stroma, are low columnar in contrast to the squamous surface cells. Note that one of the surface cells is in the process of desquamation (arrow). X280. c. A higher-magnification photomicrograph of the posterior surface of the cornea covered by a thin layer of simple squamous epithelium (corneal endothelium). These cells are in direct contact with the aqueous humor of the anterior chamber of the eye. Note the very thick Descemet's membrane (basal lamina) of the corneal endothelial cells. x280.
diminish light transmission. Instead, it has recently been shown that corneal epithelial cell nuclei contain ferritin, an iron-storage protein. Experimental studies with avian corneas have shown that nuclear ferritin protects the DNA in the corneal epithelial cells from free radical damage due to UV light exposure.
Bowman's membrane is a homogeneous-appearing layer on which the corneal epithelium rests
Bowman's membrane (anterior basement membrane) is a homogeneous, faintly fibrillar lamina that is approximately 8 to 10 |xm thick. It lies between the corneal epithelium and the underlying corneal stroma and ends abruptly at the corneoscleral limbus. The collagen fibrils of Bowman's membrane have a diameter of about 18 nm and are randomly oriented. Bowman's membrane imparts some strength to the cornea, but more significantly, it acts as a barrier to the spread of infections. It does not regenerate. Therefore, if damaged, an opaque scar forms that may impair vision. In addition, changes in Bowman's membrane are associated with recurrent corneal erosions.
The corneal stroma constitutes 90% of the corneal thickness
The corneal stroma, also called substantia propria, is composed of about 60 thin lamellae. Each lamella consists of parallel bundles of collagen fibrils. Located between lamellae are nearly complete sheets of slender, flattened fibroblasts. The fibrils measure approximately 23 nm in diameter and up to 1 cm in length. The collagen fibrils in each lamella are arranged at approximate right angles to those in the adjacent lamellae (Fig. 23.5). The ground substance contains corneal proteoglycans (lumican), which are sulfated glycosaminoglycans (chiefly, keratan sulfate and chondroitin sulfate) covalently bound to protein (decorin).
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