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The Eye
Chapter 22
The Eye The mammalian eye is a subject of intense research, and ophthalmic or visual physiology is now a well-defined and distinct subject to which textbooks and journals are devoted. In the short account provided here we have been selective; the histology of the retina is not covered in much detail, nor have we described the topographical anatomy of the intra-orbital nerves, or attempted a synopsis of the evolution of the vertebrate eye. For reviews of the evolution of the vertebrate eye and its many variations, we suggest Fernald (2000, 2004) and Gehring (2004). Readers with access to a good library may wish to consult the classic work, the Vertebrate Eye and Its Adaptive Radiation (Walls, 1963). Davson’s famous book Physiology of the Eye (1990) provides an outstanding, but rather dated account of visual physiology. Excellent accounts of the retina are available in textbooks of human histology, but beware the pitfalls of transferring the histological features of man to the rat. Books by Tansley (1965), Gregory (1966) and Pirenne (1967), though more than 50 years old, are still useful, and contain much of interest on structure, function and comparative physiology. There are also many fine up to date textbooks of human and veterinary ophthalmology. Grant’s Toxicology of the Eye (1993) is a large compendium of the effects of compounds on the eye, but does not deal with structure and function.
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DIFFERENCES BETWEEN THE RAT AND HUMAN EYE Clearly the rat eye is substantially smaller than the human eye (5.5 mm in diameter compared with 24 mm), and by way of introduction we have provided a list of differences between the eyes of man and rat. 1. The lens of the rat eye is close to spherical in shape, and unlike the lens of the human eye, cannot be brought to a shorter focal length by contraction of the ciliary muscle which is poorly developed in the rat, so the rat lacks the capacity of accommodation. 2. The depth of focus (depth of field is a better term) of the eye is defined by the minimal distance from the
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eye at which objects can be focussed without changes to the focal length of the optical system. In man objects can be brought in from infinity to 2.3 m without loss of focus if the diameter of the pupil is held at 2 mm (Campbell, 1957). A smaller pupil increases the depth of focus by reducing spherical aberration introduced by the periphery of the lens, so the rat has a greater depth of focus than man: from infinity to 7 cm (Powers and Green, 1978). This large depth of focus offsets the lack of accommodation in the rat. The lens of the rat eye is close to spherical, that of man is lenticular (meaning like a lentil or lens) and biconvex. The rat has poor visual acuity compared with man. In the rat the minimal angle subtended at the retina by distinguishable points is about 1 degree of arc, in man it is about 30 seconds of arc (0.0083 degrees of arc). The rat is hypermetropic (long sighted) compared with man. If one were thinking in terms of human ophthalmic optics then the rat would be described as 7 dioptres hypermetropic (Glickstein and Millodot, 1970). The power of a lens expressed in dioptres is the reciprocal of its focal length in metres. As a nocturnal animal, the rat has a higher ratio of rods to cones in its retina than man; 99% of the receptors in the rat retina are rods. The rat lacks a fovea, the small central area of the human retina that is occupied only by cones and where the retina is thinned. This accounts for the high visual acuity of man. Man has good colour vision based on cones that are sensitive to red, green and blue light, but in the rat, only cones sensitive to green and blue/ultraviolet light are found. Rod development depends in part on dihydroxyphenylalanine (DOPA), a compound not produced in albino animals, so as a result albino rats have poorly developed rods. Binocular stereoscopic vision is dependent on the overlap of visual fields. The laterally placed eyes of the rat have less overlap of fields then frontally placed human eyes.
Anatomy and Histology of the Laboratory Rat in Toxicology and Biomedical Research. DOI: https://doi.org/10.1016/B978-0-12-811837-5.00022-8 © 2019 Elsevier Inc. All rights reserved.
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All this means that the rat sees less well than man. Compared with man, the image produced by the rat eye is blurred with poor colour definition. Of course, the rat is a macrosmatic mammal and relies on its sense of smell to a large extent, and using its whiskers to provide it with information on close objects (see Chapter 10: Nasal Cavity).
