marks the former location of the hyaloid artery.
Following regression of the hyaloid artery, the lens receives all its nourishment from the aqueous humor.
Nutrients and waste fluids diffuse from the anterior and posterior poles and out of the equatorial regions, with the process maintained by the Na+/K+ ATPase pumps located in the equatorially cells of the lens epithelium.
Consumption of oxygen by the lens is very little as it lacks aerobic respiration, and glucose the primary energy source is metabolized by anaerobic respiration.
Approximately 80% of glucose is derived from anaerobic respiration and the remaining fraction of glucose is shunted primarily down the pentose phosphate pathway.
Lens opacities are ref2242ed to a cataracts, and they develop with aging and become proressively opaque.
As cataracts become progressive light is blocked anss vision becomes impaired.
Cataracts may be congenital or result from lens injury and is associated with diabetes.
With age presbyopia the loss of accommodation occurs and the eye is unable to focus on nearby objects occurs.
Age-related changes in the hardness, shape, and size of the lens are associated with prebyopia.
Dsplacement of the lens from its normal position is referred to as ectopic lentis.
Absence of the lens as the result of surgery or injury, or congenitally is referred to as aphakia.
Age-related change in the density of the lens nucleus is reffered to as nuclear sclerosis.