Iris and eye color
How was born the eye color?
The iris is the diaphragm of the eye. It gives color to the eyes; (Brown) Brown, green or blue. The iris is one of the components of the uveal tissue of the eye (uvea), which also includes the ciliary body and the choroid. Many questions exist as to the color eyes, and the possibilities of change. It is important to know some of the characteristics of the anatomical support of the color of the eyes: the iris.
The geometry of the iris is a circular opening called perforated disc pupille. The pupil diameter varies from 1 mm (conditions conditions of high brightness, tell photopiques, which cause a constriction of the pupil called myosis) and 9 mm (conditions conditions of low brightness, tell scotopiques, which cause dilatation of the pupil - Mydriasis). The eyes which the pupil is wide and which are taken picture Flash may appear red on the shots. Of course, this color is not related to the color of the iris but the backscatter of the illumination of the back of the eye by a richly vascularized tissue (retina).
The diameter of the iris is close to 12 mm. Its thickness varies, it reaches 1.5 mm at the iris flange, which is the thickest area and located about 1.5 mm from the pupillary edge. The base or root of the iris does in comparison only 0.5 mm thick. The pupillary edge of the iris rests on the anterior side of the Crystalline lens. The iris forms a tablecloth comparable to that of a truncated cone, as the pupillary edge is located in a plane slightly more anterior than its root (due to the vaulted of the Crystalline lens).
The iris represents the boundary between the anterior chamber and the posterior chamber of the eye. Thus, the so-called "anterior chamber" implants are inserted and placed in front of the iris. The so-called "posterior chamber" implants are inserted in the rear of the iris against the Crystalline lens.
Only the front of the iris is accessible to the review, and it is this face that gives color to the eyes.
Histology of the iris
From before backward, there are several layers.
The anterior layer of the iris
This thin layer is an extension of the stromal layer: it is the most superficial, and it is what gives its appearance and color in the iris. It is composed of various cells: fibroblasts (who make collagen), of melanocytes (which contain the pigment). These cells are inserted into a frame of collagen fibrils. Collagen fibers have a rather radiaire orientation. Fibroblasts have a particular form: these cells form many interconnections, which are partly responsible for the fine variations of corneal relief (with an alternation of small ledges and hollows or Crypts), and which are distributed of random material. These structures play also a role in color like the IRIS, because they cause the visible light diffusion, more marked in the short wavelengths (blue). The density of the network formed by the expansions of the fibroblasts varies from one eye to the other.
Under the most superficial layer of fibroblasts, the melanoma cells are located. When clustered, they form small, pigmented ridges comparable to Neavi.
The iris and her sphincter stromal layer
The stromal layer consists of collagen tissue of pigmented and non pigmented cells. At the level of the peripheral flange, the arterial circle of iris is a vascular structure where to leave the arteries of the iris, that have a radiaire trip directed to the pupillary edge.
The sphincter of the iris is a smooth muscle, whose width is about 1mm, and which is responsible for the constricting movements of the iris (reduction of the diameter of the pupil: The sphincter allows the miosis). The sphincter is innervate by parasympathetic fibers.
anterior epithelium and dilator muscle
A The deeper part of the iris is the anterior epithelium, which is composed largely of Myo epithelial cells, whose expansions directed to the Stroma form the iris dilator muscle. Because of their radial orientation, the contraction of this muscle causes a dilation of the iris (sympathetic innervation). Some eye drops stimulate the contraction of these fibers, and allow to dilate the pupil. Some of these eye drops are called "cycloplégiques" eye drops. They are used for the examination of the background, and also to paralyse the accommodation (study of the refraction Cycloplegic, which is often realized during the balance of refractive surgery).
The posterior epithelium
The posterior irien epithelium consists of a single layer of pigmented cells. The pigmentation of these cells decreases on the periphery of the iris, and this layer becomes the epithelial layer of the ciliary body. The pigmented, the deepest layer is the one that can leave its mark on the surface of the lens, when hugs repeated with the anterior side of the lens (we call these hugs of the cervix).
Color of the iris
Before studying the factors that influence the color of the iris (eyes) more specifically, it is important to remember some important information concerning the notion of colorful sensation. The color of a light perceived by the eye (or rather the brain) depends on the wavelengths that composes (nt). The color of an object depends on the light it emits or reflects and who is captured by the eye of one who observes it. The color of an object is in fact influenced by the light that "enlightens".
Under normal conditions, the light that illuminates our terrestrial environment is sunlight, which contains all of the radiations of light visible (the mixture of these radiations causes a feeling of 'white' light). Plucked from an Apple tree, an Apple is green for located in its skin pigments absorb all of the radiation colored solar light which illuminates it... except the radiation seen as green. This absorption is related to the interaction between photons and electrons of the constituent atoms of the pigments concerned and located in the skin of the fruit. Green radiation are reflected in a diffuse way in all directions. If it illuminates the same Apple by a red light, it will appear almost black, because all the incident light (red) will be absorbed by the pigments: it would be the same with a Red Apple illuminated by a green light.
Regardless of the amount of light it absorbs and reflects in a diffuse way, the color of the iris of the eye depends primarily on the type of the light it receives. It is for this reason that the eyes, regardless of their "base" color in natural light, are undergoing changes of tone depending on the light environments (whether natural or artificial, light etc.).
A dark object (piece of coal) absorbs almost all the incident light, while a white object reflects in a diffuse way without preferential absorption (ex: snow). The melanocytes of the human skin absorb ultraviolet and an important part of the visible spectrum. They darken the skin because it, absorbing more light, reflects less.
