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Figure 1.40 Frontal view SEM of the canine ICA. Fibrous pillars that attach the iris (I) to the limbus form the pectinate ligaments (PL). Arrows indicate smaller fibrous connections between these pillars and uveal trabeculae located behind the pectinate ligament. (Original magnification, 160×.)
The corneoscleral trabecular meshworks of domestic animals are characterized mainly by small trabeculae separated by small intertrabecular spaces. In carnivores, these trabeculae are incompletely lined by trabecular cells. Composition of the trabeculae varies very little among species. The core, or center, of each beam is made up of circularly and meridionally oriented collagen fibers interspersed with a modified elastin. The core is usually enveloped by a cortical zone consisting of amorphous, granular material surrounded by basement membrane‐like material. Trabecular cells are similar across species, being fibroblast‐like with slender cell processes that attach to adjacent cells and their processes. These processes allow the corneoscleral trabecular meshwork to act as a sieve, thus reducing the size of the particles that can move into the meshwork. The trabecular cell also has the ability to ingest a wide variety of particles, which can range greatly in size. The phagocytic‐like quality of the trabecular cell provides the ICA with an indigenous clearance mechanism for debris, thus reducing possibilities for an inflammatory response. An operculum is located within the canine trabecular meshwork, and comprises much of the nonfiltering portion of the anterior trabecular meshwork (Figure 1.41).
The external boundary of the corneoscleral trabecular meshwork is formed by the sclera and a plexus of aqueous humor collector vessels. In mammals and most lower vertebrates, the aqueous humor chiefly exits the eye through the trabecular meshworks into these vessels. In most mammals, these vessels consist of a small network of veins collectively termed the AAP. These vessels have radially oriented lumens, differing from the circumferentially coursing canal of Schlemm in primates. The plexiform nature of the drainage vessels in most mammals allows removal of a substantial amount of aqueous humor.
The size of the individual collector vessels (i.e., trabecular veins) and the tissue immediately adjacent to the AAP varies considerably among mammals. The trabecular veins in cattle, sheep, and water buffalo are large and extensive. Those associated with dogs, cats, pigs, and horses are less prominent but are still extensive.
The manner by which aqueous humor flows into the trabecular veins of the AAP or canal of Schlemm is not completely understood. Most of the aqueous humor is thought to move through large, vacuole‐like structures of the inner endothelial cells.
Figure 1.41 Cells associated with the operculum in the dog form clusters and can be linearly arranged (Schwalbe's line cells [SLC]) within the anteriormost regions of the corneoscleral trabecular meshwork. O, operculum. (Original magnification, 9800×.)
The area adjacent to the trabecular veins typically consists of a zone of cellular elements intermixed with irregularly arranged elastin, collagen, and basement membrane‐like material. In some species, including dogs, rats, rabbits, and humans, smooth muscle‐like cells (myofibroblastic cells) have been observed in the trabecular meshwork, especially adjacent to the aqueous humor outflow channels and along the distal or outer walls of the AAP and Schlemm's canal. In the dog, the presence of myofibroblastic cells within the ICA suggests that these cells and the smooth muscle cells of the ciliary body along the same plane of orientation function to facilitate the removal of aqueous humor and are likely to be influenced by vascular mediators.
Uveoscleral Outflow
Aqueous humor is not entirely removed by a plexus of collector vessels via the ICA. Some aqueous drains posteriorly into the vitreous humor, anteriorly within the iridal stroma and across the cornea, or exteroposteriorly along a supraciliary–suprachoroidal space into the adjacent sclera (Figure 1.42). The lattermost pathway is called the uveoscleral, or unconventional, outflow pathway (not sensitive to changes in intraocular ocular pressure). The degree of uveoscleral outflow varies remarkably between species, with cats experiencing the least drainage (3%), followed by humans (4–14%), rabbits (13%), dogs (15%), and nonhuman primates (30–65%). In the horse, the uveoscleral pathway may be just as important as the conventional route for aqueous humor removal (Figure 1.43).
Figure 1.42 The majority of aqueous humor flows from the posterior chamber (PC) into the anterior chamber (AC), where it is removed via the ICA by the trabecular meshwork and AAP. Other drainage routes include exchange across the vitreous face (V), iris vessels (I), and corneal endothelium (C), and via the uveoscleral (US) pathway.
Innervation
As mentioned previously, the ciliary musculature is innervated both sympathetically and parasympathetically. Cholinergic and adrenergic nerve endings have been observed in the various components of the ciliary body, including the trabecular meshwork and within the ICA. In the dog, cholinergic activity is most intense in the musculature, ciliary processes, and epithelium.
Choroid
The choroid is the posterior portion of the uveal coat. It is composed primarily of blood vessels (mainly thin‐walled veins) and pigmented support tissues (Figure 1.44a and b). It is the main source of nutrition for the outer layers of the retina. In most domestic animals, the anterior margin of the choroid joins the ciliary body along a regular, non‐serrated junction called the ora ciliaris retinae. In primates, the junction is irregular and serrated and termed the ora serrata. The choroid tends to thicken along the posterior pole, becoming thinner toward the globe equator.
Figure 1.43 Located between the ciliary body meshwork and the sclera (i.e., supraciliary space), the supraciliary meshwork likely represents a major pathway for aqueous humor drainage in the horse via uveoscleral outflow. SCT, supraciliary trabecula; TC, trabecular cell. (Original magnification, 3500×.) Inset: Light micrograph of the meshwork. S, sclera. (Original magnification, 200×.)
For morphological discussions, the choroid is divided externally to internally, into the suprachoroidea, the large‐vessel layer, the medium‐sized vessel and tapetum layer, and the choriocapillaris (Figure 1.45). The tapetal layer varies among species, and it is absent in pigs, squirrels, rodents, kangaroos,
