The lens of the eye: role and Anatomy
The lens is a natural optical lens of the eye that is involved in focusing light on the retina and accommodation (focus in near vision). The young lens is flexible, transparent, and has a form of elliptical edge in Cup. It is located in the posterior Chamber of the eye, in front of the vitreous cavity, and at the back of the iris. The lens is suspended from the ring formed by the ciliary body through the cristallinienne zonule, ligament consists of fibers which fit to the equator of the lens. Cataract is the result of the loss of transparency of the lens for visible light.
Role of the lens
The role of the lens is twofold: it allows the light captured by the eye and emitted from distant objects to converge the retina to form a focused, sharp image. When the ciliary muscle contracts, the zonular fibers relax and crystalline, naturally elastic, adopts a more convex shape. This increases the optical power of the lens, and allows accommodation: the rays emitted by objects close to the eye being so focused on the retina.
Dimensions of the lens
The overall shape of the lens is a biconvex lens, the rear is more arched than the front.
The center of the front corresponds to the "anterior pole', the center of the rear corresponds to the"posterior pole. The equator of the lens is the largest circumference located, between the anterior and posterior poles. The thickness of the lens is the distance between the poles. The thickness of the lens to the rest (without accommodation) varies between 3.5 and 5 mm. The thickness of the lens increases the lifetime of about 0.02 mm per year (1). The earlier (at the pole) radius of curvature varies between 8 and 14 mm, the rear RADIUS varies between 5 and 8 mm. The diameter of the lens is the distance between the edge of nasal and temporal (at the Equator) edge of the lens. This diameter varies between 6.5 mm at birth, and 9 mm in adolescence. The curvatures of the crystalline gradually changes with time (2).
The optical power of the lens depends on the curvature of its surface, its refractive index and thickness. The curvature of its surface varies depending on the State of the accommodation: it is of approximately 20 diopters out accommodation. The lens is a lens that has a special feature: its refractive index is not uniform (the lens is "anisotropic"). The value of the refractive index of the lens decreases from the Center to the Equator and the poles. This variation is due to (gradient) changes of the concentration of the protein cristallinienne, and is in part the product of the mechanisms that govern the growth of the crystalline. A page is devoted to the modelling of the Optical power of the lens. The
Accommodation results in an increase of the power of the lens, as its anterior surface becomes more convex. The maximum amplitude of accommodation is 14 diopters, between the ages of 8 and 12. Then accommodation decreased with age, to become zero around the age of 55, as the lens becomes more distorted.
It is interesting to understand the development of the lens, because it allows to understand the particular structure of the fabric. The lens is a structure of epithelial origin (the cell layer concerned during embryogenesis of the Crystal is the ectoderm, which also gives the skin, hair, etc.). It is formed during Embryological Development by a layer of cells that is invagine (widening closes on itself), and then a cristallinienne sphere initially hollow form. Epithelial cells at the level of the posterior differ and are getting longer, forming the first cristalliniennes fibres. Their full growth gradually the empty central cristallinienne sphere, and the lens fibers then come in contact with the previous epithelial cells. These will ensure the further growth of the crystalline lens. There is indeed an area just in front of Ecuador, under the cristallinienne capsule, where germ cells multiply and grow stretching forward and backward, covering most ancient fibers.
This particular development to understand the anatomy and structure of the adult lens, which is made up of concentric layers: the spatial arrangement of the cristalliniennes fibres is particular, think of the slats of an onion. Each slat is made up of a layer of epithelial fibers. The arrangement into "strips" regular spacing and without preferred orientation is interesting on the optical plan (she sassuretransparence of the lens) and Biomechanics (necessary deformation during the accommodation properties).
We're understandable as this cristallinien development abnormality, occurring during pregnancy can cause congenital cataract, which appears at birth or shortly after. Rubella is a classic cause of cataracts: contracted before the 7th week of pregnancy, when the capsule is permeable to the rubella virus, it hinders the normal process of cristallinien development. The aging of the lens occurs by the formation of protein aggregates that disorganize the regular arrangement of proteins of cristalliniennes cells and cause an increase of the light diffusion.
The layers of the lens
The main layers that can be individualized are:
-the embryonic nucleus of the lens: it corresponds to the primary, developed fibers from the posterior epithelium during embryonic development.
-The fetal nucleus: it comes from the fibers formed by the previous cell divisions during fetal life.
-The adult nucleus: it is between birth and adolescence.
