The cornea is a transparent, avascular tissue that consists of 5 layers (Fig 1-4): epithelium, Bowman, stroma, Descemet membrane, and endothelium; these are discussed in the following subsections.
In adults, the cornea measures 11–12 mm horizontally and 10–11 mm vertically. It is approximately 500–600 μm thick at its center and gradually increases in thickness toward the periphery. The cornea is aspheric, although the central portion of the anterior corneal surface is often described as a spherocylindrical convex mirror. The general refractive index of the cornea is 1.376. The average radius of curvature of the anterior central cornea is 7.8 mm, which would produce a dioptric power of 43.25 D for the front surface of the cornea, using the keratometer calibration index of 1.3375. The total dioptric power of a normal human eye is 58.60 D, to which the cornea contributes 74%. The cornea is also the major source of astigmatism in the human optical system. For further discussion of corneal optics, see Evaluation of Corneal Curvature in Chapter 2.
Figure 1-4 The layers of the normal cornea. The epithelium is composed of 4–6 cell layers, but it can increase in thickness to maintain a smooth surface (hematoxylin-eosin, ×32).
For its nutrition, the cornea depends on diffusion of glucose from the aqueous humor and of oxygen through the tear film. In addition, the peripheral cornea is supplied with oxygen from the limbal circulation.
The density of nerve endings in the cornea is among the highest in the body, and the sensitivity of the cornea is 100 times that of the conjunctiva. Sensory nerve fibers extend from the long ciliary nerves and form a subepithelial plexus.
The corneal epithelium is composed of 4–6 layers, which include 1–2 layers of superficial squamous cells, 2–3 layers of broad wing cells, and an innermost layer of columnar basal cells. It is 40–50 μm thick (see Fig 1-4; also see the Pachymetry section in Chapter 2). The epithelium and tear film form an optically smooth surface. Tight junctions between superficial epithelial cells prevent penetration of tear fluid into the stroma. Continuous proliferation of limbal stem cells gives rise to the other layers, which subsequently differentiate into superficial cells. With maturation, these differentiated cells become coated with microvilli on their outermost surface and then desquamate into the tears. The process of differentiation takes approximately 7–14 days. Basal epithelial cells secrete a continuous, 50-nm-thick basement membrane, which is composed of type IV collagen, laminin, and other proteins. Corneal clarity depends on the tight packing of epithelial cells, which results in a layer with a nearly uniform refractive index and minimal light scattering.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.