LITMIR.BIZ

Figure 1.19 Histological view of the four layers in the equine cornea: anterior epithelium (AE), stroma (S), Descemet's membrane (DM), and endothelium (E). Inset: Basal cells (B), wing cells (W), and squamous cells (S).

      Corneal Epithelium

The corneal epithelium is a nonkeratinized, stratified squamous epithelium that covers the anterior corneal surface. The epithelium is approximately 25–40 μm thick in the domestic carnivore and two to four times thicker in the ungulate. In the dog, cat, and birds, the anterior epithelium consists of a single layer of basal cells that lie on a thin basement membrane (Figure 1.20a and b), two or three layers of polyhedral (i.e., wing) cells, and two or three layers of nonkeratinized squamous cells. In larger animals, the layers of polyhedral and squamous cells are more numerous. The cells are arranged to provide orderly replacement of the surface cells during desquamation.

There are several layers of outer flattened superficial squamous cells. The cells appear to be flat and polygonal with straight borders on scanning electron microscopy (SEM) (Figure 1.21). Both light and dark cell types can be identified. The light cells contain more microvillae and microplicae. These numerous projections scatter electrons and, as a result, produce a lighter appearance of the cell. The darker cells are older and are occasionally seen to be desquamating (see Figure 1.15b). Cells in the central cornea have more projections (i.e., microplicae and microvillae) than those in the periphery. It has been proposed that the fine microplicae and microvillae that considerably expand the cells' surface area enable movement of oxygen, potential nutrients, and various metabolic products across the exposed cell membranes of the outermost squamous epithelial cells. Also more likely, the microprojections of the squamous epithelial cells, which can be sometimes intricate in their patterns, allow mucin of the PTF to adhere firmly to the anterior epithelium, which aids in stabilizing the tear film on the corneal surface.

Beneath the epithelium is a basement membrane, which stains positively with PAS (see Figure 1.20). The basal cells are firmly attached to the basal lamina of the basement membrane (i.e., anterior limiting lamina) by hemidesmosomes, anchoring collagen fibrils, and the glycoprotein laminin. Ultrastructurally, the basement membrane consists of a 30–55 nm thick osmiophilic layer that is separated from the basal cell plasma membrane by a 25 nm wide, electron‐lucent zone (see Figure 1.15b). Hemidesmosomes attach the basal cells to the basement membrane, which in turn anchors the epithelium to the stroma. The arrangement of hemidesmosomes varies among different animals, being linear among mammals and amphibians, in rosettes among birds and reptiles, and punctate without arrangement, or completely absent, among fish. The epithelial cells have strong regenerative abilities (basal cell turnover time is approximately seven days), but after removal of the basal lamina, weeks to months may be necessary for it to completely reestablish.