Despite constant exposure of the corneal epithelium to UV light, cancer of this epithelium is extremely rare. Unlike the epidermis, which is also exposed to UV light, melanin is not present as a defense mechanism in corneal epithelium. The presence of melanin in the cornea would 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 caused by 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 m to 10 m 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 signifcantly, 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 are as long as 1 cm. The collagen fibrils in each lamella are arranged at approximately right angles to those in the adjacent lamellae (Fig. 24.5). The ground substance contains corneal proteoglycans, which are sulfated glycosaminoglycans— chiefly, keratan sulfate (lumican) and chondroitin sulfate covalently bound to protein (decorin). Lumican regulates normal collagen fibril assembly in the cornea and is critical in the development of a highly organized collagenous matrix.
It is believed that the uniform spacing of collagen fibrils and lamellae, as well as the orthogonal array of the lamellae (alternating layers at right angles), is responsible for the transparency of the cornea. Proteoglycans (lumican), along with type V collagen, regulate the precise diameter and spacing of the collagen fibrils. Swelling of the cornea after injury to the epithelium or endothelium disrupts this precise array and leads to translucency or opacity of the cornea. Lumican is
FIGURE 24.5 • Electron micrograph of the corneal stroma. This electron micrograph shows parts of three lamellae and a portion of a corneal fibroblast (CF) between two of the lamellae. Note that the collagen fibrils in adjacent lamellae are oriented at right angles to one another. 16,700.
overexpressed during the wound-healing process following corneal injury.
Normally, the cornea contains no blood vessels or pigments. During an infammatory response involving the cornea, large numbers of neutrophilic leukocytes and lymphocytes migrate from blood vessels of the corneoscleral limbus and penetrate the stromal lamellae.
Descemet’s membrane is an unusually thick basal lamina.