Chapter 14: The Eye
The vascular tunic is comprised of three distinct regions, (1) the iris, (2) the ciliary body, and (3) the choroid. The vascular tunic is mesodermal in origin and is situated between the outer fibrous tunic and the inner nervous tunic. The vascular tunic is also refered to as the uvea.
The iris is the anterior most portion of the vascular tunic and functions as a moveable diaphragm between the anterior and posterior chambers. Embryologically, the iris is comprised of components that originate from neural ectoderm, the sphincter and dilator muscles and the 2-layered posterior epithelium, and mesodermal components that form the iris stroma. The pupil varies in shape between species and can be circular (dog, rabbit), oval (horse, cow), or vertical (cat). This central portion of the iris normally rest against the anterior lens surface and in the absence of this support, the iris will tremble. The color of the iris can vary with species, age, and sex, and can even differ between eyes or within eyes in the same animal. The color of the iris depends on the pigmentation of both the stromal melanocytes and the posterior epithelium. In animals with blue eyes the pigment is absent in the stroma, while in the albino it is lacking in both the stroma and the posterior epithelium.
The anterior border of the iris is often discussed as possessing an epi- or endothelial cell layer. This is incorrect. The anterior border is formed by fibroblasts and melanocytes. The peripheral portion of the iris inserts as the pectinate ligaments in the area where Descemet’s membrane ends. This angle formed by the cornea and the root of the iris is termed the iridocorneal angle and is the entrance to the trabecular meshwork, the site of outflow of the aqueous humor.
The iris stroma is a loose tissue comprised of collagen bundles, blood vessels, nerves, melanocytes, fibroblasts, and the sphincter muscle of the iris. The blood vessels are endothelial lined structures with a basement membrane, smooth muscle, and pericytes. The capillary endothelium is not fenestrated and is part of the blood-eye-barrier.
The sphincter muscle is found within the iris stroma. It is smooth muscle in mammals and striated in birds and lower vertebrates. As stated previously, it has its origin from neural ectoderm. It is situated in the pupillary zone and varies in shape depending on the shape of the pupil. Contraction of the sphincter muscle results in a decrease in the pupil diameter, termed miosis. The sphincter muscle is innervated by parasympathetic fibers from the Edinger-Westphal nucleus, the fibers of which travel with the third cranial nerve (oculomotor).
The posterior epithelium of the iris is actually 2 cell layers, both of neuroectodermal origin. Because of their origin from the invaginating optic cup these cells are situated apex to apex and remnants of the optic vesicle may occasionally persist between these cell layers. The posterior most cell layer is pigmented and is continuous with the non-pigmented layer of the ciliary body and ultimately with the neural retina. It is separated from the posterior chamber by a basal lamina. Adjacent, posterior epithelial cells are attached by desmosomes and terminal bars to each other. The anterior epithelial layer consists of a pigmented apical portion and a myoepithelial basal portion which forms the dilator muscle of the pupil. The dilator muscle is a radially-arranged, smooth muscle and is innervated primarily by sympathetic fibers.
In herbivores a structure termed the corpora nigra or granula iridica is found. This an extension of the posterior epithelium into the pupil both dorsally and ventrally. These appear as 3-4 pigmented, irregular masses. The superior ones are usually larger. They are a normal finding.
The ciliary body, like the iris, contains both neurectodermal and mesodermal tissue. It is divided into 2 parts, the anterior pars plicata and the posterior pars plana. In saggital section the ciliary body is triangular in shape with its base at the iris and the apex posteriorly at the ora ciliaris retinae. The pars plicata possesses 70-100 major ciliary processes and between these ciliary processes are valleys in which can be found smaller minor processes. Each ciliary process contains a vascular core and is covered by a 2-layer epithelium. The ciliary processes are the source of production of the aqueous humor.
The posterior two layered epithelium of the iris continues over the ciliary body as the outer pigmented and inner non-pigmented epithelia. As is the case in the iris, these epithelial cells are arranged apex to apex. At the junction of the ciliary body and the retina, the single layered non-pigmented epithelium (NPE) continues as the multi-layered retina and the pigmented epithelium of the ciliary body continues as the retinal pigment epithelium. Unlike in the retina and the iris, the two cell layers of the ciliary epithelium are strongly attached to each other by terminal bars. In addition, there are zonulae adherentes and zonulae occludens between the apices of the non-pigmented cells. This is the site of the ciliary body blood-aqueous-barrier.
The lenticular zonules that support the lens originate in the pars plana and travel forward in the valleys between the ciliary processes. The zonules ensheath the ciliary processes as they insert on the lens capsule both anterior and posterior to the lens equator. The zonules appear to be elastic microfibrils.
The pigmented cells are joined to each other by desmosomes and the intercellular space of these cells is therefore permeable. The basal portion of the cell faces the ciliary body stroma and is covered by a basal lamina. Beneath the pigmented cell layer is the stroma of the ciliary body. This is mesodermal in origin and contains vessels, nerves, collagen bundles, smooth muscle, melanocytes, and fibroblasts. The stroma of the ciliary processes is a highly vascularized connective tissue with capillaries containing large fenestrations, 300-1000A. The ciliary muscle is smooth and in all primates is comprised of 3 parts: the longitudinal, circular, and radial portions. This muscle is poorly developed in most other mammals, with the longitudinal portion predominating. This muscle is primarily innervated by parasympathetic fibers, but also has some adrenergic innervation. This is the muscle of accommodation and serves to change the shape of the lens.
The choroid is the third component of the uvea. It extends from the edge of the optic nerve to the pars plana. The choroid is loosely attached to sclera. The choroidal stroma varies from being darkly pigmented to a complete absence of pigmentation depending on the species, breed, and individual. In those animals lacking pigmentation, the fundic reflex appears red and on ophthalmoscopic examination the choroidal vessels are visible. The stroma itself is composed of collagen fibrils, melanocytes, fibroblasts, nerves, and larger blood vessels. The majority of the vessels are veins with arteries situated amongst them.
The capillary layer of the choroid, the choriocapillaris, is found in the inner portion just below the retinal pigment epithelium (RPE). It is responsible for the nutrition of the RPE and the outer retina and, in some species, for the entire retinal nutrition. The capillaries form a lobular network and are comprised of typical fenestrated endothelial cells surrounded by a basal lamina that surrounds and is shared by pericytes and smooth muscle cells. These lobules are supplied by a central arteriole and are surrounded by a ring of postcapillary venules. Venous drainage from the choroid occurs in four quadrants where blood collects in an ampulla and then drains via one of four vortex veins that penetrate the sclera.
In addition to above structures, some species also contain a layer in the choroid, the tapetum lucidum, situated between the larger choroidal vessels and the choriocapillaris. Because of its location, the vascular communications between the larger vessels and the choriocapillaris must traverse the tapetum. The tapetum is responsible for the so called “eye-shine” or bright, colored reflection seen in certain species. It is situated in the superior one-half to one-third of the choroid. The color of the tapetum varies with and within species and can be green, blue, yellow, orange, or a variation of these. The tapetum is found in dogs, cats, ferrets, horses, ruminants, and nocturnal animals. It is absent in humans, other primates, pigs, rabbits, rats, guinea pigs, and mice. The tapetum can be cellular (dog, cat, ferret, and other carnivores) or fibrous (horse, ruminants, and other ungulates).