Cornea collagen

White J, Werkmeister JA, Ramshaw JA, Birk DE. Organization of fibrillar collagen in the human and bovine cornea collagen types V and III. Connect Tissue Res 1997 36 165-174. 

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. 

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]. 

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],... 

Another 3-D cornea model, comprising rabbit primary cultures of epithelial and stromal cells as well as mouse immortalized endothelial cells, was described in 1994 by Zieske and coworkers [70], They showed the influence of endothelial cells on the formation of a tightly packed, multilayered epithelium as well as the expression of laminin, type VII collagen, a6 integrin, keratin K3, and a-enolase. Furthermore, their findings suggested that the formation of an in vivo-like epithelium requires the cultivation of the 3-D corneal construct under AIC conditions. By contrast, LCC methods of cultivating corneal equivalents in the absence of endothelial cells failed to promote the expression of differentiation markers and basement membrane components. 

Minami Y, Sugihara H, Oono S. Reconstruction of cornea in three-dimensional collagen matrix culture. Invest Ophthalmol Vis Sci 34 2316-2324 (1993). 

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]. 

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. 

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]. 

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. 

Substances that guarantee the chemical and mechanical stability of the cornea are mostly collagen type I and X that are mainly made of proline and hydroxy-proline. Other substances that are present in a considerable amount are proteins (pK between 5.3 and 9), vitamin C (pK = 4.2), bicarbonate, glutathione, and lactate. (For more details about pK refer to Chap. 3, Sect. 3.2.4.1)... 

Collagen Like the a-keratins, collagen has evolved to provide strength. It is found in connective tissue such as tendons, cartilage, the organic matrix of bone, and the cornea of the eye. The collagen helix is a unique... 

Collagen and elastin are examples of common, well-characterized fibrous proteins that serve structural functions in the body. For example, collagen and elastin are found as components of skin, connective tissue, blood vessel walls, and sclera and cornea of the eye. Each fibrous protein exhibits special mechanical properties, resulting from its unique structure, which are obtained by combining specific amino acids into reg ular, secondary structural elements. This is in contrast to globular proteins, whose shapes are the result of complex interactions between secondary, tertiary, and, sometimes, quaternary structural elements. 

Type I, II, and III collagens are fibrillar, and are found in tendon, skin, bone, cornea, carti lage, vitreous body, and blood vessels. Types IX and XII are fibril-associated, and are found in cartilage, tendon, and ligaments. Type IV and VII form networks in basement membrane and beneath stratified squamous epithelia. 

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