Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 39

The corneal stroma (i.e., the substantia propria) constitutes 90% of the corneal thickness. It consists of transparent lamellae of collagenous tissue, and these lamellae lie in sheets and separate easily into planes (Figure 1.22). Between the lamella are fixed cells and infrequent wandering cells. The fixed cells are fibrocytes, which are called keratocytes, and their extensions contribute to the formation and maintenance of the stromal lamellae. The keratocytes have thin nuclei, ill‐defined borders, and delicate cell membranes (Figure 1.23a and b). Similar to lens fibers, these cells possess crystallins, which are believed to facilitate tissue transparency. Keratocytes can transform into myofibroblasts when deep corneal injury occurs, and they can form scar tissue that is not transparent. While healing (re‐epithelization) of the corneal epithelium is relative fast (7–10 days for the complete corneal surface), replacement of large stromal defects may require weeks, even months!

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.