INTRODUCTION

The eye is one of the primary sense organs. This unique organ is composed of highly specialized tissues: an outgrowth of the central nervous system, the retina, on the posterior end and a highly modified transparent skin, the cornea, on the anterior end (Figure 31.1). In the simplest terms, the primary function of the eye is to convert light energy into nerve action potentials. This essential and evolutionarily conserved function directly depends on maintaining (1) the refractive properties of the cornea, lens, aqueous humor, and vitreous humor; (2) formation and reabsorption of fluids that keep intraocular pressure in the fluid-filled eyeball constant; and (3) the function of retinal neurons. The eye, as an organ that faces the external environment, provides a unique opportunity to directly access highly specialized and diverse tissues such as the immune-privileged avascular cornea and the conjunctiva, a mucosal tissue that directs host defense and immune response of the ocular surface. Moreover, since light has to pass through the eye, physiological processes and cells in the anterior (cornea), interior (lens, uvea), and posterior (neural and vascular retinal layers) tissues of the eye (Figure 31.1) can be studied by noninvasive procedures in animals and humans [1 – 7].

The visual axis is a delicate sensory organ that has to maintain an avascular and transparent state in order to preserve ocular function. Hence, in the eye, especially in the cornea and retina, the threat of inflammation is incompatible with good vision.