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Eye Anatomy Introductory article Article Contents Thomas C Litzinger, Miami University, Oxford, Ohio, USA Katia Del Rio-Tsonis, Miami University, Oxford, Ohio, USA . Introduction The eye is a small yet multifaceted unit of anatomical machinery in which each structure works in accord with the next, refracting, constricting, dilating and chemically reacting to convert patterns of light into discernible images. Eyes can be divided into two broad categories: the ‘simple’ eye of vertebrates and the compound eye of invertebrates. . An Overview of the Basic Structures and Functions of the Simple Eye: Human as Primary Model . The Retina as a Part of the Central Nervous System . Metabolic Support for Photoreceptor Cells from the Retinal Pigment Epithelium . 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 would be incorrect to say that one is superior to the next. This is the perfection that the eye beholds; each eye has evolved to suit precisely the necessities of its possessor. Though numerous and intricate, the many eyes of the world can be placed into two very general categories: simple and compound. Though different in appearance, these two models of the eye are actually quite similar in their most elementary functional components. One particularly well-conserved molecule between organisms is the light-absorbing protein opsin, which essentially initiates the sequence of events leading to image formation. Though the most basic visual molecules such as opsin have not been selectively altered in a drastic way by environmental pressures, the anatomical morphology of the eye has. This divergent evolution has led to the formation of such dissimilar eyes as that of the human (simple eye), the fly (compound eye), and many in between. An Overview of the Basic Structures and Functions of the Simple Eye: Human as Primary Model Along light’s journey through the eye it is slowed down, bent, absorbed, and converted by various structures (Figure 1a). As light approaches the eye it first comes in contact with the cornea. The cornea refracts the light, causing the image to converge on its way to the iris and pupil. Depending on the intensity and availability of the light, the iris will contract or expand adjusting the pupil size. In situations of low light, the pupil will be larger, allowing for the passage of enough light to form a discernible image. The opposite is true in situations of abundant light, for an excess of light results in poor imaging as well. Once through the gate of the pupil, the light is received by the lens. With the aid of auxiliary muscles, the lens possesses the ability to change shape. Depending on its form, objects at various distances can be brought into focus. The lens also slightly improves the already refined image from the cornea, and projects it onto the retina. The retina, which literally means ‘net’, catches the light via its photoreceptor and pigment epithelial cells. The photoreceptor cells’ photopigment molecules absorb the light, causing a change in the photoreceptor’s membrane potential. This initiates a series of signals that travel through the neurons of the retina, and into the optic nerve leading to the brain. This signal is then received and processed by the brain as an interpretable image. A closer look at the structures involved in the entry of light into the eye The cornea 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 homogeneous elastic lamina and a single layer of endothelial cells. The cornea is the primary contributor in the focusing of light on the retina. Following the basic laws of refraction, as incident light encounters a medium possessing a greater refractive index than that of air, propagation slows down, thus bending the beam’s path. The cornea would be an example of such a medium, possessing refracting capabilities. When light hits the surface of the cornea, it slows down and converges towards the centre of the eye, thus reducing the image that has been reflected to the eye. Though the cornea bends light, its transmission is very characteristic of the transparent media that it is, the main characteristic of transparency being the minimal scattering of light, and the continuing transmission of light in its original direction, both of which contribute to discernible image formation. These intrinsic properties of the cornea are made possible by the spatial 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 makes up the first of many critical components of the functioning eye. The pupil and iris The light must cross through the aqueous humour, the body of fluid that fills the anterior chamber between the cornea and the lens, so that it can reach the next group of 2 structures: the iris and pupil. The two structures work as the regulators of the amount of light passing through the 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, respectively. This contraction and dilation regulates the aperture of the eye, the pupil. In cases of abundant light, the iris lessens the pupillary aperture with the aid of the sphincter muscles, trying to avoid the admittance of too 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 of a muddled blur. The opposite is true when light is lacking. The pupil becomes greatly dilated in an attempt to gather as many photons as possible for imaging. The lens Once the correct amount of light has entered the eye through the pupil, it encounters the lens. The lens, composed of a lens epithelium layer covering a mass of lens fibres, is primarily made up of proteins called crystallins, which further refine the image from the cornea. Like the cornea, the molecules of the lens are densely packed and uniformly spaced. This is necessary for its transparency. The lens has an inherently greater index of refraction than that of the cornea, based