cell types: fibroblasts and melanocytes. The anterior cells, which lack a basement membrane, form an almost continuous layer with their cellular processes, but frequent small openings with large intercellular spaces and extension of underlying melanocyte processes break this continuity. This anterior fibrocytic layer can be exquisitely thin and easily overlooked histologically. Particles measuring up to 200 μm in diameter can diffuse into the iris stroma through the anterior portion of the iris. For the most part, the melanocytes are oriented parallel to the iris surface, and their processes intermingle with other melanocytes and anterior fibroblasts with no intercellular junctions. The shape of the melanin granules in the stroma varies between species and with the maturity of the granules. The pigment granules in the cat and dog are lanceolate to ovoid in shape, whereas they are round to ovoid in the horse. In addition to the scattered melanocytes in the anterior stroma of most dog irides, a dense band of melanocytes can be present in the ciliary zone anterior to the dilator muscle, extending centrally to the sphincter muscle. The granules are generally smaller and more rod‐like than the pigmented granules of the posterior epithelium. Particularly in the horse and the dog, large cells containing pigment are associated with capillaries and venules near the sphincter muscle. The iris stroma is composed of fine collagenous fibers, chromatophores, and fibroblasts. The stroma is loosely arranged except around blood vessels and nerves, where it can form dense sheaths.
Figure 1.29 Equine iris (I) and anterior ciliary body (CB). The arrow points to the granula iridica, which continues posteriorly as the posterior pigment epithelium (PE).
Iridal color varies considerably among individuals, breeds, and species. The variation of iridal color results from the amount and type of pigmentation present. The coloration of irides in most domestic animals is dark brown, golden brown, gold, blue, or blue‐green. Several avian species have brightly colored irides. Historically, these bright colors were thought to result from the presence of carotenoids; however, purines and pteridines may be the major iridal pigments in a variety of avian species, including doves and great‐horned owls. Combinations of purines, pteridines, and carotenoids probably occur in the irides of avian species.
The major arterial circle is located at the peripheral iris root or the anterior ciliary body (Figure 1.30a and b), and generally avoided during intraocular surgeries. The arteries enter at the 9‐ and 3‐o'clock positions of the iris as terminations of the medial and lateral branches of the long posterior ciliary arteries. Each artery branches dorsally and ventrally to pass circumferentially toward the opposite artery and forms an incomplete arterial circle in most species. In primates, the major arterial circle forms a completely enclosed ring. The arteries radial to the pupil are tortuous in most animals to accommodate changes in the iridal stroma during pupillary changes. A capillary network near the pupillary margin connects the terminal arterioles with the venules, which pass to the base of the iris behind the arterioles in the posterior stroma. The capillary endothelium is not fenestrated (hence more permeable), but the type of intercellular junctions varies with species. Venous drainage of the iris occurs through tortuous, radial vessels that empty directly into the anterior choroidal veins and out the vortex veins. These vessels typically number four in humans, pigs, and cats, but may vary in other species. In horses, a unique variation of iridal venous drainage exists where branches of the intrascleral venous plexus empty into the bases of the iridal veins, which in turn empty into the anterior choroidal venous circulation.
Figure 1.30 (a) In many canine irides, melanocytes are concentrated in a wide band anterior to the dilator muscle (DM), as seen in the lower half of this iris. MAC, major arterial circle. (Original magnification, 100×.) (b) Photograph of a cat demonstrating the MAC in the peripheral iris.
The iridal sphincter muscle, which is a flat band of thin, circular bundles of smooth muscle fibers in mammals and striated muscle fibers in nonmammalian species, is located in the iris stroma near the pupil. In the dog and cat, it lies in the posterior stroma, separated from the pigmented epithelium and subjacent dilator muscle by a thin layer of connective tissue (Figures 1.31a and b and 1.32a and b). In the horse, the sphincter occupies the main portion of the central stroma and is capped by the granular iridica when present. The shape of the sphincter muscle varies among species according to the pupillary shape (see Figure 1.32). The sphincter muscle is innervated primarily by parasympathetic nerve fibers.
The posterior iridal surface is covered by two layers of epithelium. The anterior layer, which forms the dilator muscle, is continuous with the pigmented epithelium of the ciliary body, whereas the posterior layer, which is densely pigmented, is continuous with the nonpigmented epithelium of the ciliary body.
The iridal dilator muscle is a single layer of smooth muscle fibers in the posterior iridal stroma extending from the iris sphincter to the iris periphery. These muscle fibers apically (i.e., posteriorly) contain pigment around their nuclei and are innervated by sympathetic nerve fibers. The basal regions of each cell, which contain the myofilaments, overlap one another in a shingle‐like fashion. The basal aspect of the posterior epithelium of the iris faces the posterior chamber and has numerous surface projections.
The apical portion of the cells of the anterior epithelium (iris dilator muscle) contains the nucleus and is located adjacent to the apical portion of the posterior epithelium. Melanin granules are predominately present in the apical portion of the cell. The myoepithelial (basal) portion has scattered melanin granules, forms irregular projections into the stroma, and is covered by a basement membrane.
In avian species and other lower vertebrates, the iris muscles are striated. In addition to controlling the amount of light that enters the back of the eye, the iris of birds is thought to contribute to lenticular accommodation. Changes in the pupil diameters of chickens and pigeons result in changes in the positioning of their lenses.
Figure 1.31 Sphincter muscle (SM) location in the dog (a) and in the horse (b). The sphincter muscle in the horse is capped by the granula iridica (GI), which is a proliferation of the posterior epithelium (PE). (Original magnification, 200×.)
Figure 1.32 (a) Iris sphincter muscles that create a slit pupil when the pupil is constricted as found in domestic cats, bobcats, and lynx. (b) The circular iris sphincter muscle as found in primates, birds, dogs, and pigs. (c) Iris sphincter muscle in an ungulate with a horizontal pupil.
The iris contains numerous myelinated and nonmyelinated nerves for autonomic innervation. The myelinated fibers do not specifically follow the iris vessels, but they have a similar pattern
