The corneal epithelium (Fig. 24.4) consists of approximately five layers of nonkeratinized cells and measures about 50 m in average thickness. It is continuous with the conjunctival epithelium that overlies the adjacent sclera. The epithelial cells adhere to neighboring cells via desmosomes 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 squamous 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. 24.4b). As the cells migrate to the surface, the cytoplasmic organelles gradually disappear, indicating a progressive decline in metabolic activity. The corneal epithelium 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 this stem cell niche 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, which is 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 fll the defect.
Numerous free nerve endings in the corneal epithelium provide it with extreme sensitivity to touch. Stimulation of
FIGURE 24.4 • Photomicrograph of the cornea. a. This photomicrograph of a section through the full thickness of the cornea shows the corneal stroma and the two corneal surfaces covered by different types of epithelia. The corneal stroma does not contain blood or lymphatic vessels. 140. b. A higher magnification of the anterior surface of the cornea showing the corneal stroma covered by a stratified squamous (corneal) epithelium. The basal cells that rest on Bowman’s membrane, which is a homogeneous condensed layer of corneal 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). 280. 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. 280.
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.