Cornea structure

The cornea is the first structure of the eye to be in contact with incident light. It is composed of five distinct layers lying parallel to its surface the outer epithelium, which is continuous with the epithelial layers of the conjunctiva the epithelial basal lamina the keratocyte-containing stroma, which is a collagen structure arranged so that it is transparent Descemet s membrane and, finally, the endothelium adjacent to the aqueous humour. 

The most common means of administering drugs to the eye is by topical administration of agents capable of penetrating the cornea and targeting the appropriate tissue for either physiological or medicinal effect [159,160]. The trilaminar structure of the transparent avascular cornea has been described previously. The... 

SD Klyce, RW Beuerman. (1988). Structure and function of the cornea. In HE Kaufman, BA Barron, MB McDonald, SR Waltman, eds. The Cornea. New York Churchill Livingstone, pp 3-54. 

In order to achieve the firm fixation of the artificial cornea to host tissues, composites of collagen-immobilized poly(vinyl alcohol) hydrogel with hydroxyapatite were synthesized by a hydroxyapatite particles kneading method. The preparation method, characterization, and the results of corneal cell adhesion and proliferation on the composite material were studied. PVA-COL-HAp composites were successfully synthesized. A micro-porous structure of the PVA-COL-HAp could be introduced by hydrochloric acid treatment and the porosity could be controlled by the pH of the hydrochloric acid solution, the treatment time, and the crystallinity of the HAp particles. Chick embryonic keratocyto-like cells were well attached and proliferated on the PVA-COL-HAp composites. This material showed potential for keratoprosthesis application. Further study such as a long-term animal study is now required [241]. 

Microscopically, the cornea shows a rather simple and multilayered structure that can be divided into six layers the epithelium, basement membrane, Bowman s layer, stroma, Descemet s membrane, and endothelium. The corneal tissue consists of three different cell types epithelial cells, keratocytes (corneal fibroblasts), and endothelial cells. The outermost corneal surface is covered with the preocular tear film, which is functionally associated with the cornea. The epithelial surface must be kept moist and smooth, a role played by the tear film in conjunction with a spreading function of the eyelids during blinking motions. Furthermore, the tear film provides a protection against infectious agents that may gain access into the eye. 

As another extracellular component in the cornea, the Bowman s layer is an acellular and amorphous band between the corneal epithelium and stroma. The layer is about 8-12 [im thick and consists of randomly arranged collagen fibers (types I and III) and proteoglycans. The physiological function of Bowman s layer is not yet completely understood, since not all animal species exhibit this membrane in the corneal structures, but an important role in the maintenance of the corneal epithelial structure is expected or probable, since a damaged Bowman s membrane usually results in scarring during wound repair [16],... 

In another approach, Parnigotto and coworkers reconstructed corneal structures in vitro by using corneal stroma containing keratocytes to which corneal epithelial cells from bovine primary cultures were overlaid [73], However, this particular corneal model did not contain an endothelial layer. This model was histochemically characterized and the toxicity of different surfactants was tested using MTT methods. This stroma-epithelium model has been reported to show a cornea-like morphology, where a multilayered epithelial barrier composed of basal cells (of a cuboidal shape) and superficial cells (of a flattened shape) is noted. Furthermore, the formation of a basement membrane equivalent and expression of the 64-kDa keratin were reported, indicating the presence of differentiated epithelial cells. The toxicity data for various surfactants obtained with this model correlate well with those seen by the Draize test [73], However, this corneal equivalent was not further validated or used as a model for permeation studies. 

Liquid anhydrous ammonia in contact with the eyes may cause serious injury to the cornea and deeper structures and sometimes blindness on the skin it causes first- and second-degree burns that are often severe and, if extensive, may be fatal. Vapor concentrations of 10,000 ppm are mildly irritating to the moist skin, whereas 3 0,000 ppm or greater causes a stinging sensation and may produce skin burns and vesiculation. With skin and mucous membrane contact, burns are of three types cryogenic (from the liquid ammonia), thermal (from the exothermic dissociation of ammonium hydroxide), and chemical (alkaline). ... 