on its environment needs. Since the lens is surrounded by the fluid of the aqueous humour and the vitreous humour, which have a relatively high index of refraction, the index of the lens must be higher still if it is to focus the image further and contribute to the optical system. Though the lens has an inherent refractive index, it actually has the ability to change its degree of refraction with the aid of ciliary muscles. When discussing the process of accommodation, the active altering of the shape of the lens to bring close objects into focus, it would be appropriate to start with the ciliary zonule. The ciliary zonule consists of a series of thin, peripheral ligaments that suspend and hold the lens in place (also known as suspensory ligaments). These ligaments, or fibres, are attached to the area of the ciliary muscle called the ciliary body. The ciliary body and the zonule fibres work in conjunction to alter the focal point of the eye. When the eye is in its most relaxed state, it is focusing at distances beyond 6 metres (20 feet). In this state, the ciliary muscle is relaxed, and the zonular fibres are taut, thus pulling outward on the lens forcing it to assume a rather flattened shape. When the eye focuses on an object within 6 metres, the ciliary muscle must contract or close, as the tension in the zonular fibres is reduced. This results in a thickening and bulging of the lens, in turn increasing optical power, bringing the focal point closer and creating a clear image of an object within 6 metres of the viewer. The Retina as a Part of the Central Nervous System The viewer would never perceive this image if it were not for the retina. The retina is the light-processing centre of the eye, where light signals are transformed into neural signals that can be perceived and processed by the brain. The neural cells involved in this process are remarkably similar to those of the brain, which supports the common assertion that the visual system is an outgrowth of the central nervous system. The retina is in fact the only part of the human central nervous system that is exposed to stimuli from the outside environment. Organization of the retina into the different cell and synaptic layers The retina can be divided into many distinguishable layers (Figure 1b). The first layer to interact with light coming from the lens is the retinal pigment epithelium (RPE) layer. The RPE cells do not contribute directly to the transformation and transduction of information in the retina, but do provide supportive functions to the photoreceptor cells, which lie just above this layer. The next set of cells, making up the photoreceptor layer, are the first of three neural cell types (photoreceptor, bipolar cells and ganglion cells) that contribute to the vertical transferring of signals in the retina. This photoreceptor layer consists of the outer and inner segments of the rods and cones, which receive and transform photons of light. The nuclei of these photoreceptor cells reside in the outer nuclear layer and their axons and cell terminals in the outer plexiform layer and the outer synaptic layer, respectively. The outer synaptic layer represents the site where the photoreceptors first interact with the bipolar cells and other retinal neurons and marks the transition between the ‘outer and inner layers’ of the retina. Like the outer layers, the inner layers can be divided into nuclear and plexiform layers. The inner nuclear layer contains the nuclei of bipolar cells, horizontal cells, and the majority of the amacrine cells. The inner nuclear layer is followed by the inner plexiform layer, where vertical communication between the bipolar cells and the ganglion cells takes place, thus making up the second synaptic contact layer. The next layer, the ganglion cell layer, contains the cell bodies of the ganglion cells. The dendrites of the ganglion cells actually extend into the inner plexiform layer, whereas their axons extend in the opposite direction towards the nerve fibre layer. It is through this layer that all of the ganglion cells’ axons travel in the direction of the optic nerve. Five different types of neurons Now that the groundwork of the retina has been laid out, the cells already mentioned can be discussed further, starting with the five different kinds of neurons in the retina. The first three neurons are involved in the vertical transmission of information through the retina, beginning with the photoreceptor cells. These cells are responsible for initiating the cascade of events that takes an image projected onto a layer of tissue (retina), and converting it from photons to an electrochemical signal capable of being read by the brain. This conversion of light energy to informative chemical energy is called phototransduction. The two types of cells involved in this process are the photoreceptor cells: rods and cones. ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 3 Eye Anatomy Rods and cones Of the 130 million photoreceptors, about 120 million are long cylindrical structures known as rods. Rods are extremely sensitive to light, and only send shades of grey to the brain. Cones are a thicker, usually shorter version of rods that register fine detail and colour, provided they receive enough light. The most critical element in this process of phototransduction is the photopigments contained within the rods and cones. Both cells contain the light sensitive protein opsin, as mentioned previously. In rods this protein binds to a straight chain of vitamin A, assuming a bent position. When in this conformation, the complex is called rhodopsin. When as much as a single photon of light strikes this compound, the energy absorbed causes the bent vitamin A chain to snap back into its original straightened form. This occurrence consequently disrupts the electrical field within