Cornea, central thickness

Rachymetry measures central corneal thickness thin corneas (less than 540 microns) are considered a glaucoma risk factor. 

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. 

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 aqueous phase of the tears comprises more than 95% of the total volmne and covers the cornea with a layer that averages approximately 7 mm thick. This layer is inherently unstable, however, and begins to thin centrally at the end of each blink.The tear film in healthy subjects has a breakup time that averages between 25 and 30 seconds. 

The corneal epithelium is 5 to 6 cell layers thick centrally and 8 to 10 cell layers thick at the periphery. It is composed of a basal germinative layer, intermediate wing cells, and a surfece squamous layer that possesses structures that are known as zonula occludens, or tight junctions. These junctions constitute a continuing border between epithelial cells formed by the fusion of the outer plasma membrane. Mucopolysaccharides bound to the outer plasma membrane stabilize the tears. The cornea... 

Histologic cross-section of the cornea reveals five identifiable layers epithelinm. Bowman s layer, stroma, Descemet s membrane, and endothelium. Fluid surrounds the cornea in the forms of the tear film in front and the aqneons behind. The various corneal layers combine to form a structure that is approximately 633 mcm thick at the inferior periphery, 673 mcm at the superior periphery, and 515 mcm thick centrally. The adult corneal diameter measmes 11 to 12 mm horizontally and 9 to 11 mm vertically, creating a horizontally oriented ellipse. The radius of ciuvature of the central 3-mm optical zone ranges between 7.5 and 8.0 mm. 

In animals of the control and experimental groups after eye treatment with 0.5% dicain solution, a mechanical trauma in the central part of the cornea was produced by a trepan. Wound depth achieved was half of the corneal thickness diameter was 4 mm. 

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