DEVELOPMENT OF THE EYE Development of the eyes begins with the evagination of the right and left optic vesicles, which arise from the ventral aspect of the forebrain. The optic vesicles and the stalks that connect the vesicles to the brain are hollow. As the vesicles grow, they come into contact with the ectoderm of the lateral aspect of the head. Interaction between neuroectoderm and surface ectoderm induces formation of the lens placode from which the lens of the eye will develop. The developing lens sinks below the surface and invaginates the optic vesicle, which assumes the shape of a cup. The cup has an outer layer that will give rise to the pigment epithelium of the retina and an inner layer that gives rise to all the other elements of the retina. Mesenchyme (mesoderm) condenses around the developing eyeball and forms the choroid layer and the sclera. The collagen of the sclera is produced by fibroblasts, so although the sclera and the cornea are essentially continuous, they are produced by cells of different embryological origins. The choroid provides the bulk of the ciliary body, and is covered by two layers of epithelium derived from the retina that continue over the ciliary body and form the posterior epithelium of the iris. The remainder of the iris and its anterior surface are thought to be formed from mesenchyme. In man the ciliary body contains the smooth muscle cells that allow accommodation; in the rat these are poorly developed, the rat has minimal powers of accommodation. The iris contains the constrictor and dilator muscles of the pupil, also thought to be derived from mesenchyme. The epithelium of the anterior surface of the cornea is derived from surface ectoderm, and the posterior epithelium (often called endothelium), Descemet’s membrane and the cells that produce the collagen fibres of the corneal stroma from neural crest tissue that infiltrates between the developing lens and the anterior surface of the developing cornea. The collagen fibres of the corneal stroma are produced by keratocytes, a cell similar to the fibroblast, but not to be confused with the keratin producing keratinocytes, although both words have the same derivation the Greek, kerato, meaning horny. With the exception of the microglial cells and endothelium of the capillaries and other blood vessels, the retina develops from the retinal progenitor cells of the optic vesicle.
Differentiation, migration and maturation of retinal cells are closely controlled by regulatory genes. In the mouse, development of the cones, ganglion cells, horizontal cells and amacrine cells begins at gestational day (GD) 11. Development of the cones and horizontal cells is complete at birth, with ganglion and horizontal cells continuing to develop for a few days after birth. Rods, bipolar cells and Mu¨ller cells begin to develop between GD 12 and 16 and continue to do so after birth until the eyes open. The axons of the ganglion cells grow into the stalk of the optic vesicle and form the optic nerve (Bassett and Wallace, 2012). The development of the eye of the rat has not been documented in such detail but it is thought to be similar, beginning on GD 11, with the eyelids forming at about GD 17 and fusing shortly thereafter, with the result that rats are born with their eyes closed, only opening at around day 12 of postnatal life. The period of eye-closure is important for the normal development of the rat eye (Meng et al., 2014). The eye of the rat is much less well developed at birth than the human eye, and development continues for around 3 weeks after birth. Corneal development continues for a week or so after the eyes have opened, with the anterior epithelium of the cornea increasing in thickness from two to six layers of cells. At birth the lens of the eye is biconvex, and development into the almost spherical lens of the adult rat occurs after birth. The blood vessels that run through the hyaloid canal of the vitreous to the lens disappear between 7 and 22 days of postnatal life; in man they have disappeared by birth. There are two sets of hyaloid vessels in the rat, a central group of three to five arteries running to the posterior pole of the lens, and a peripheral set that runs over the surface of the retina to reach the equator of the lens (Heywood, 1973). The development of the eye is substantially more complicated than described above and for a detailed account we suggest Walling and Marit (2016), who have provided a short account of prenatal development, and a more detailed account of postnatal development. Publications by Bassett and Wallace (2012), Kuszal et al. (2004), Zieske (2004), Meng et al. (2014), Braekevelt and Hollenberg (1970) and Ju et al. (2012) should also be consulted for further details.
THE GENERAL ANATOMY OF THE RAT EYE Although the rat eye is similar in general structure to other mammalian eyes, it differs from man in that it is not completely surrounded by bone, so is held in place by the Harderian and intra-orbital lacrimal glands and the extraocular muscles, with the lateral aspect of the eye being relatively unprotected. The rat eye bulges forwards from the face, in human terms the rat is exophthalmic. The bulging of the eyes is increased during chewing by contraction of
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that part of the masseter muscle that runs through the orbit (see Chapter 6: The Musculature of the Rat). The optic nerve runs from the eye to the optic foramen.
