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Home , Compound eye, Human eye, Inner plexiform layer

Eye 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 : Human as Primary Model The eye is a small yet multifaceted unit of anatomical machinery in which each structure . The as a Part of the Central works in accord with the next, refracting, constricting, dilating and chemically reacting to . Metabolic Support for Photoreceptor Cells from the convert patterns of into discernible images. can be divided into two broad Pigment Epithelium categories: the ‘simple’ eye of vertebrates and the 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 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 , 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 leading to the brain. This signal is then received and , 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 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 , 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 and the centre of the eye, thus reducing the image that has been . 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 . (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 and dilator muscles, The pupil and iris respectively. This contraction and dilation regulates the The light must cross through the , the 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 called RPE cells do not contribute directly to the transformation , 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 and their Though the lens has an inherent refractive index, it axons and cell terminals in the 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 , 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 called the ciliary nuclear layer is followed by the , body. The 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. The and the zonular fibres are taut, thus pulling outward on the dendrites of the ganglion cells actually extend into the inner lens forcing it to assume a rather flattened shape. When the plexiform layer, whereas their axons extend in the opposite eye focuses on an object within 6 metres, the ciliary muscle direction towards the nerve fibre layer. It is through this must contract or close, as the tension in the zonular fibres is layer that all of the ganglion cells’ axons travel in the reduced. This results in a thickening and bulging of the direction of the . lens, in turn increasing , bringing the focal point closer and creating a clear image of an object within 6 metres of the viewer. Five different types of neurons Now that the groundwork of the retina has been laid out, the cells already mentioned can be discussed further, The Retina as a Part of the Central starting with the five different kinds of neurons in the Nervous System retina. The first three neurons are involved in the vertical transmission of information through the retina, beginning The viewer would never perceive this image if it were not with the photoreceptor cells. These cells are responsible for for the retina. The retina is the light-processing centre of initiating the cascade of events that takes an image the eye, where light signals are transformed into neural projected onto a layer of tissue (retina), and converting it signals that can be perceived and processed by the brain. from photons to an electrochemical signal capable of being The neural cells involved in this process are remarkably read by the brain. This conversion of light energy to similar to those of the brain, which supports the common informative chemical energy is called phototransduction. assertion that the is an outgrowth of the The two types of cells involved in this process are the central nervous system. The retina is in fact the only part of 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 lateral body, the . These cells form links Of the 130 million photoreceptors, about 120 million are between vertical pathway neurons in the inner layers, and long cylindrical structures known as rods. Rods are sometimes the ganglion layer of the retina. Their effects are extremely sensitive to light, and only send shades of grey not entirely clear, but they are thought to contribute to the effect of contrast. to the brain. Cones are a thicker, usually shorter version of rods that register fine detail and colour, provided they Retinal ganglion and output from the retina receive enough light. The most critical element in this process of phototransduction is the photopigments con- The last neurons of the network to receive the signals are tained within the rods and cones. Both cells contain the the retinal ganglion cells. When activated by an incoming light sensitive protein opsin, as mentioned previously. In signal, the ganglion cells produce an action potential that rods this protein binds to a straight chain of vitamin A, begins its journey down the cells’ axons. The axons of the assuming a bent position. When in this conformation, the ganglion cells of the retina converge, forming the optic complex is called . When as much as a single nerve. The optic nerve represents a highway for electrical photon of light strikes this compound, the energy absorbed signals en route to the brain. 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 Metabolic Support for Photoreceptor the brain. The cones possess three different forms of Cells from the Retinal Pigment capable of binding to vitamin A. Each compound eventually is responsible for the creation of one of the Epithelium three primary colours (red, or yellow), as interpreted The RPE is located underneath the neural retina and it is by the brain. However, as mentioned earlier, cones are characterized by having tight junctions, forming the much less sensitive to low intensities of light, and therefore blood–retinal barrier. The RPE regulates and transports require a very specific wavelength of light to initiate the ions, water, growth factors and nutrients such as glucose electrical impulse. This is why our daylight environment is and amino acids to the outer portions of photoreceptors. full of brilliant colours, whereas our rod-dominated night The RPE is also involved in the maintenance of retinal cell vision produces various shades of grey. Essentially, the adhesion by supporting the interphotoreceptor matrix world is colourless. Colours are merely biochemical (IPM). This extracellular matrix is bound to the outer interpretations of wavelengths of light, whose identity is limiting membrane and the apical membrane of the RPE dependent on the biochemical make-up of the particular (membrane facing photoreceptors). The IPM is critical for organism in question. the metabolic exchanges between the photoreceptors and the RPE. Its bonding properties and viscosity are regulated Bipolar cells by the RPE, which tightly controls the ionic environment The next set of neurons that propagate the vertical, or in that region. The RPE cells are essential for the direct, communication pathway are the bipolar cells. As regeneration of photopigments, since they uptake, store stated earlier, their cell bodies reside in the inner nuclear and reisomerize vitamin A, which is necessary for the layer while their dendrites receive signals from the future synthesis of rhodopsin used by photoreceptors. The photoreceptors at the first synaptic junction. On the RPE also phagocytoses the tips of the outer segments of opposite end of the cell body the signal travels through photoreceptors on a regular basis, then it digests the the bipolar cell’s axon to synapse with the next vertical absorbed material to finally recycle it. Melanin, the visual neuron, the ganglion cell. pigment present in the RPE, reduces scatter to the photoreceptors and shields them from excessive light Lateral neurons: horizontal and amacrine cells exposure. 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 Focusing of Light onto the Fovea in lateral neuron is the horizontal cell. Horizontal cells are Primates found in the of the retina. These cells are commonly linked to more than one photoreceptor, mean- As mentioned earlier, the cones of the eye are responsible ing that the subsequent bipolar cells receive signals from for discerning minute details. The highest concentration of more than one photoreceptor. This pathway would cone photoreceptors is found at the centre of an area of the intuitively seem to lessen , but in most cases retina called the fovea (Figure 1a). The fovea is about serves a useful purpose, as it increases the of 1500 mm in diameter, a third of which comprises cone contrast. The final type of neuron in the retina is another photoreceptors. This area contains the highest frequency

