Eye Anatomy Introductory article Thomas C Litzinger, Miami University, Oxford, Ohio, USA Article Contents Katia Del Rio-Tsonis, Miami University, Oxford, Ohio, USA . Introduction . An Overview of the Basic Structures and Functions of the Simple Eye: Human as Primary Model The eye is a small yet multifaceted unit of anatomical machinery in which each structure . The Retina as a Part of the Central Nervous System works in accord with the next, refracting, constricting, dilating and chemically reacting to . Metabolic Support for Photoreceptor Cells from the convert patterns of light into discernible images. Eyes can be divided into two broad Retinal Pigment Epithelium categories: the ‘simple’ eye of vertebrates and the compound eye of invertebrates. Focusing of Light onto the Fovea in Primates . The Compound Eye Introduction . Evolutionary Trends of Eye Structures . Summary The eye has been described by Charles Darwin as both perfect and complex. There are several structural and functional variations that exist between organisms, yet it muscles, the lens possesses the ability to change shape. would be incorrect to say that one is superior to the next. Depending on its form, objects at various distances can be This is the perfection that the eye beholds; each eye has brought into focus. The lens also slightly improves the evolved to suit precisely the necessities of its possessor. already refined image from the cornea, and projects it onto Though numerous and intricate, the many eyes of the the retina. The retina, which literally means ‘net’, catches world can be placed into two very general categories: the light via its photoreceptor and pigment epithelial cells. simple and compound. The photoreceptor cells’ photopigment molecules absorb Though different in appearance, these two models of the the light, causing a change in the photoreceptor’s eye are actually quite similar in their most elementary membrane potential. This initiates a series of signals that functional components. One particularly well-conserved travel through the neurons of the retina, and into the optic molecule between organisms is the light-absorbing protein nerve leading to the brain. This signal is then received and opsin, which essentially initiates the sequence of events processed by the brain as an interpretable image. leading to image formation. Though the most basic visual molecules such as opsin have not been selectively altered in A closer look at the structures involved in the a drastic way by environmental pressures, the anatomical morphology of the eye has. This divergent evolution has entry of light into the eye led to the formation of such dissimilar eyes as that of the The cornea human (simple eye), the fly (compound eye), and many in between. As mentioned, along the path of light into the eye, it will first encounter the cornea, which is a transparent body consisting of an epithelium, a thick fibrous structure made up of connective tissue and extracellular matrix, a An Overview of the Basic Structures and homogeneous elastic lamina and a single layer of endothelial cells. The cornea is the primary contributor Functions of the Simple Eye: Human as in the focusing of light on the retina. Following the basic Primary Model laws of refraction, as incident light encounters a medium possessing a greater refractive index than that of air, Along light’s journey through the eye it is slowed down, propagation slows down, thus bending the beam’s path. bent, absorbed, and converted by various structures The cornea would be an example of such a medium, (Figure 1a). As light approaches the eye it first comes in possessing refracting capabilities. When light hits the contact with the cornea. The cornea refracts the light, surface of the cornea, it slows down and converges towards causing the image to converge on its way to the iris and the centre of the eye, thus reducing the image that has been pupil. Depending on the intensity and availability of the reflected to the eye. Though the cornea bends light, its light, the iris will contract or expand adjusting the pupil transmission is very characteristic of the transparent media size. In situations of low light, the pupil will be larger, that it is, the main characteristic of transparency being the allowing for the passage of enough light to form a minimal scattering of light, and the continuing transmis- discernible image. The opposite is true in situations of sion of light in its original direction, both of which abundant light, for an excess of light results in poor contribute to discernible image formation. These intrinsic imaging as well. Once through the gate of the pupil, the properties of the cornea are made possible by the spatial light is received by the lens. With the aid of auxiliary uniformity of its cells, which contribute to the acuity of ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 1 Eye Anatomy Figure 1 (a) Three-dimensional representation of the structures of the human eye. (b) Cross-section of the human eye, and an enlarged view of the various layers of the retina. light transmission. With these elements present, the cornea structures: the iris and pupil. The two structures work as makes up the first of many critical components of the the regulators of the amount of light passing through the functioning eye. system. The iris is a pigmented sheet of tissue that lies directly in front of the lens, and has the ability to restrict and dilate with the aid of sphincter and dilator muscles, The pupil and iris respectively. This contraction and dilation regulates the The light must cross through the aqueous humour, the aperture of the eye, the pupil. In cases of abundant light, body of fluid that fills the anterior chamber between the the iris lessens the pupillary aperture with the aid of the cornea and the lens, so that it can reach the next group of sphincter muscles, trying to avoid the admittance of too 2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Eye Anatomy much light which would eventually result in the processing the human central nervous system that is exposed to stimuli of a muddled blur. The opposite is true when light is from the outside environment. lacking. The pupil becomes greatly dilated in an attempt to gather as many photons as possible for imaging. Organization of the retina into the different The lens cell and synaptic layers Once the correct amount of light has entered the eye The retina can be divided into many distinguishable layers through the pupil, it encounters the lens. The lens, ( ). The first layer to interact with light coming from composed of a lens epithelium layer covering a mass of Figure 1b the lens is the retinal pigment epithelium (RPE) layer. The lens fibres, is primarily made up of proteins called RPE cells do not contribute directly to the transformation crystallins, which further refine the image from the cornea. and transduction of information in the retina, but do Like the cornea, the molecules of the lens are densely provide supportive functions to the photoreceptor cells, packed and uniformly spaced. This is necessary for its which lie just above this layer. The next set of cells, making transparency. The lens has an inherently greater index of up the photoreceptor layer, are the first of three neural cell refraction than that of the cornea, based on its environ- types (photoreceptor, bipolar cells and ganglion cells) that ment needs. Since the lens is surrounded by the fluid of the contribute to the vertical transferring of signals in the aqueous humour and the vitreous humour, which have a retina. This photoreceptor layer consists of the outer and relatively high index of refraction, the index of the lens inner segments of the rods and cones, which receive and must be higher still if it is to focus the image further and transform photons of light. The nuclei of these photo- contribute to the optical system. receptor cells reside in the outer nuclear layer and their Though the lens has an inherent refractive index, it axons and cell terminals in the outer plexiform layer and actually has the ability to change its degree of refraction the outer synaptic layer, respectively. The outer synaptic with the aid of ciliary muscles. When discussing the process layer represents the site where the photoreceptors first of accommodation, the active altering of the shape of the interact with the bipolar cells and other retinal neurons and lens to bring close objects into focus, it would be marks the transition between the ‘outer and inner layers’ of appropriate to start with the ciliary zonule. The ciliary the retina. Like the outer layers, the inner layers can be zonule consists of a series of thin, peripheral ligaments that divided into nuclear and plexiform layers. The inner suspend and hold the lens in place (also known as nuclear layer contains the nuclei of bipolar cells, horizontal suspensory ligaments). These ligaments, or fibres, are cells, and the majority of the amacrine cells. The inner attached to the area of the ciliary muscle called the ciliary nuclear layer is followed by the inner plexiform layer, body. The ciliary body and the zonule fibres work in where vertical communication between the bipolar cells conjunction to alter the focal point of the eye. When the eye and the ganglion cells takes place, thus making up the is in its most relaxed state, it is focusing at distances beyond second synaptic contact layer. The next layer, the ganglion 6 metres (20 feet). In this state, the ciliary muscle is relaxed, cell layer, contains the cell bodies of the ganglion cells.