The equine cornea is a highly innervated, nonvascular, simple structure that is 0.8 to 1.0 mm thick (FIGURE 1). It protrudes beyond the bony orbit of the skull, where it is protected only by the eyelid. The normal equine tear film contains corneal nutritional elements, mostly gram-positive bacteria, and, occasionally, fungal organisms (TABLE 1). The corneal epithelium repels tears and keeps organisms from entering deeper layers. The stromal layer, which is approximately 90% of the corneal thickness, has few cells (keratocytes) and is mainly composed of water, with the remaining portion made of collagen fibrils, glycosaminoglycans, and glycoproteins.4 The Descemet membrane is a basement membrane that is resistant to penetration by bacteria, but not fungal organisms.5 The corneal endothelium, a single layer of hexagonal cells, forms a semiocclusive barrier that allows careful regulation of water and nutrients from the aqueous into the cornea.6 Corneal innervation is through cranial nerve V, and nerve endings are located in the anterior stroma and basement membrane of the corneal epithelium.6 For this reason, a superficial corneal ulceration is more painful than a descemetocele.
The equine cornea is a highly innervated, nonvascular, simple structure that is 0.8 to 1.0 mm thick (FIGURE 1). It protrudes beyond the bony orbit of the skull, where it is protected only by the eyelid. The normal equine tear film contains corneal nutritional elements, mostly gram-positive bacteria, and, occasionally, fungal organisms (TABLE 1). The corneal epithelium repels tears and keeps organisms from entering deeper layers. The stromal layer, which is approximately 90% of the corneal thickness, has few cells (keratocytes) and is mainly composed of water, with the remaining portion made of collagen fibrils, glycosaminoglycans, and glycoproteins.4 The Descemet membrane is a basement membrane that is resistant to penetration by bacteria, but not fungal organisms.5 The corneal endothelium, a single layer of hexagonal cells, forms a semiocclusive barrier that allows careful regulation of water and nutrients from the aqueous into the cornea.6 Corneal innervation is through cranial nerve V, and nerve endings are located in the anterior stroma and basement membrane of the corneal epithelium.6 For this reason, a superficial corneal ulceration is more painful than a descemetocele.
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