4 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 Many structures of the are analogous to structural oddity goes beyond its compositional homo- those of the simple eye. On the surface of an ommatidium is geneity; it actually lacks many of the common retinal layers the cornea, followed by a conical lens. The two structures as well. The only stratifications present are the pigment may be physically separated, or fused depending on the epithelium cells, photoreceptor layer, the outer nuclear organism. Either way, the lens cannot be adjusted; layer, and a bit of the outer plexiform. Owing to its therefore the compound eye is a fixed-focus eye. Only the compositional nature and resolving capabilities, the fovea position of the organism can determine what objects are in is an obvious target for light as it enters the eye. The cornea focus. Just below the cornea and lens there are nerve cells and lens make it possible to focus light onto this small area called retinula cells, which contain photoreceptors called in order to produce images possessing the finest details we rhabdomes. There are anywhere between one and eight are capable of visualizing. 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 The Compound Eye neural fibres to the brain. 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 two types of compound eyes the cornea and lens). On the other , the eyes of most and a few are considered compound, There are two types of compound eyes: the apposition and with each eye possessing multiple components or optical superposition eye. The apposition eye is characterized by systems. The surface of the compound eye is divided into the optical system being continuous with the photorecep- separate circular or hexagonal facets that act as individual tor cells, and is usually a trait of insects adapted to a well-lit refractive units, called ommatidia. Each ommatidium does environment. The lens and rhabdomes are constantly not receive an entire image, but rather a small part of the surrounded by pigment, not allowing transmittance into whole. Each parcel of the perceived image travels through adjacent ommatidia. This results in many isolated imaging the optical system of the organism, and is eventually fused systems, whose photoreceptors only receive the light that to some degree, creating the overall image (Figure 2). 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 no wonder that there is such a diversity of eye structures between the optical system and the photoreceptor cells, found in nature. The planarian or flatworm eye represents and is commonly found in nocturnal organisms. Being the most primitive invertebrate eye, made up of visual cells creatures of the night, they must gather as much light as within a pigmented mantle. In contrast, have possible, so the ommatidia are often not completely complex eyes consisting of up to 800 ommatidia, each surrounded by pigment cells. The intended function of containing all the basic eye components. These basic this is to enable the ommatidia to share incoming light in an components of an ommatidium include the cornea and/or attempt to form an image. The effort is often aided by the lens for focusing light, pigment cells with absorbing and/or convergence of light from many optical systems onto a reflecting properties, and retinula cells essential for light single rhabdome. processing. It is believed that the vertebrate eye evolved indepen- dently from its invertebrate counterpart, keeping the basic Evolutionary Trends of Eye Structures eye function but increasing its complexity to accommodate the needs of the organisms. This adjustment resulted in the It is believed that eyes have evolved over 40 times, production of a complex eye that contained elaborated independently, during the course of their evolution. It is 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; Â 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; Â 648 magnification. Evident structures include the cornea (C), and ommatidium (O). The thin, -like structures are setae (S), and are believed to reduce glare. (c) Top: cross-section of the (invertebrate) eye, illustrating the striking resemblances to the vertebrate eye. Bottom: scanning electron microscope composite image of a cross-section of the squid eye; Â 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 Each structure works in accord with the next, refracting, machinery made up of neural retina cells and the important constricting, dilating, and chemically reacting to convert supportive cells, the RPE cells. It is amazing, then, how the patterns of light into interpretable images. Not only are the (an invertebrate eye) developed so mechanisms numerous, but they occur involuntarily and similarly to the vertebrate eye (see Figure 3). Overall, the with extremely high frequency. The functions of the eye basic eye function of detecting and transforming light represent a symphony of activity that has been perfected signals into neuronal signals has been conserved regardless over millions of years, resulting in each organism’s detector of the origin of the eye. of light, their sculptor of subjective reality, their own respective evolutionary masterpiece.

Summary Further Reading Following the light path through the vertebrate and invertebrate eye, we have compared the light-focusing Dawkins R (1996) The forty-fold path to enlightenment. Climbing structures as well as the light-transforming cells in both the Mount Improbable, pp. 138–197. New York: WW Norton. vertebrate eye and the compound eye of invertebrates. The Kessel D and Kardon RH (1979) Tissues and Organs: A Text-atlas of different evolutionary trends that shaped the eyes of the Scanning Electron Microscopy. San Francisco: WH Freeman. Marmor MF and Wolfensberger TJ (1998) The Retinal Pigment world have also been discussed. Epithelium. New York: Oxford University Press. Charles Darwin asserted that the eye is both perfect and Oyster CD (1999) The Human Eye. Sunderland, MA: Sinauer complex. The eye is a small yet multifaceted unit of Associates. anatomical machinery, with intricate design and function. Sinclair S (1985) How Animals See. New York: Facts On File.

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