An object that lets or reflects light by diffusing it (scattering in all directions) can have an effect on the colour perception, if it diffuses the light more or less selectively towards the wavelength. Depending on the size of the particles that diffuse the light to that of the wavelengths considered, some coloured radiation will be more diffused (deviating from their path) than others: the smaller the object is to the length And the less it affects the path of the incident ray and vice versa.
For a light of 'normal' composition (ex: natural light), the color of the iris is influenced by several factors:
– The structure and density of collagen fibers of the anterior stroma and the anterior layer
– Density in melanoma cells (pigmented cells)
– The density of pigments contained in melanoma cells
– The higher the number of melanoma cells, the more pigment they contain, the more brown and dark the iris appears; In fact, the incident light is absorbed by these structures, which reflect it little, and thus appear dark.
– Blue eyes have an anterior layer devoid in pigmented cells : light is not absorbed, and she is thoughtful and broadcast more significantly by structures which the microscopic dimensions make the short wavelengths (blue) are more distributed than the long (known as Rayleigh scattering: Blue is broadcast about 16 times more than red). This type of scattering also explains the color of the sky: the molecules and atoms present in the atmosphere more diffuse radiation of wavelengths blue wavelengths than red. Thus, blue light is more diffused in all directions, toward the ground, as the light of wavelengths higher and (red). When you look up to the sky, in proportion received more bluish radiation, and the sky seems blue. The Sun appears yellow because the rays of light reaching the eye are "depleted" blue (by higher diffusion towards the longer radiation). Clouds are white because they broadcast in a non-preferential manner all visible colors.
– Earlier iriennes structures also preferentially diffuse blue light: In the absence of marked absorption (not or little melanocytes), it is a light more intense and "enriched" in blue (relatively) which returns to the eye of The Observer.
– The green eyes are fitted with medium-pigmented irises, which absorb some of the incident light. The mixture between reflected and diffused light and partial absorption leads to a colorful perception of clear to green hazelnut.
Color variations within the same iris are explained by variations in the scale of enabling structures, fluctuations in density of pigmented cells, etc.
The light that is not absorbed by the earlier iris or diffused layer in reflection goes more in depth is absorbed by pigmented posterior epithelium cells. Albino eyes, whose color is gray to bluish, are devoid of pigmentation not only at the front of the iris (anterior layer) but also to his back. The albino patients are photophobes, because light entering the eye is too important.
Changes in eye color
Many patients express the wish to change forever eye color: in the overwhelming majority of the case, it's patients to brown eyes, wishing to have blue eyes.
Laying of prostheses of iris, in front of the natural iris which is then hidden, to change the color of the eyes in a spectacular way. Implants of artificial iris have been proposed and put (a technique recently proposed and called "Bright ocular" is based on this approach).
The illusion is quite successful, even if short distance (or slit lamp examination), the appearance of the prosthesis greatly differs from the front of the iris. In addition, the diameter of the pupil (artificial since formed by a hole about 4 mm in the center of the prosthesis) is fixed. This reduces the expression in the eyes that appear more frozen, especially in low light conditions.
The comparison between a review of pupillometry performed before and after the placement of implants of iris shows the fixity of the diameter of the artificial pupil of the iris prosthesis:
In a significant number of documented case, this site has Ocxxx_xxx_5163ionne of the irreparable damage (severe glaucoma inducing a legal blindness in some case). Other complications, which seem inevitable after a certain delay, associate chronic Uveitis (friction between the iris and the implant), and the loss of endothelial cells (cells at the posterior face of the cornea).
Unfortunately, he is no reliable, effective and devoid of risk technique to change the eye color of a permanent and definitive.
Studies on the use of impacts of Yag laser on the anterior layer of the iris are underway; These impacts cause inflammation of this layer superficuelle, and cellular phenomena involving the action of macrophages recruited (and then ingesting the melanin and pigment cells). The effects of this technique seem limited on the cosmetic front, iris taking that rather grayish blue next to the box of the impacts. Its effectiveness, but also its safety remain to demonstrate.
It is of course possible to wear contact lenses 'of color' to get, the time of day, the eyes of a different color. Recall that color lenses, as correction lenses (there are also colorful correction lenses) are devices that should be taken as under medical control and contactologique, and by rules d' use and very strict maintenance.
The cosmetic result of these processes is often considered disappointing: the "blue" color of the eyes is not necessarily easy to reproduce, since the optical mechanism originally this bluish coloration (diffraction and not selective pigmentation).
Some eye conditions cause a change of the color of the iris: the follows of Fuchs is responsible, on the side of the achievement, a clarification of the color of the iris, due to the gradual disappearance of the pigmented cells. One speaks then of IRIS heterochromia.
The heterochromia can be congenital, as in the following example:
Eye drops containing prostaglandins or derivatives, used to reduce intra ocular pressure in patients with glaucoma (ex: latanoprost) cause increased IRIS pigmentation, and can thus make less blue eyed (green or brown). This pigmentation is final. It is preferable to use another class of eye drops anti glaucomatous patients to the blue iris, who want to retain this coloration.
In eye surgery, especially cataract, a lesion of the iris may cause a local discoloration of the iris.
Permanent changes in the diameter of the pupil can also give an impression of changing the color of the affected eye. A famous example is that of the late English artist David Bowie, whose eyes were not truly minnows; victim of local trauma in childhood, the pupil of the left eye of Bowie remained in Mydriasis (Permanent dilation), which darkened the aspect of this eye to the dilated pupil, towards the other eye to clear iris. This feature gave the illusion of wall eyes, and a feeling of particular strangeness related to the more or less subliminal perception of a difference in pupil size (anisocoria).
Finally, remember that surgery refractive corneennne (LASI, PKR, etc) will cause no change to the color of the eyes.