LITMIR.BIZ

Figure 1.22 The corneal epithelium and anterior stroma. Nonkeratinized squamous cells (two to three layers), wing cells (two to three layers), basal cells (single layer), basal lamina, and corneal nerves.

      The lamellae are parallel bundles of collagen fibrils, with each lamella running the entire diameter of the cornea. All the collagen fibrils within a lamella are parallel, but between lamellae, they vary greatly in direction. The lamellae of the posterior stroma are more regular in arrangement than those of the anterior third of the stroma. The anterior lamellae are more oblique to the surface, and they have more branching and interweaving. The precise organization of the corneal stroma is the most important factor in maintaining corneal clarity, which involves the select integration of collagen and amorphous ground matrix, consisting of select proteoglycans such as lumican, keratocan, osteoglycin, and decorin. The collagen in the human cornea has a periodicity of 100 nm. This special arrangement of the collagen in the stroma is believed to permit 99% of the light entering the cornea to pass without scatter. When the corneal stroma is replaced, this orderly lamellar organization is absent!

Collagen fibrils, along with the proteoglycans and their associated GAGs and glycoproteins, constitute 15–25% of the stroma, and they are the principal support structure of the cornea. These collagen fibrils form the matrix for a specialized population of proteoglycans within the corneal stroma. The cornea is 75–85% water, and it is relatively dehydrated compared to other body tissues. This state of dehydration is termed deturgescence and is, in part, a function of the endothelium and epithelium. These cells move water out of the stroma via energy‐dependent Na⁺/K⁺ adenosine triphosphatase (ATPase) pumps, being most active in the endothelium. Other “pumps” for deturgescence might also exist, including carbonic anhydrase. These cells pump Na⁺ and HCO³⁻ ions outward, into the aqueous humor and tears. An osmotic gradient is established, and water flows down the gradient from the corneal stroma into the aqueous humor. Experimentally, removal of the epithelium produces an increase of 200% in corneal thickness after 24 h because of the influx of water. Removal of the endothelium produces an increase of 500% or more in thickness as the permeability increases sixfold, so the endothelium appears to be more important in maintenance of corneal deturgescence. Figure 1.24 illustrates the primary roles the endothelium plays, both as a pump and as a barrier. The barrier component is provided by the tight junctions occurring apically along the lateral faces of adjoining cells next to the anterior chamber. These tight junctions are sensitive to calcium exposure, and they break down when excess free Ca²⁺ exists in the aqueous humor. The Na⁺/K⁺ ATPase pump is located along the lateral membranes of neighboring cells. A breakdown of the pump, the barrier, or both will result in rapid movement of water into the highly hydrophilic stroma, causing corneal edema to develop.

Figure 1.23 (a) SEM of corneal stroma in the dog. (b) TEM of corneal stroma in the horse consists of layers or lamellae (L) of collagen, which are sparsely interspersed with keratocytes (K). (Original magnification: a, 7400×; b, 10 000×.)

      The anteriormost stroma has a thin, cell‐free zone corresponding in location with the anterior‐limiting membrane, also known as Bowman's layer (anterior lamina), in humans and nonhuman primates. Bowman's layer is also present in birds, giraffes, dolphins, some whales, and large herbivores. In avian and human corneas, Bowman's layer is 10–15 μm thick, relatively acellular, and composed of collagen fibrils of various types. Bowman's layer fibrils are smaller in diameter and less uniform than those of the stroma. Bowman's layer is not elastic, and when damaged it is replaced with scar tissue. Bowman's layers of the land‐based species share similarities in size, morphology, and histochemistry, differing substantially from that of marine mammals, which may reflect a variation of roles that this structure plays.

      Descemet's Membrane

Descemet's membrane is a PAS‐positive homogeneous, acellular membrane that is actually the basement membrane of the posterior endothelium. Descemet's membrane is produced throughout life, thus forming a thicker membrane as individuals age. Clinically, the membrane shows elasticity, but it contains only fine collagen fibrils. Descemet's membrane is normally under some tension and when ruptured it tends to curl like a scroll. Descemet's membrane ends at the apex of the trabecular meshwork in the limbal region. To some degree, its composition is similar to that of the trabeculae of the ICA. Ultrastructurally, Descemet's membrane is distinctly layered in most animals, usually having a relatively thin anterior, unbanded zone next to the stroma, followed by a broad‐banded zone and then by another broad, posterior unbanded zone located next to the endothelium. Healing and replacement of Descemet's membrane can result in duplication of this structure.

Figure 1.24 SEM of a four‐year‐old canine corneal endothelium reveals occasional variability in cell size (a) and the lateral surface interdigitations (arrows) between cells (b). The most prominent feature of the endothelial cell is the nucleus (N), which bulges slightly into the anterior chamber. (Original magnification: a, 960×; b, 3500×.)

      Corneal Endothelium

      The corneal endothelium is a single layer of flattened cells lining the inner (posterior) cornea (see Figure 1.24). The regenerative ability of the endothelium varies with species

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