-The cortex is formed of cristalliniennes fibers formed after adolescence. These structures do not have the same refractive index, which is all the more important that the structure is central.
We can observe these layers of the lens to the slit lamp examination, using a thin line of light, and a slightly oblique lighting.
The sutures of the lens
They are born from the development of the fibers of the lens, and their meeting to the poles (6). The junction between these fibrous cells forms a suture. The previous stitches is logically formed by apical fibers portions, while the posterior suture is formed by the basal portions of the same fibers.
The fibers formed during Embryological Development meet by forming three branches, drawing a shape in Y. The previous stitches form a Y in the place, then the posterior suture form a Y in reverse.
During the growth of the lens, this arrangement changes, the sutures using a form and a more variable orientation, and the number of branch from 3 to more than a dozen (3).
Capsule of the lens
The capsule of the lens is a transparent envelope that surrounds the lens. Its thickness is variable depending on the location: it is thinner at the poles and at the equator, and thicker in an annular region surrounding the anterior pole (4).
The lens capsule is histologically a basal membrane, and its anterior surface thickens to the over time, becoming the basal membrane the thickest of the body. It is made up of collagen, devoid of elastic fibers, but has properties of elasticity because of the lamellar arrangement of the crystalline lens fibers.
Without the influence of the zonule fibers, which put it into voltage, the capsule of the lens tends to adopt a circular shape. The zonule fibers fit on the capsule, anterior and posterior, the equator to the pole. This property says that the accommodative mechanism is based on a relaxation of the zonule, allowing the anterior capsule to adopt a more spherical shape and thus increase the optical power of the lens (5).
The capsule of the lens also plays a metabolic role: it forms a barrier against some macromolecules such as albumin and hemoglobin.
The loss of flexibility of the capsule of the lens with age greatly explains the appearance of presbyopia: the contraction of the ciliary muscle causing a loosening of the zonule, but the capsule deforms less, in all case insufficient to adequately increase the power of the lens.
During cataract surgery, must achieve a circular cutout of the anterior capsule, called capsulorexhis. This cut allows access to the cortex and nucleus of the lens, to suck them. The rest of the capsule is preserved to serve as support for the implant intra ocular (artificial lens).
The lens epithelium
It is only located in the anterior capsule, as the posterior epithelium is "consumed" during Embryological Development to form the posterior fibres of crystalline. This anterior epithelium secretes the anterior capsule, which explains the thickening of it over time.
The growth of the crystalline in existence is ensured by cristallinien epithelium germ cells, which are located in the anterior pre equatorial region. These cells lengthen their basal part migrating to the posterior pole, and their apical (top of the cell) to the anterior pole. This process is so accomplished on both sides of the equator of the lens, and the newer cells to insinuate between the capsule and the oldest cells. At the end of this process of growth, the cells lose their nucleus and organelles: then they become the cristalliniennes fibers. The latest fibers are the most superficial.
The lens fibers
The production of the lens fibers continues over the existence. The lens consists of a concentric stack of adjacent fibers. These fibers have a hexagonal section, and an overall shape of crescent tapered (6).
The inside of these fibers (cytoplasm) consists of a high concentration of proteins called "crystalline proteins". The distribution of these proteins and their concentration decreases from the kernel to the superficial cortex, which contributes to the variation of the refractive index of the lens. The formation of protein aggregates causes the space regularity of protein loss and a reduction in transparency of the cristallinien tissue. The recent experimental work suggest that the lanosterol could re - solubilize the protein aggregates and increase the transparency of the lens.
The lens Zonule
The lens zonule (called zonule of Zinn) is a fibrous structure attached to the ciliary body. It is made up of microfibrils grouped Microfiber whose diameter is of the order of a micron: these Microfiber gather itself in fibers with thickness of 50 microns. It forms the suspensory ligament of the lens, and have elastic properties, although the fiber itself are not composed of elastin. The so-called primary zonule fibers fit on cristallinienne capsule in the regions located at the front and at the rear of the equator of the lens. The so-called secondary zonule consists of fibers that connect some fiber between them. The zonule is inserted to the ciliary body, she undergoes the action of the ciliary muscle, which, by contracting, release the front part of the zonule fibers, which relaxes the lens capsule and made this dish.
