Transparency, corneal

Lerman, S. (1984). Biophysical aspects of corneal and lenticular transparency. Curr. Eye Res. 3, 3-14. 

However, an intrinsic physiological requirement for the cornea is the maintenance of its transparency, ensuring proper vision. This clarity as well as maintenance of the corneal shape (surface smoothness and total thickness) is critical to refraction. The unique transparency of corneal tissues is strongly dependent on their avascularity and a functionally intact endothelium, which are crucial for the maintenance of both stromal clarity and thickness by regulating corneal hydration. Macroscopically, the cornea is a transparent and avascular tissue... 

It is well recognized that in vitro angiogenesis assays can have clear advantages. However, the major drawback of all of these assays is that they require the endothelial cells to be removed from their natural microenvironment, which alters their physiological properties. To study angiogenesis in vivo, the most frequently used assay systems exploit chicken chorio-allanto-ic membrane (CAM) [28,60], the corneal pocket [61], transparent chamber preparations such as the dorsal skin fold chamber [62,63], the cheek pouch window [64] and polymer matrix implants [65,66]. 

They secure and maintain the transparency of the cornea, mainly thanks to type V collagen, which influences the diameter of the collagen fibers and thus plays a part in the corneal transparency [3]. 

Located on the edge of the transparent cornea, it maintains the junction with the opaque sclera. On the level of the epithelium, it is the transition between a multilayer scale-like corneal epithelium and a cylindrical conjunctival epithelium with two cellular bases, with continuity of the basement membranes. At the epithelial level, the cells of the comeal epithelium are gradually replaced by a conjunctival epithelium made of two layers of cylindrical cells accompanied by calyciform cells. 

Its arrangement in even and regular fibrillae, parallel to each other, is a fundamental element of the corneal transparency. 

In normal state, the cornea has no vascularity. After lesions, scarring and the modification of the physiological conditions cause a vascularization with diminution of the corneal transparency. 

Mechanism of Recovery of the Corneal Transparency in case of a Burn... 

There is a parallel to draw between the Thill and assistant study and the study by Kubota and Fagerlhom [15] who have demonstrated that the importance of the initial corneal edema, resulting from a bum, is correlated to the importance of the sequelar cicatricial leukoma that causes the drop of vision. The stromal lacunae, fonned by the edema, will be colonized by the keratocytes. After the resorption of the edema and at the level of these lacunae, the keratocytes form a zone of cicatricial tissue, which is the origin of the leukoma. These keratocytes also produce an unorganized network of collagen fibrillae, thus causing the drop of transparency of the cornea. 

The inner layer of the corneal stroma is a dense membrane of collagen like the basal membrane of the monolayer of corneal endothelium. Descemets membrane is transparent with a thickness varying from 7 to 20 pm, according to the age of the individual. Conjunctiva and cornea host nerve endings of high density in the snperhcial and basal layers. The cornea at the limbns smoothly changes to sclera with interconnected nontransparent collagen hbrils. 

Kubota, M., Pagerholm, P Corneal alkali burn in the rabbit. Waterbalance, healing and transparency. Acta Ophthalmol (Copenh) 69(5), 635-640 (1991)... 

A passive flux of water continually flows across the endothelial layer toward the stroma, which has a tendency to swell. An active pump mechanism pulls an aqueous flux in the opposite direction which controls corneal turgescence [13]. Corneal deturgescence is an ATP-dependent process of the endothelial cells and as such any disruption of the endothelium may result in corneal oedema, thereby affecting corneal transparency. The specific distribution of different proteoglycans across the cornea has recently been implicated in water gradients across the cornea. This water gradient serves to diminish dehydration of the front of the cornea, which is exposed to the atmosphere. 

The sclera is the outer white tough part of the eye, which is an important structural element, with the site of insertion of extraocular muscles. It covers 80% of the exterior surface and is white and nontransparent. It borders the transparent cornea at the pars planar. The sclera is divided into three layers episclera, stroma, and lamina fusca. Only a limited number of blood vessels, originating from arteriolar branches of the anterior ciliary vessels, are found and superficial vessels are mainly confined to the loose outer episclera. Scleral permeability approximates that of the corneal stroma and has been shown to be permeable to solutes up to 70 kDa in molecular weight [14]. 

Precorneal Tear Film Corneal transparency and good visual function require a uniform eye surface. This is achieved by the tear film, which covers and lubricates the cornea and the external globe. It is about 7-8 pm thick and is the first structure encountered by topically applied drugs. The trilaminar structure of the tear film is shown in Figure 2. 

It plays an important role in the maintenance of corneal hydration and transparency via active ion and fluid transport mechanisms. 

Sclera The sclera is the outermost firm coat of the eye that serves as a protective barrier for the sensitive inner parts. It is composed of the same type of collagen fibers as the corneal stroma. However, the fibers are arranged in an irregular network rather than a lattice pattern, which makes the tissue appear opaque compared to the transparent cornea. The white sclera constitutes the posterior five-sixths of the globe, whereas the transparent cornea comprises the anterior one-sixth [17],... 

Whenever swelling takes place, transparency is lost in the region where the edema occurs. Because the corneal epithelium and tear film constitute the most anterior optical surface of the eye, epithelial edema can exert a major detrimental influence on vision because it induces anterior irregular astigmatism. 

The cornea is the avascular, transparent, richly innervated anterior-most snrface of the globe, which is the eye s primary refracting snrface. As a result of these characteristics, diseases and disorders of the cornea can result in symptomatology, snch as loss of vision, pain, and photophobia, that generally prompts the patient to seek care. Both the prevalence and potential severity of corneal conditions obligates the eye care provider to be hilly versed in the diagnosis, treatment, and management of corneal diseases and disorders. 

The stroma constitutes approximately 90% of the total corneal thickness and is primarily composed of collagen fibers, keratocytes, and glycosaminoglycans. The imiform arrangement of the collagen fibers is the major determinant of corneal transparency, in contrast to the opaque and less regularly arranged fibers of the sclera. Disruption of the stromal layer regularity results in loss of corneal transparency and potential scar formation. 

Corneal dystrophy of Bowman s layer type 1 Reis-Biicklers TGBFI AD Comeal surface appears rough and irregular with accumulation of opacities at Bowman s layer in annular, crescent, polygonal, or map-like formations. Opacities are confined to central and mid-peripheral cornea, whereas the extreme periphery remains transparent. RCE common with surgery often required in second or third decades due to severe vision loss. 

The endothelium functions as both a barrier and pump and is responsible for maintaining corneal transparency by regulating stromal hydration. The endothelium imder-goes an age-related decrease in cell density due to a reduced proliferation rate that does not keep pace with cell loss. As a result, the endothelium becomes fragile and its function can potentially be compromised as a result of trauma or disease. 

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