the photoreceptor, initiating an electrical impulse that begins its journey to the brain. The cones possess three different forms of opsins capable of binding to vitamin A. Each compound eventually is responsible for the creation of one of the three primary colours (red, blue or yellow), as interpreted by the brain. However, as mentioned earlier, cones are much less sensitive to low intensities of light, and therefore require a very specific wavelength of light to initiate the electrical impulse. This is why our daylight environment is full of brilliant colours, whereas our rod-dominated night vision produces various shades of grey. Essentially, the world is colourless. Colours are merely biochemical interpretations of wavelengths of light, whose identity is dependent on the biochemical make-up of the particular organism in question. Bipolar cells The next set of neurons that propagate the vertical, or direct, communication pathway are the bipolar cells. As stated earlier, their cell bodies reside in the inner nuclear layer while their dendrites receive signals from the photoreceptors at the first synaptic junction. On the opposite end of the cell body the signal travels through the bipolar cell’s axon to synapse with the next vertical neuron, the ganglion cell. Lateral neurons: horizontal and amacrine cells The electrical impulses running through the vertical neurons are not completely independent of one another, because most are linked by lateral neurons. One type of lateral neuron is the horizontal cell. Horizontal cells are found in the inner nuclear layer of the retina. These cells are commonly linked to more than one photoreceptor, meaning that the subsequent bipolar cells receive signals from more than one photoreceptor. This pathway would intuitively seem to lessen visual acuity, but in most cases serves a useful purpose, as it increases the perception of contrast. The final type of neuron in the retina is another 4 lateral body, the amacrine cell. These cells form links between vertical pathway neurons in the inner layers, and sometimes the ganglion layer of the retina. Their effects are not entirely clear, but they are thought to contribute to the effect of contrast. Retinal ganglion and output from the retina The last neurons of the network to receive the signals are the retinal ganglion cells. When activated by an incoming signal, the ganglion cells produce an action potential that begins its journey down the cells’ axons. The axons of the ganglion cells of the retina converge, forming the optic nerve. The optic nerve represents a highway for electrical signals en route to the brain. Metabolic Support for Photoreceptor Cells from the Retinal Pigment Epithelium The RPE is located underneath the neural retina and it is characterized by having tight junctions, forming the blood–retinal barrier. The RPE regulates and transports ions, water, growth factors and nutrients such as glucose and amino acids to the outer portions of photoreceptors. The RPE is also involved in the maintenance of retinal cell adhesion by supporting the interphotoreceptor matrix (IPM). This extracellular matrix is bound to the outer limiting membrane and the apical membrane of the RPE (membrane facing photoreceptors). The IPM is critical for the metabolic exchanges between the photoreceptors and the RPE. Its bonding properties and viscosity are regulated by the RPE, which tightly controls the ionic environment in that region. The RPE cells are essential for the regeneration of photopigments, since they uptake, store and reisomerize vitamin A, which is necessary for the future synthesis of rhodopsin used by photoreceptors. The RPE also phagocytoses the tips of the outer segments of photoreceptors on a regular basis, then it digests the absorbed material to finally recycle it. Melanin, the visual pigment present in the RPE, reduces scatter to the photoreceptors and shields them from excessive light exposure. Focusing of Light onto the Fovea in Primates As mentioned earlier, the cones of the eye are responsible for discerning minute details. The highest concentration of cone photoreceptors is found at the centre of an area of the retina called the fovea (Figure 1a). The fovea is about 1500 mm in diameter, a third of which comprises cone photoreceptors. This area contains the highest frequency ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Eye Anatomy of cones per unit area in the entire retina. However, this structural oddity goes beyond its compositional homogeneity; it actually lacks many of the common retinal layers as well. The only stratifications present are the pigment epithelium cells, photoreceptor layer, the outer nuclear layer, and a bit of the outer plexiform. Owing to its compositional nature and resolving capabilities, the fovea is an obvious target for light as it enters the eye. The cornea and lens make it possible to focus light onto this small area in order to produce images possessing the finest details we are capable of visualizing. The Compound Eye The human and other vertebrate eyes are considered to be simple not because of any restraints of function, but because they consist of a singular optical system (primarily the cornea and lens). On the other hand, the eyes of most insects and a few crustaceans are considered compound, with each eye possessing multiple components or optical systems. The surface of the compound eye is divided into separate circular or hexagonal facets that act as individual refractive units, called ommatidia. Each ommatidium does not receive an entire image, but rather a small part of the whole. Each parcel of the perceived image travels through the optical system of the organism, and is eventually fused to some degree, creating the overall image (Figure 2). Many structures of the ommatidium are analogous to those of the simple eye. On