Transparency of the lens
The lens is a natural lens, which allows to refract the light to the retina. The lens must be transparent. The transparency of the lens is provided by:
-The absence of vessels blood (the lens is avascular)
-The rarity of the intracellular organelles
-The regularity of the arrangement of the fibers of the lens
-The contiguity of the areas which the refractive index varies
To divide, the germ cells need energy; the nutrients necessary for their energy metabolism are brought by aqueous humor, which bathes the side front of the lens. As for any metabolic process, the activity of cells produces free, potentially toxic radicals as able to cause cell damage. The lens is rich in gluthatione, which is an antioxidant molecule synthesized by the cells of the cortex. With the development of the lens, this molecule could disseminate more difficult way to the nucleus, causing an increase in oxidative phenomena which can then lead to the development of a cataract. The lens and aqueous humor are also rich in Ascorbic acid (vitamin C). This anti oxidant participates in cell protection against oxidative mechanisms, and toxicity of ultraviolet radiation: the cornea stop radiation whose wavelength is less than 300 nm; the lens absorbs the UV radiation strip between 300 and 400 nm.
Maintaining the transparency of the lens is based on a particular balance between oxidative phenomena, and anti-oxidizing molecules. Many conditions threaten this balance and can lead to early a cataract onset, they are metabolic (diabetes), physical (trauma, radiation), etc. For example, in the case of diabetes, the increase in the rate of glucose in aqueous humor led to its transformation into sorbitol, which produces an osmotic gradient and excessive moisture in the lens fibers, which swell and cause a loss of transparency of the lens.
Curvature of the lens
The determination of the front and back of the lens curvatures have no easy thing, because these change throughout the accommodation. Due to the continuous growth of the crystalline lens, these data are also likely to vary over time. The study of the curvature of the back is subject to some uncertainty, because we cannot accurately determine the value of the gradient of the refractive index of the lens.
It is more relevant to this type of study way "in vivo", because the lens adopts a more spherical shape once removed (in its natural state, the lens is "turned on", through the zonular fibers: the relaxation of this tension allows the accommodation).
Analysis of the specular reflection issued by different refractive surfaces of the eye (Purnkinje images) to calculate the radius of curvature at the level of the anterior and posterior of the crystalline poles. Brown et al. found an average anterior curvature of 12.4 + / 2.6 mm for the front, and 8.1+/-1.6 mm for the back (7).
The asphericity of the face front of the lens is a hyperbolic (Q < 1), but there is a wide dispersion of the data: Q = - 1.08 + / 9.41 according to Smith et al, the posterior is also aspherical (-0.12 +/-1.74) (8)
It is often set around an average value of 3.6 mm, but increases steadily with time. It varies also in accommodation.
It varies between 1,406 at the Center, and according to Gullstrand 1.386. This variation influences the rate of eye spherical aberration. To optical calculations and models of eyes, we usually choose an iniquitous value of 1.42.
It varies between 8.5 and 10 mm: artificial lens implants have an overall diameter slightly greater than this value (11-12 mm in general). So they put the cristallinienne capsule in tension.
Temporal variations of the cristalliniennes dimensions
The magnification of the lens during life imply a change in its geometry: the anterior radius decreases (increase of camber), and the radius of curvature later also. This should produce an increase in the power of the lens and a myopisation. However, there is no significant refractive change during adulthood: the increase in the thickness of the lens, and a possible gradual change of refractive index seems to compensate for the effect of camber and maintain a relatively constant power crsitallinienne. In case of nuclear cataracts, a mopisation is however commonly observed.
(1) Dubbelman M et al. The thickness of the human lens obtained from corrected Scheimpflug images. Optom live Sci, 2001; 78 (6): 411
(2) Koretz JF et al. Analysis of human crystalline lens curvature as a function of accommodative state and age. Res vis, 1984:24:11 - 41
(3) Hogan MJ et al. Histology of the human eye, Philadelphia, Saunders, 1971, p 638
(4) Sasaki K et al. In vivo observation of the crystalline lens capsule. Ophthalmic Res, 1988; 20 (3): 154
(5) Garner the et al. Changes in equivalent and gradient refractive index of the crystalline lens with accommodation. Opt Vis Sci 1997; 74 (2): 114
(6) Kuszak et al. Electron microscopic observations of the crystalline lens. Microsc Res Tech, 1996; 33:441
(7) Brown N. The changes in lens curvature with age. Exp Eye Res, 1974; 19:175 - 183
(8) Smith G et al. The optical modelling of the human lens. Ophthalmic B.j. Opt, 1991; 11:359 - 69