Epithelium and Basal Lamina
The anterior surface of the cornea is covered by a lipophilic, nonkeratinized, stratified squamous epithelium that is composed of 4–6 cell layers and is typically 40–50 μm thick (Fig 2-3). The basal cells have a width of 12 μm and a density of approximately 6000 cells/mm2. They are attached to the underlying basal lamina by hemidesmosomes. Trauma to the epithelium disrupting this layer can lead to recurrent corneal erosion due to improper re-formation of these hemidesmosomes.
Overlying the basal cell layer are 2 or 3 layers of polygonal “wing” cells. Superficial to these layers are 1–2 layers of corneal epithelial “surface” cells that are extremely thin (30 μm) and are attached to one another by tight junctions. The tight junctions allow the surface epithelial cells to act as a barrier to diffusion. Microvilli make the apical membranes of the surface cells highly irregular; however, the precorneal tear film renders the surfaces optically smooth.
Although the deeper epithelial cells are firmly attached to one another by desmosomes, they migrate continuously from the basal region toward the tear film, into which they are shed. They also migrate centripetally from their stem cell source at the limbus. Division of the slow-cycling stem cells gives rise to a progeny of daughter cells (transient amplifying cells), whose division serves to maintain the corneal epithelium (see also Chapter 8). Diffuse damage to the limbal stem cells (eg, by chemical burns or trachoma) leads to chronic epithelial surface defects.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.