Lens (anatomy)

The lens ( phakos gr ) of the eye is a crystal clear elastic body both at the front and at the rear - is curved convexly - more here. It is referred to in the art as crystallina lens as a converging lens and focuses the incoming light through the pupil of the eye on the rear side such that on the retina a sharp image can be produced. In ophthalmology, the presence of the natural lens is called a phakic.

Embryology

The lens is of ectodermal origin. It develops from a lens vesicle which consists of a capsule and the underlying immature cells, but otherwise is hollow. Subsequently, the cells of the posterior wall take on an elongated shape and thus becoming lens fibers, which eventually fill the lens vesicle. This process is called primary lens fiber differentiation and leads to the formation of the embryonic lens nucleus.

Construction

The mature lens itself is next to the capsule of the lens cortex and lens nucleus. An epithelium is located just under the anterior capsule of the lens. In the equatorial region of the lens capsule, new cells are formed in a growth zone ( Zona germinativa ) by cell division throughout life. These form in the further course into elongated fibers around. Here they produce and accumulate transparent proteins Crystalline, and then lose their organelles. The newly formed fibers are finally similar to the skins of an onion, the older fibers from the outside. Through this process, the lens cortex increases to the detriment of the lens nucleus with age becoming, and the originally very elastic lens becomes more rigid. To a correspondingly stronger lens curvature for near vision finally is often hardly more capable ( presbyopia ), which requires the wearing of reading glasses.

Although the lens is a very metabolically active organ, it has neither nerves nor blood vessels, but relies entirely on the nutrients, electrolytes and other substances contained in the aqueous humor. Thus the glass-clear transparency of the lens is assured.

The suspension of the lens to the ciliary body via the side radiating to the equator of the lens capsule zonular fibers ( Fibrae zonulares ).

Monofocal and multifocal lenses

Monofocal lenses and multifocal lenses: During evolution, two different lens types emerged in vertebrates. In parallel, developed depending on the particular lens type round or slit-shaped pupils, optimal supplement for the purpose of the specific optical properties of the lens type. For slit-shaped pupils are found only in animals with multifocal lenses, eg in cats, geckos or some snakes. Species with round pupils ( eg, human, dog ) in turn have monofocal lenses.

Multifocal lenses focus light of different wavelengths by different concentric ( annular ) areas. In this way, a sharper image than in eyes with round pupils whose lenses focus incident light onto a single point in the center arises. In a multifocal lens has a round pupil would cover in their contraction entire circular regions of the lens, but that would be necessary for the bundling of certain wavelengths of light. In contrast, in the slot-shaped pupil, the light can always pass through any portion of the concentric ring of the lens, so that an optimum focusing of the different wavelengths is ensured.

Accommodation

For accommodation ( local and remote setting ), the refractive power of the lens - it is in a healthy human eye in the setting for distance vision usually about 19 dpt - by reducing the radius of curvature depends on age be increased to around 33 dpt. This is done by the ciliary body, the muscle ( musculus ciliaris, ciliary ) regulates the degree of curvature of the lens. The contraction of smooth muscle this results in a relaxation of the zonules, thereby the lens to assume a more curved, spherical shape due to the inherent elasticity of the lens fibers and thus focused on close range. For relaxation of the muscle of the train the zonular fibers to a flattening of the lens and thus a setting on distance vision leads. The elasticity of the lens can gets older and eventually leads to presbyopia.

Transparency of the eye lens

The eye lens is composed of cells that ensure the transparency due to various factors without color distortion ( in the healthy eye):

• No organelles and no nucleus
• Regular and tight alignment of the hexagonal in cross-section fibers
• Production of transparent proteins ( Krystalline )
• Low water content
• Practically the same refractive index of the cell membranes, such as the cytoplasm of the cells in the interior of the lens

The transparency of the eye lens is an example of extreme specialization by biological evolution, on the one hand, the cells of the eye lens can live despite the lack of organelles and no nucleus, on the other hand, this lack only allows transparency. In the formation of this particular cell mechanisms of programmed cell death (apoptosis ) seem to play a role.

Disorders and alterations of the lens

A clouding of the lens due to different causes is referred to as cataract or glaucoma. Although this disease is successfully treated in industrial countries in large numbers by surgery and the implantation of intraocular artificial lenses, it is considered as the most common cause of blindness worldwide. Cancers of the lens does not occur.

The absence of a natural or an artificial lens is referred to in medicine as aphakia. The state after the removal or loss of the natural lens and its replacement by an implant called pseudophakic.

Moves the lens from its original position in the anterior chamber or the vitreous of the eye, it is called a lens luxation.

The deviation of the lens surface from a purely spherical shape can lead to a lenticular astigmatism.