FIBROUS TUNIC

The anterior (corneal) epithelium is nonkeratinized stratified squamous, between 4 and 12 layers in thickness (Fig. 17-2).

In the dog, cat, and bird, the corneal epithelium consists of a single layer of basal cells, two to three layers of polyhedral (wing) cells, and two to three layers of nonkeratinized squamous cells. Large animals have more layers of polyhedral and squamous cells. The epithelial cells are tightly packed, interdigitate profusely, and adhere through numerous desmosomes. The numerous microvilli on the surface of the superficial cells function to retain the tear film on the corneal surface. Numerous free nerve endings are present among the epithelial cells.

The regenerative capability of injured corneal epithelium is pronounced; mitotic divisions, along with cell movements, ensure a rapid return of injured epithelium to normal structure. An intact epithelium is necessary for maintenance of corneal transparency.

Subepithelial Basement Membrane

The subepithelial basement membrane consists of a basal lamina and a layer of reticular fibers. The basal cell layer of the epithelium is firmly attached to the basal lamina by numerous hemidesmosomes, collagen fibrils, and laminin (a glycoprotein). Frequently, this layer can be distinguished with the light microscope (Fig. 17-2). It should not be confused with the anterior limiting lamina (Bowman’s membrane), a modified outermost layer of the substantia propria that is present in primates and avians.

FIGURE 17-2 Cornea (dog). 1. Corneal epithelium (A); substantia propria (B); posterior limiting lamina (C); posterior epithelium (D). Hematoxylin and eosin (×250). 2. The corneal nonkeratinized stratified squamous epithelium is separated from the substantia propria by a rather thick basement membrane (arrowhead). Hematoxylin and eosin (×500). 3. Substantia propria with fibrocytes and collagen fibers (A), posterior limiting lamina (B), and the posterior epithelium (C). Hematoxylin and eosin (×500).

Substantia Propria (Stroma)

The corneal substantia propria, or stroma, consists of varying numbers (approximately 100 in the cat) of layers or lamellae and constitutes 90% of the corneal thickness (Fig. 17-2). Within one layer, the collagen fibers are always parallel with the corneal surface and traverse the entire diameter of the cornea; in successive layers, the fibers cross each other at right angles. Adjacent lamellae are held together firmly by fibers that deviate from their parallel course. Occasional elastic fibers are observed at the periphery of the cornea. The precise arrangement (periodicity of 620 to 640 angstroms) of the lamellae in the corneal stroma allows 99% of the light entering the cornea to pass into the globe without scatter. This accounts for the transparency of the cornea.

The predominant cell type of the corneal substantia propria is the fibrocyte (keratocyte), located mainly between the collagen layers rather than within them. These cells are elongated and branched, with little cytoplasm (Fig. 17-2). If the cornea is injured, the fibrocytes can transform into fibroblasts and form scar tissue.

There are at least five different types of collagen in the stroma, with type I being the most common. The amorphous ground substance stains metachromatically owing to the presence of sulfated glycosaminoglycans (chondroitin sulfate, keratan sulfate). The ground substance plays an essential role in the transparency of the cornea by maintaining an optimal degree of hydration; excessive water content causes opacification of the cornea.

Posterior Limiting Lamina (Descemet’s Membrane)

With the light microscope, the posterior limiting lamina (Descemet’s membrane) appears as a highly refractile, thick amorphous layer that gives a positive periodic acid-Schiff (PAS) reaction (Fig. 17-2). At the fine structural level, the lamina consists of three regions: an anterior unbanded zone, an anterior banded zone, and a posterior unbanded zone. The anterior unbanded zone has types V and VI collagen, the anterior banded zone has types IV and VIII collagen, and the posterior unbanded zone has types III and IV collagen. The posterior limiting lamina is continually produced throughout life by the posterior epithelial cells, resulting in a thicker membrane in aging animals.

Posterior Epithelium (Corneal Endothelium)

A single layer of flat hexagonal cells covers the posterior surface of the cornea as the posterior epithelium and is often referred to as the corneal endothelium (Fig. 17-2). The cells interdigitate heavily and contain numerous mitochondria and pinocytotic vesicles. The epithelium functions in the maintenance of the transparency of the cornea. Both the anterior and posterior epithelial cells actively move water out of the stroma by Na⁺-K⁺ ATPase (adenosine triphosphatase) and carbonic anhydrase pumps. Defects in the epithelium can cause edema and opacification of the cornea. The regenerative ability of the posterior epithelium is limited and varies with species and age. It is generally accepted that active mitosis only occurs in immature animals.

Corneoscleral Junction (Limbus)

At the corneoscleral junction or limbus, the sclera overlaps the cornea. The corneal epithelium gradually changes into conjunctival epithelium, which rests on a lamina propria of loose connective tissue. The characteristically layered collagen fibers of the substantia propria assume a more irregular arrangement, become associated with elastic fibers, and are continuous with the equatorial bundles of the sclera. The posterior limiting lamina of the posterior epithelium ends at the limbus near the apex of the trabecular meshwork.

The only blood vessels supplying the cornea are located at the level of the limbus; the normal cornea is completely devoid of blood vessels. Corneal nerves originate from a dense, marginal nerve fiber plexus at the same level as the blood vessels or from the ciliary plexus of the vascular tunic.

VASCULAR TUNIC

The choroid is a thick, highly vascularized layer that is continuous with the stroma of the ciliary body anteriorly and extends posteriorly around the globe (Figs. 17-3 and 17-4). The external surface of the choroid is connected to the sclera; the internal surface is adjacent and intimately attached to the pigmented epithelium of the retina. The choroid is subdivided into five layers.