Acute epithelial herpes simplex keratitis (dendritic keratitis) fungal diseases of ocular structures vaccinia, varicella and most other viral diseases of the cornea and conjunctiva ocular tuberculosis hypersensitivity after uncomplicated removal of a superficial corneal foreign body mycobacterial eye infection acute, purulent, untreated eye infections that may be masked or enhanced by the presence of steroids. 

Pilocarpine is a naturally occurring cholinomimetic alkaloid that is structurally distinct from the choline esters. It is a tertiary amine that crosses membranes relatively easily. Therefore, it is rapidly absorbed by the cornea of the eye, and it can cross the blood-brain barrier. Pilocarpine is a pure muscarinic receptor agonist, and it is unaffected by cholinesterases. Muscarine is an alkaloid with no therapeutic use, but it can produce dangerous cholinomimetic stimulation following ingestion of some types of mushrooms (e.g., Inocybes). 

The immediate or secondary necrosis (by more progressive diffusion into depth or by concomitant toxic effect) explain the macroscopic lesions of the tissues of the various eye structures (eyelids, conjunctiva, cornea, even iris or the ocular lens) that are specific to the chemical bum. 

Nevertheless, the lesions provoked by HF are dreadful and may even endanger the vital prognosis of the individual because of the cardiac complications that it can engender in case of associated facial projection. Why Simply because to the initial destruction of the structures of the eye by its acid potential (ion H ), is added the chelating and toxic action of the fluoride ion (F ) (Fig. 3.57). This action will develop gradually, in situ in the layers of the cornea, as the HF breaks up. This results in a deep damage with a necrotic character. 

The chemical bum is the expression of a chemical ability to react between two molecules, a xenobiotic one and a biochemical tissues one. An acid can only affect the eye if it finds some chemical structures of basic nature in the cornea. It s the same for the attack of an oxidizing agent toward reducing cellular molecules. Generally speaking, in a chemical reaction, there is always a donor entity and an acceptor entity. For more details see Sect. 3.4.2. 

The comeal epithelium is made of stratified and keratinless scale-like and junction-like cells. These cells are arranged on 4-6 layers at the central part and 4-8 layers on the edge of the cornea. Their combined thickness is about 50-60 pm representing about 10% of the whole thickness of the cornea. The structure and function of the epithelium strictly depends on the lacrymal secretion that covers it and plays a big part in the preservation of the cells and in their transparency. 

Cintron, C. The molecular structure of the comeal stroma in health and disease. In Chandler, J.W., Sugar, J., Edelhauser, H.F. (eds.) External Diseases Cornea, Conjonctiva, Sclere, Eyehds, Lacrimal System, vol. 8. Mosby, London (1994)... 

The construction of the eye is completely different. The outer layer of this mucosa consists of a tiny tear layer of lipids and water which covers a superficial epithelium closed by double layer lipid membranes of 30-70 nm size interconnected by tight jnnctions. Three to seven layers of epithelial cells cover the stromal structures of conjunctiva or cornea. The conjnnctival surface has interposed cells secreting small amonnts of mucin, the so-called goblet cells, which are typically missing within the corneal epithelinm. The regeneration of epithelial structnres is dne to the limbal stem cells located deep in the Vogt s crypts, for the cornea. 

These vessels end in arcade-like structures at the limbus. The corneal stroma is made of three different main layers that differ in density of collagen and type of packing. The Bowman s membrane of the anterior stroma is part of the basal membrane of the corneal epithelium and accounts for 5% of the thickness of the central 500-600 pm cornea. The corneal stroma consists of highly ordered, horizontally organized and noninterconnected coUagen I and X fibriUae that are kept in a hydrated state with a water content of 72-78% and an osmolarity of 420 mOsmol/kg [1]. 

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