Conventional Axes and Planes of Reference An easy way to remember the axes and planes of reference of the eye is to imagine the planet earth lying on its side. The poles are thus anterior and posterior, and the equator runs vertically around the globe. The optic nerve is not quite in line with the centre of the cornea, it leaves below and medial to the point where the optical axis, running horizontally from the centre of the cornea to the retina, meets the back of the eyeball. An equatorial section divides that the globe into anterior and posterior halves is not much use for histological work, but if the vitreous is removed from the anterior half the posterior surface of the lens and the surrounding ciliary processes can be seen. Planes passing through the axis are described as meridians. For histological and pathological purposes, sections that are taken in either the horizontal or vertical meridian will show approximately the same appearance as the eye is close to symmetrical around the optical axis. Fig. 22.1 shows a diagram of a section taken through the vertical meridian.
The Eyeball The wall of the eyeball comprises three layers, a tough outer sclera, a vascular middle layer called the uveal tract
(more often called the choroid membrane or the choroid), and the retina. The sclera is made up of collagen and some elastic fibres, and is continuous with the cornea at the front of the eye, although the radius of curvature of the cornea is smaller than that of the sclera. Human anatomists use the analogy of a watch glass sitting on a cricket ball to represent the eye. At the back of the eye, the dura mater that surrounds the optic nerve blends into the sclera, and where the optic nerve penetrates the sclera, bundles of Type IV collagen fibres are arranged as a lattice through which the interstices of the bundles of nerve fibres run. The nuclei of the fibroblasts that produce the collagen and elastic fibres of the sclera can be seen pressed between these bundles of collagen fibres. The uveal tract (or vascular tunic) can be divided into the choroid, which underlies the retina, the ciliary process and the iris, which forms the diaphragm that defines the pupil of the eye. The light-receptive retina occupies all the inner surface of the eye posterior to the lens. The anterior portion runs forwards over the ciliary body and its processes onto the posterior surface of the iris. Here the retina is thin and does not function as a photoreceptor. The interior of the eyeball can be divided into three parts. The anterior chamber lies between the cornea and the iris and has a volume of around 15 µL. The posterior chamber lies between the posterior surface of the iris, the lens and the suspensory ligaments that link the lens with the ciliary processes. The anterior and posterior chambers contain aqueous humour, which is produced by the ciliary
Lining of eyelid Vitreous humour
Posterior chamber Anterior chamber
both filled with aqueous humour
Sclera Choroid Retina Lens Conjunctiva Cornea Blind spot Pupil Optic nerve Sheath
Iris Suspensory ligament Ora serrata
FIGURE 22.1 Eyeball (vertical section).
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Ciliary body (double ciliary processes)
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processes of the posterior chamber and percolates between the iris and the lens into the anterior chamber where it is absorbed at the irido-corneal angle. The vitreous body (the vitreous) fills the space between the lens, its suspensory ligaments and the retina. The vitreous is a gel with a volume of about 35 µL (Hebel and Stromberg, 1976). Substances found in the aqueous humour can diffuse into the vitreous, but beyond that little is known of its turnover (Davson, 1972). Note that the vitreous does NOT occupy the posterior chamber. One of the most important things to grasp about the structure of the eye is that the iris and ciliary body are circular structures. Similarly, the suspensory ligaments of the lens form a circle around the periphery of the lens. The canal of Schlemm, see below, runs in a circle around the irido-corneal angle.
The Anterior Parts of the Eye The Conjunctiva The surface of the cornea is covered by a nonkeratinised, stratified squamous epithelium that is continuous with the conjunctival epithelium covering the sclera (bulbar conjunctiva) and the inner surfaces of the eyelids (palpebral conjunctiva), blending into the skin at the edges of the eyelids. The bulbar and palpebral parts of the conjunctiva meet at the fornices of the conjunctival sac (Fornix is Latin for an arch). The conjunctival sac sometimes causes confusion. If you shut your eye the sac is a closed space limited by the cornea, sclera and the posterior or inner surfaces of the eyelids. The conjunctiva is more than an epithelium, it includes a subepithelial lamina propria that contains many blood and lymphatic vessels and many nerve fibres. In cases of conjunctivitis, blood vessels dilate and the inner surfaces of the eyelids and the bulbar conjunctivae become reddened. Although no capillaries enter the cornea, severe damage to the cornea may lead to capillaries growing into it from the conjunctiva. Sensory nerve fibres running in the conjunctiva mean the cornea is exquisitely sensitive to touch. Secretions from the lacrimal and Harderian glands flow into the lateral part of the conjunctival sac, and are drained by the two lacrimal ducts at the medial ends of the upper and lower eyelids. These join to form the lacrimal sac from where the fluid drains via the nasolacrimal duct to the nasal cavity.