FIGURE 17-3 Schematic drawing of the organization of the retina and choroid. The rod spherules (r) have synaptic contact with rod bipolar cells (rb), which synapse with ganglion cells. The cone pedicles (c) have synaptic contact with midget bipolar cells (mb), which contact a single cone, and flat bipolar cells (fb), the dendrites of which contact several cones and the axon of which then synapses with amacrine (a) and ganglion cells (g). Horizontal cell (h) processes contact rod spherules and cone pedicles. Amacrine cells (a) contact the axons of bipolar cells and dendrites and perikarya of ganglion cells. Radial glial (Müller) cells (rg) provide support to the retina; their cytoplasmic processes extend between and around the other cells, and their foot processes participate in the formation of the internal limiting membrane. Retinal capillaries (cp) are located in the inner nuclear, ganglion cell, and nerve fiber layers. A cellular tapetum lucidum (t) is shown in the left half of the schematic drawing.

FIGURE 17-4 The choroid layer (C) is located between the retina (R) and sclera (S). The choroid is highly vascularized and many pigmented cells are present.

Suprachoroid Layer

The suprachoroid layer is the most external layer (Fig. 17-3) of the choroid and is the transition between the sclera and the choroid. It consists of bundles of collagen and some elastic fibers, fibrocytes, and numerous melanocytes.

Vascular Layer

Numerous large arteries and veins, separated by a stroma similar to that of the suprachoroid layer, make up the vascular layer (Fig. 17-3). These vessels provide a major source of oxygen and nutrients to the retina.

Tapetum Lucidum

A layer of medium vessels and connective tissue lies internal to the vascular layer. The dorsal portion of this layer contains the tapetum lucidum, which acts as a light-reflecting layer, supposedly increasing light perception under conditions of poor illumination. In herbivores, the tapetum is fibrous (tapetum fibrosum), consisting of intermingling collagen fibers and a few fibrocytes. In carnivores, the tapetum consists of a varying number of layers of flat polygonal cells (tapetum cellulosum) that appear bricklike in cross section (Figs. 17-3 and 17-5; see also Fig. 17-9). The thickness of the tapetum varies, being multilayered at its center (up to 15 cell layers thick in dogs and 35 cell layers thick in cats) and thinning to a single cell at its periphery. The tapetal cells are packed with bundles of parallel small rods, all of which are oriented with their long axes parallel to the retinal surface. In cats, the rods in the inner tapetal cells may be modified melanosomes, which are present along with typical melanosomes in the outer tapetal cells near the sclera. Zinc is associated with the rods in dogs and riboflavin with the rods in cats. Diffraction of light as a result of the spatial orientation of the rods (or of the collagen fibrils in herbivores) is probably responsible for producing the light reflection of the tapetum. In swine and camelids, the tapetum is absent.

FIGURE 17-5 Electron micrograph of the feline tapetum lucidum illustrating the bricklike arrangement of cells and bundles of parallel rods (arrows) oriented in various directions with their long axes perpendicular to the angle of incident light and a tapetal cell nucleus (n) (×3780). (Courtesy of E. J. King.)

Choriocapillary Layer

The choriocapillary layer (lamina choroidocapillaris) is a dense network of capillaries immediately adjacent to the pigmented epithelium of the retina (Fig. 17-3). Wide capillaries often deeply indent the pigmented epithelial cells. The endothelium is fenestrated, and endothelial nuclei and pericytes are located toward the choroidal side of the capillaries only. The basal laminae of the capillary and pigmented epithelial are fused. Capillaries provide nutrients to the pigmented epithelium and photoreceptors (rods and cones).

Basal Complex

The basal complex (complexus basalis) is also referred to as Bruch’s membrane. The complex serves as a barrier between the blood in the choriocapillary vessels and the retinal pigmented epithelium. Species variation occurs among domestic animals with respect to the degree of development and thickness of the basal complex. In species without a tapetum, the basal complex has five layers: basal lamina of the retinal pigmented epithelium, two layers of collagen with an intervening band of elastic fibers, and the basal lamina of the choriocapillary endothelium. In species with a tapetum, the complex has three layers: the two basal laminae separated by a layer of collagen.

Ciliary Body

The ciliary body is the direct anterior continuation of the choroid (Fig. 17-1). It begins posteriorly at the ora serrata, a sharply outlined dentate border that marks the transition between the optic part (pars optica retinae) and the blind part (pars caeca retinae) of the retina. Anteriorly, the ciliary body is continuous with the iris and participates in the formation of the trabecular meshwork of the iridocorneal angle. All layers of the choroid extend into the ciliary body, except the tapetum lucidum and the choriocapillary layer.

Anteriorly, the ciliary body projects ciliary processes into the posterior chamber (Fig. 17-1). Collectively, the ciliary processes form a region of the ciliary body referred to as the pars plicata (corona ciliaris). The processes greatly increase the surface area for production of aqueous humor and also serve as the origin for zonular fibers, which attach to the lens. The posterior portion of the ciliary body is flat and smooth and is referred to as the pars plana (orbiculus ciliaris). Histologically, the ciliary body consists of ciliary epithelium, a vascular layer, and the ciliary muscle.

Ciliary Epithelium

The ciliary body is covered by two layers of cuboidal epithelial cells of neuroepithelial origin. Cells of the epithelial layers are joined apex to apex by cell junctions, with the basal laminae facing toward the outsides of the fused epithelial layers.

FIGURE 17-6 This canine ciliary process is covered by pigmented and nonpigmented epithelial cells. Remnants of zonular fibers (arrow) are present in the posterior chamber. Masson’s trichrome (×600).

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