the surface of an ommatidium is the cornea, followed by a conical lens. The two structures may be physically separated, or fused depending on the organism. Either way, the lens cannot be adjusted; therefore the compound eye is a fixed-focus eye. Only the position of the organism can determine what objects are in focus. Just below the cornea and lens there are nerve cells called retinula cells, which contain photoreceptors called rhabdomes. There are anywhere between one and eight rhabdomes in each retinula cell, which are encased by a periphery of pigment cells that absorb any excessive light. However, it is in the rhabdomes that an image ‘forms’. As in most organisms, this image is sent through a series of neural fibres to the brain. The two types of compound eyes There are two types of compound eyes: the apposition and superposition eye. The apposition eye is characterized by the optical system being continuous with the photoreceptor cells, and is usually a trait of insects adapted to a well-lit environment. The lens and rhabdomes are constantly surrounded by pigment, not allowing transmittance into adjacent ommatidia. This results in many isolated imaging systems, whose photoreceptors only receive the light that enters their respective cornea and lens. Figure 2 Cross-section of a compound eye illustrating a group of ommatidia, and an enlarged view of a single ommatidium. ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 5 Eye Anatomy The superposition eye is characterized by a separation between the optical system and the photoreceptor cells, and is commonly found in nocturnal organisms. Being creatures of the night, they must gather as much light as possible, so the ommatidia are often not completely surrounded by pigment cells. The intended function of this is to enable the ommatidia to share incoming light in an attempt to form an image. The effort is often aided by the convergence of light from many optical systems onto a single rhabdome. Evolutionary Trends of Eye Structures It is believed that eyes have evolved over 40 times, independently, during the course of their evolution. It is no wonder that there is such a diversity of eye structures found in nature. The planarian or flatworm eye represents the most primitive invertebrate eye, made up of visual cells within a pigmented mantle. In contrast, arthropods have complex eyes consisting of up to 800 ommatidia, each containing all the basic eye components. These basic components of an ommatidium include the cornea and/or lens for focusing light, pigment cells with absorbing and/or reflecting properties, and retinula cells essential for light processing. It is believed that the vertebrate eye evolved independently from its invertebrate counterpart, keeping the basic eye function but increasing its complexity to accommodate the needs of the organisms. This adjustment resulted in the production of a complex eye that contained elaborated focusing equipment including a cornea, lens, pupil and iris. Figure 3 (a) Top: cross-section of the newt (vertebrate) eye. Bottom: scanning electron micrograph of a cross-section of the newt eye; 35 magnification. Evident structures include the retina (R), iris (I), cornea (C), and lens (L). (b) Top: cross-section of a small area of the Drosophila compound eye. Bottom: scanning electron micrograph of a cross-section of the Drosophila eye; 648 magnification. Evident structures include the cornea (C), and ommatidium (O). The thin, hair-like structures are setae (S), and are believed to reduce glare. (c) Top: cross-section of the squid (invertebrate) eye, illustrating the striking resemblances to the vertebrate eye. Bottom: scanning electron microscope composite image of a cross-section of the squid eye; 5 magnification. Evident structures include the retina (R) and lens (L). 6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Eye Anatomy These eyes also contained an intrinsic light-processing machinery made up of neural retina cells and the important supportive cells, the RPE cells. It is amazing, then, how the cephalopod eye (an invertebrate eye) developed so similarly to the vertebrate eye (see Figure 3). Overall, the basic eye function of detecting and transforming light signals into neuronal signals has been conserved regardless of the origin of the eye. Each structure works in accord with the next, refracting, constricting, dilating, and chemically reacting to convert patterns of light into interpretable images. Not only are the mechanisms numerous, but they occur involuntarily and with extremely high frequency. The functions of the eye represent a symphony of activity that has been perfected over millions of years, resulting in each organism’s detector of light, their sculptor of subjective reality, their own respective evolutionary masterpiece. Summary Following the light path through the vertebrate and invertebrate eye, we have compared the light-focusing structures as well as the light-transforming cells in both the vertebrate eye and the compound eye of invertebrates. The different evolutionary trends that shaped the eyes of the world have also been discussed. Charles Darwin asserted that the eye is both perfect and complex. The eye is a small yet multifaceted unit of anatomical machinery, with intricate design and function. Further Reading Dawkins R (1996) The forty-fold path to enlightenment. Climbing Mount Improbable, pp. 138–197. New York: WW Norton. Kessel D and Kardon RH (1979) Tissues and Organs: A Text-atlas of Scanning Electron Microscopy. San Francisco: WH Freeman. Marmor MF and Wolfensberger TJ (1998) The Retinal Pigment Epithelium. New York: Oxford University Press. Oyster CD (1999) The Human Eye. Sunderland, MA: Sinauer Associates. Sinclair S (1985) How Animals See. New York: Facts On File. ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 7