The Cornea The rat cornea is about 250 µm in thickness, somewhat thicker than the sclera (Fig. 22.2). The surface epithelium is continuous with that of the bulbar conjunctiva and rests
FIGURE 22.2 The cornea.
on a basement membrane. In man, but not in the rat, this basement membrane is supported by a well-defined layer of fine collage fibres, Bowman’s membrane. Basal cells of the epithelium divide in response to injury and areas of denudation are covered by sliding of epithelial cells. This sliding of epithelial cells is an interesting phenomenon as these cells interdigitate with one another and are connected to one another by desmosomes. The bulk of the corneal stroma is made up of bundles of collagen fibres produced by fibroblasts and arranged as lamellae or sheets. Nuclei can be seen in the bundles of fibres in adjacent lamellae running at 90 degrees to one another. The most important functional characteristic of the cornea is that it is transparent. Transparency is maintained by the actions of endothelium of the posterior surface of the cornea that controls the amount of fluid that lies between the bundles of fibres; if the cornea becomes oedematous then blurring of vision occurs. These endothelial cells lie on a thickened basement membrane known as Descemet’s membrane, which is present in both man and the rat. These cells pump ions from the interstitium of the cornea and water follows. The water and ions are subsequently secreted into the aqueous humour of the anterior chamber.
The Choroid A number of names and eponyms have been given to the layers that can be defined in the choroid. In essence, there is an outer layer containing large blood vessels, a middle layer containing fenestrated capillaries and an inner membrane. The inner membrane is known as Bruch’s membrane, a combination of the basement membrane of the pigment epithelium of the retina and the endothelial cells of the capillaries of the choroid with an intervening layer of fine collagen and elastic fibres. It is frequently described as vitreous or glassy because of its glistening
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appearance in histological sections. Bruch’s membrane has been subdivided into five layers that can be defined at electron microscopy (Fawcett, 1994). Many melanocytes are found in the choroid of pigmented animals but not in the albino rat.
The Ciliary Body The bulk of the ciliary body (Fig. 22.3) is made up from connective tissue containing a wealth of fenestrated capillaries. In histological sections the ciliary body looks like a wedge, but is, in fact, a ridge that runs round the inner surface of the eyeball. Extending from the surface of the ring are about 100 finger-like ciliary processes reaching from the apex of the ring towards the lens. From these ‘fingers’, fine collagenous filaments run to the capsule of the lens to make up its suspensory ligament. Ciliary processes are less well developed in the rat than they are in man, and not much smooth muscle is found in the ciliary body of the rat, accounting for its poor powers of accommodation. The epithelium that covers the ciliary body and processes is both interesting and unique. Two layers of cuboidal cells, each layer resting on a basement membrane, are arranged with their apices facing each other across a narrow gap. Aqueous humour is secreted by this double layer of epithelium. This unique arrangement has an embryological basis, the outer layer being derived from the outer wall of the original invaginated cup, and the inner layer from the inner wall of the cup, and the narrow gap between the apical surfaces of the two layers represents the remains of the original optic vesicle. The outer layer is thus no more than a forward extension of the pigment epithelium of the retina and the inner layer a forward extension of the retina itself. However, the 10 layers of the retina have been scaled down to a single
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layer of cuboidal cells and a basement membrane that probably represents the inner limiting membrane of the retina proper. Note that this basement membrane is in contact with the aqueous humour of the posterior chamber. The apices of the cells are characterised by microvilli and their basal surfaces by marked infolding of the basal cell membrane. In pigmented animals, the cells of the outer layer are laden with melanin granules. Similar granules appear in the cells of the inner layer as it runs forwards from the ciliary processes to the posterior surface of the iris, but again only in pigmented animals.
