Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

species, active mitosis of the endothelium occurs primarily in only immature animals. Specular microscopy and SEM of adult eyes reveal that the cells are usually hexagonally shaped. Closer inspection by SEM reveals that the surface is spotted with small microvillae and pores, and that the lateral edges of one cell interdigitate with another. In young canines (i.e., one to four weeks of age), many of the cells do not have the typical hexagonal shape. Pronounced pleomorphism has also been observed in kittens and rabbits.

      Transmission electron microscopy (TEM) reveals the extensive, lateral, convoluted interdigitations between adjacent cells in the dog. The cell junctions, including zonulae occludentes and maculae adherentes, are located at the lateral cell margins. The abundance of mitochondria, smooth and rough endoplasmic reticulum, and a variety of vesicles indicates that these cells are metabolically active. There is gradual loss of the hexagonal shape in older animals due to a gradual decrease in the overall cell density of the endothelium. In young dogs, endothelial density is greater than 3000 cells/mm² with approximately 3600 cells/mm² in dogs less than one year old. As animals age, endothelial density can gradually decrease to 50% or less of that number.

With a smaller population of cells, the endothelial cells spread out and produce more pump sites to compensate for increasing leakage. An age‐related decrease in the density of corneal endothelial cells results in little change in overall corneal thickness, but if the cell density continues to decrease, however, the cells become too attenuated, resulting in the pumps being unable to withstand the increasing leakage with concomitant corneal thickening and loss of optical clarity. This point is known as corneal decompensation, and it usually occurs when the endothelial cell density decreases to between 500 and 800 cells/mm².

Sclera

The sclera comprises the remainder of the fibrous tunic of the globe. Anteriorly, it merges with the peripheral cornea and the bulbar conjunctiva to form a transition zone, the limbus (Figure 1.25a and b). The limbus and now the peripheral cornea are the main entry sites for incisions into the anterior chamber (as for cataract surgery). At the limbus, the sclera is variably pigmented, and the overlying epithelium is thicker, with pigmented epithelial cells (see Figure 1.25b). The stroma loses the regular arrangement characteristic of the cornea and takes on a less organized appearance of irregular, dense connective tissue. Numerous blood vessels (i.e., the anastomosing branches of the anterior ciliary arteries) terminate in the loops of the marginal plexus, and then drain back into the conjunctival venules. Any incision in the sclera will result in profuse hemorrhage!

Along the outer portion of the anterior scleral stroma is an interconnecting network of veins, the intrascleral plexus, which receives aqueous humor from the veins that drain the angular aqueous plexus (AAP) (Figure 1.26). In domestic animals, the intrascleral plexus is variably connected with the choroidal venous system, the vortex system. The intrascleral plexus is variable in size and depth within the sclera. In rabbits and primates, the plexus is formed on the outer side of circumferentially coursing canals, and it is composed of its small vessels deep in the sclera. In carnivores, the intrascleral plexus is prominent and composed of two to four large, anastomosing vessels in the mid‐sclera. The intrascleral plexus also receives afferent channels superficially via the episcleral network at the limbus. In the horse, the plexus, which is less prominent, collateralizes entirely with the anterior vortex system, because it is oriented radially to facilitate unidirectional flow outward from the angle region toward the vortex veins (posterior or uveoscleral aqueous humor outflow).

Figure 1.25 Photomicrographs of canine limbus. (a) The irregular connective tissue of the sclera (S) merges with the highly organized connective tissue of the cornea (C). (b) Close‐up of the outer limbus reveals an anterior epithelium that is markedly thickened, and contains small blood vessels (BV) and melanocytes. (Original magnification, 250×.)

The color of the sclera depends on the thickness of its stroma, appearing blue when thin (less than 0.2 mm) or yellow with increased fat content (carotenoids). The inner surface, which is referred to as the “lamina fusca,” is brown because of the adherent suprachoroidal pigment. The sclera contains elastic fibers that are interlaced among the collagen fibers, as are melanocytes (anteriorly) and fibrocytes (the sclera is probably more elastic than the cornea). The collagen fibers, fibrocytes, and occasional melanocytes are arranged meridionally, obliquely, and radially in an irregular fashion. The most notable emissaria accommodate the optic nerve, long and short ciliary nerves, long posterior ciliary arteries, vortex veins, and anterior ciliary vessels and represent weak areas in the sclera and can enlarge and protrude (called staphylomas) when IOP is chronically elevated.

Figure 1.26 The intrascleral plexus (ISP) of a dog is located within the mid‐sclera (S), and is interconnected to the AAP by aqueous veins (AV). ESV, episcleral veins. (Original magnification, 125×.)

      The episclera is a collagenous, vascular, and elastic tissue that is between the sclera and the conjunctiva and attaches to Tenon's capsule. Tenon's capsule consists of small, compact bundles of collagen that lie parallel to the surface of the episclera.

Figure 1.27 Scleral ossicles (SO) in birds vary in size and shape. (a) Screech owl with large intraosseous spaces. (Original magnification, 40×.) (b) Chicken with smaller scleral ossicles and considerable overlap between adjacent ossicles. (Original magnification, 100×.) CM, ciliary body musculature (Crampton's muscle); TM, trabecular meshwork.

Besides dense connective tissue, the sclera can be largely composed of cartilage in some species, as in fish, lizards, chelonians, some amphibians, and birds. When cartilage is found in the sclera, it usually forms a complete cup that extends to the margin of the cornea or, in birds and lizards, to a ring of bony plates or ossicles. Scleral ossicles are located external to the ciliary body (Figure 1.27a and b). Ossicles are believed to have evolved for retaining ocular rigidity. The number of ossicles that comprise a ring can vary within the same species; in individual eyes with fewer ossicles, the single ossicle area increases, resulting in a constant scleral ring area, which must be avoided when this area is incised.

      Between the inner sclera and outer uveal tissues is the sclerociliary/sclerochoroidal space, which is the exit pathway for the posterior uveoscleral aqueous humor drainage; this extends all the way to the optic nerve head.

Uvea

The iris, ciliary body, and choroid form the uvea. Unlike the fibrous coat, the uveal coat is highly vascular and usually pigmented. The ciliary body and choroid are loosely attached to the internal surface of the sclera (Figure 1.28). The iris originates from the anterior portion of the ciliary body, and it extends centrally to form a diaphragm (the pupil) anterior to the lens. The