The cornea consists of five layers: three cellular layers and two noncellular layers
The transparent cornea (see Figs. 23.1 and 23.2) is only 0.5 mm thick at its center and about 1 mm thick peripherally. It consists of three cellular layers that are distinct in both appearance and origin. These layers are separated by two important membranes that appear homogeneous when viewed in the light microscope. Thus, the five layers of the cornea seen in a transverse section are
• Corneal epithelium
• Bowman's membrane (anterior basement membrane)
• Corneal stroma
• Descemet's membrane (posterior basement membrane)
• Corneal endothelium
The corneal epithelium is a nonkeratinized stratified squamous epithelium
The corneal epithelium (Fig. 23.4) consists of approximately five layers of nonkeratinized cells and measures about 50 fx in in average thickness. It is continuous with the conjunctival epithelium that overlies the adjacent sclera. The epithelial cells adhere to neighboring cells via desmo-somes that are present on short interdigitating processes. Like other stratified epithelium, such as that of the skin, the cells proliferate from a basal layer and become squa mous at the surface. The basal cells are low columnar with round, ovoid nuclei; the surface cells acquire a squamous or discoid shape and their nuclei are flattened and pyknotic (see Fig. 23.4b). As the cells migrate to the surface, the cytoplasmic organelles gradually disappear, indicating a progressive decline in metabolic activity. The cornea has a remarkable regenerative capacity with a turnover time of approximately 7 days.
The actual stem cells for the corneal epithelium reside at the corneoscleral limbus, the junction of the cornea and sclera. The microenvironment of the limbus is important in maintaining the population of stem cells that also act as a "barrier" to conjunctival epithelial cells and normally prevent their migration to the corneal surface. The corneal epithelial stem cells may be partially or totally depleted by disease or extensive injury, resulting in abnormalities of the corneal surface that lead to "conjunctivalization" of the cornea, characterized by vascularization, appearance of goblet cells, and an irregular and unstable epithelium. These changes cause ocular discomfort and reduced vision. Minor injuries of the corneal surface heal rapidly by inducing stem cell proliferation and migration of cells from the corneoscleral limbus to fill the defect.
Numerous free nerve endings in the corneal epithelium provide it with extreme sensitivity to touch. Stimulation of these nerves, e.g., by small foreign bodies, elicits blinking of the eyelids, flow of tears, and, sometimes, severe pain. Microvilli present on the surface epithelial cells help retain the tear film over the entire corneal surface. Drying of the corneal surface may cause ulceration.
DNA in corneal epithelial cells is protected from UV light damage by nuclear ferritin
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
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