The Iris The choroid continues forwards from the ciliary body to form the iris. Like the ciliary body, the posterior surface of the iris is covered by a double layer of epithelial cells that is heavily pigmented in non-albino animals. The constrictor muscle of the iris is arranged in a ring around the inner edge that defines the pupil; the dilator muscles are placed peripherally and arranged radially. The constrictor muscle is under parasympathetic control, and the dilators under sympathetic control, although a good deal of dilation of the pupil results from a reduction of parasympathetic input to the constrictor muscle. The bulk of the iridial stroma comprises a loose vascular connective tissue in which two arterial circles, one near the margin of the pupil, and one at the periphery of the iris, can be seen. The anterior surface of the iris is described by some authors as being covered by endothelium that is continuous with the posterior surface of the cornea, and by others as comprising a surface made up of fibroblasts and connective tissue fibres.
The Irido-Corneal Angle This angle, which can be seen in histological sections, is a cross section of a narrow groove spanned by trabeculae of connective tissue, and runs round the inner surface of the eye. Aqueous humour percolates between the trabeculae and enters the canal of Schlemm (seen in Fig. 22.3 as several small spaces) that runs around the eyeball at this location. Fluid flows from the canal into the unique aqueous veins of the sclera (we are now beyond the periphery of the cornea) and thence to the veins that drain the sclera (Fig. 22.3).
The Lens
FIGURE 22.3 The ciliary body and the canal of Schlemm.
The lens is enclosed by the thickened (11 18 µm) basement membrane of the epithelial cells that cover the anterior surface of the lens: the lens capsule. The lens forms as a vesicle, the cells of the posterior wall of the vesicle lengthen and form long curved ribbons. These cells lose their nuclei and become the lens fibres, becoming arranged as a series of intricately spiralling lamellae
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FIGURE 22.4 The lens in the area of the bow.
(Kuszal et al., 2004). The lumen of the vesicle is lost, and the adult lens comprises the lens fibres and an anterior epithelium. At the equator or bow of the lens (an upright equator in a plane parallel to the equator of the eye itself), nuclei can be seen turning in from the anterior layer (Fig. 22.4). Sivak and Dovrat (1983) have shown that the optical properties of the rat lens change with age. They noted that the lens of the rat was over corrected for spherical aberration, indeed it showed negative spherical aberration that increased with age. This has been regarded as an adaption for nocturnal living (Walls, 1963).
The Vitreous The transparent vitreous is a gel made up of a liquid phase comprising mostly hyaluronic acid, and a solid phase made up of fine collagen fibres. Hyalocytes produce both phases, but are difficult to detect in histological sections, although occasional nuclei are found at the periphery of the vitreous. In young rats the remains of embryonic blood vessels can be seen, these having reached the lens via the hyaloid canal during development.
The Retina The structural complexity of the structures described earlier is as nothing compared with that of the retina (Fig. 22.5). Detailed accounts of the structure of individual types of cells may be found in textbooks of histology and in Bron et al. (1997). The retina comprises the pigment epithelium that develops from the outer wall of the invaginated optic cup, and the many layers of cells that develop from the inner wall (these may become separated and form a ‘detached retina’). As the rat retina ages, the thickness and the area of the internal surface of the eyeball covered by retina decreases. It appears that ganglion cells are more affected by age than other cells of the retina,
FIGURE 22.5 The retina.
and are reduced in number by 20% 25% (Cavallotti et al., 2001). Most accounts divide the retina into no less than 10 layers. These are set out, for reference, in Table 22.1. The albino rat has no pigment in the ‘pigmented layer’ of the retina, but the rods and cones of the albino rat do contain visual pigment. The time-honoured 10 layers of the retina do scant justice to the complexity of the tissue. Many accounts refer to: The vertical system of neurons: photoreceptors bipolar cells ganglion cells. The horizontal system of neurons: horizontal cells and amacrine cells that connect across the junctions of the vertical system and sharpen the coding of the retinal image. The supporting cells: those of Mu¨ller than run vertically through the retina and the astrocytes and microglial cells that occur amidst the other cells. The details of the cytoarchitecture of the retina inspire a feeling of awe. Masland (2001) reported 55 distinct cell types, including 29 types of amacrine cells and 10 15 types of ganglion cells. In 2000, Masland and Raviola asked whether it was necessary to treat so many types of cells as distinct entities and pleaded ‘surely nature is not malicious’! Upon detailed consideration they concluded that the distinctions were necessary. In addition to the 55 types of cells, there are at least 6 neurotransmitters, mainly amino acids, acting at conventional chemical synapses and gap junctions serving as electrical synapses. Reading recent accounts of the retina is likely to inspire feelings of elation or despair, depending on the reader’s familiarity with the field. It seems likely that when the cytoarchitecture is eventually fully understood and the ‘wiring diagram’ properly modelled, the nonspecialist will be unable to understand, let alone remember, these details.
