Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 51

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, llamas, alpacas, and many nonhuman primates. The suprachoroidea consists of elastic, heavily pigmented connective tissue that forms a transition between the sclera and the choroid, and functions as the posterior component for uveoscleral outflow. Aqueous humor that has moved along this narrow junction of the sclera and choroid diffuses into the sclera and, subsequently, the systemic circulation. The layers of melanocytes and fibrocytes and the interspersing collagen and elastic fibers may produce resistance to uveoscleral drainage, even though a cellular barrier has not been found. The long posterior ciliary nerves and arteries course their way anteriorly in the suprachoroidea along the horizontal meridian.

Figure 1.44 (a) The canine choroid (C) consists of the suprachoroidea (1), large‐vessel layer (2), medium‐sized vessel and tapetum layer (3), and choriocapillaris (4). R, retina; BV, blood vessel. Asterisk denotes a nerve within the sclera. (b) SEM shows a close‐up view of the outer choroid, where the suprachoroidea (Su) forms fine collagenous attachments (arrows) with lamina fusca of the sclera (S). (Original magnification, 850×.)

Figure 1.45 SEM of the posterior canine eye shows that the choroid (C) is composed mostly of large, cavernous veins (V) that drain the choriocapillaris (arrow), which nourishes the outer retina (R). S, sclera. (Original magnification, 25×.)

Immediately internal to the suprachoroidea or the large vessel layer is a vascular plexus embedded in loose connective tissue containing melanocytes and fibrocytes. This plexus is composed mostly of large veins and scattered arteries. Four or more prominent vortex veins are located obliquely near the globe equator between the horizontal and vertical meridians. In cross section, the veins are cavern‐like, occupying 50% or more of the total volume of the choroid. The large arteries, which are much fewer in number, are mostly branches of the short posterior ciliary arteries, which enter the globe in the vicinity of the optic nerve and supply the retina, optic nerve, and choroid.

In addition to providing the major source of oxygen and nutrients for the retina, the large vessels may act as a “cooling system,” dissipating the heat produced from light absorption. The osmotic pressure created by high levels of plasma proteins in the choroidal tissue fluid might also assist in keeping the retina attached to the RPE, by allowing retinal fluids to pass into the choroid, and then subsequently into the suprachoroidea, sclera, and episcleral tissues.

A small layer of medium‐sized vessels and pigmented reticular connective tissue lies internal to the large‐vessel layer. These vessels are emissaries between a single sheet of capillaries and the layer of large blood vessels. The medium‐sized vessels, especially the arteries, dichotomously branch, radiating slightly inward in a fanlike manner from the larger vessels.

In most domestic animals, the dorsal portion of the choroid at the medium‐sized vessel layer contains a layer of reflective tissue called the tapetum lucidum. The tapetum is roughly triangular in shape when viewed funduscopically, and it varies in color (Figure 1.46a and b). It reflects light that has passed through the retina and thus restimulates the photoreceptor cells. The tapetum lucidum is responsible both for the “eyeshine” seen at night when the eye is illuminated and for the variable background color of the ocular fundus when viewed ophthalmoscopically during fundic examination. Animals without a tapetum lucidum have diurnal habits and red or orange to pale gray (depending on the amount of choroidal pigmentation) fundic reflections. The tapetal layer is composed of regularly arranged collagenous fibers in herbivores (i.e., the tapetum fibrosum in horses, cattle, sheep, and goats) and of specific polyhedral cells, or iridocytes, containing reflecting crystals in carnivores (i.e., the tapetum cellulosum in the dog and cat). Histologically, the tapetum cellulosum is composed of rectangular‐shaped cells with a species‐dependent variability in number of cell layers. The tapetal layer is thickest centrally and thins toward the periphery until the tapetum cellulosum is replaced by regular choroidal stroma. From the underlying choroidal stroma, numerous small vessels penetrate the tapetal layer to form a single‐layered capillary bed, known as the choriocapillaris network, on the inner surface of the tapetum.

Figure 1.46 The carnivorous tapetum lucidum consists of layers of cells, called iridocytes, which vary in number, size, and composition. (a) The dog. (b) The cat. (Original magnification: all, 200×.)

In ungulates, closely and regularly arranged collagen fibers comprise the tapetum, which is often referred to as a fibrous tapetum. The fibrous tapetum is basically acellular, except for an occasional fibrocyte. The collagen fibrils are organized into well‐ordered lamellae that branch and interconnect with adjacent lamellae at the same level, parallel to the retinal surface.

Small blood vessels, typically capillaries, penetrate the tapetum at right angles to the long axis of the iridocytes in carnivores, and to the collagen lamellae in herbivores, directly interconnecting the medium‐sized blood vessels with the choriocapillaris. When observed ophthalmoscopically, these end‐on vessels are sometimes called the “stars of Winslow.” The choriocapillaris is the innermost layer of choroidal vessels, forming a thin layer of capillaries separated from the RPE by a basement membrane complex known as Bruch's membrane.