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TABLE 22.1 Structure of the Retina Layer Number
Layer Name
Description
1
Pigmented epithelium
Cuboidal, heavily pigmented cells with processes that extend between the tips of the outer segments of the rods and cones
2
Bacillary layer
Outer segments of the rods and cones containing the stacks of membranous discs that contain the visual pigment
3
Outer limiting membrane
Not a true membrane but formed by the peripheral processes of the cells of Mu¨ller that support the rods and cones. These processes are held together by junctional complexes
4
Outer nuclear layer
Nuclei of the rods and cones. The segments of the rods and cones containing the nuclei are connected to the outer segments containing the photoreceptors by modified, intracellular, cilia
5
Outer plexiform layer
Synapses between the photoreceptors (rods and cones) and the bipolar cells and horizontal cells
6
Inner nuclear layer
Nuclei of the bipolar cells
7
Inner plexiform layer
Synapses between the bipolar cells and the ganglion cells and the amacrine cells
8
Ganglion cell layer
Nuclei of the ganglion cells
9
Nerve fibre layer
Unmyelinated axons of the ganglion cells running to the optic disc when they form the nerve fibre bundles of the optic nerve
10
Inner limiting membrane
Basal foot processes of the Mu¨ller cells linked by junctional complexes
The Blood Retinal Barrier The retina can be regarded as a displaced part of the brain, and like the brain, is defended by a barrier that allows only selected substances to reach its neurones. The barrier comprises the pigment epithelium (unpigmented in the albino rat), the epithelium of the ciliary processes, and the tight junctions between the endothelial cells of the retinal capillaries. Damage to the blood-retinal barrier can be caused by a number of toxic substances and by light. Phototoxic damage promotes the formation of free radicals that damage lipidrich structures such as cell membranes and the membranous stacks that make up the photoreceptors of the rods and cones. The pigment epithelium is much more than merely a dark coat that prevents reflection of light, playing an important role in ‘nibbling away’ at the distal ends of the outer segments of rods and cones, and removing ‘worn out’ photoreceptor membrane. New photoreceptor discs are synthesised continuously by the rods and cones with a rod turning over its complement of photoreceptors every 10 days (Fawcett, 1994). The pigment epithelium also cycles visual pigment cycles to and from the rods and stores the Vitamin A that is an essential component of the pigment.
The Extraocular Muscles The rat has all the extraocular muscles found in man, plus the retractor bulbis muscle, which lies beneath the eyeball
and pulls it back into the orbit. The four rectus muscles (lateral, medial, superior and inferior) arise from just below the optic foramen, and are inserted into the sclera of the eyeball just behind the equator. The lateral (or external) and medial (or internal) muscles turn the eye laterally and medially; the superior rectus turns the eye upwards and a little inwards, the inferior rectus turns the eye downwards, and again, a little inwards. The superior oblique muscle runs a rather unusual course. From the orbital aspect of the frontal bone it forms a tendon that runs through a tiny cartilaginous or bony hook attached to the bone, before swinging back to the eyeball as a second muscle belly that is inserted into the sclera. It turns the eye downwards and a little outwards. The inferior oblique runs from medial to lateral beneath the eyeball and turns the eye upwards, and again, a little outwards. Acting in concert the combination of their actions permits movement of the eyeball in all directions. The splendidly named levator palpebrae superioris muscle that lifts the upper eyelid lies in the roof of the orbit and is inserted into the connective tissue of the upper lid. The extraocular muscles are supplied by different cranial nerves that enter the orbit through the foramen orbitorotundum (see Chapter 21(3): The Autonomic Nervous System). The nerve supply of the individual muscles is as follows: Superior oblique by the trochlear nerve, CN IV (trochlea means pulley).
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Lateral rectus by the abducens nerve, CN VI (so named because it supplies the muscle that abducts the eyeball). Superior and inferior recti, the medial rectus, levator palpebrae superioris and inferior oblique by the oculomotor nerve, CN III.
Autonomic Supply of the Eyeball and Associated Structures Preganglionic parasympathetic fibres travel from the Edinger Westphal nuclei of the midbrain to the orbit via CN III, the oculomotor nerve. Here they leave the oculomotor nerve and run to the ciliary ganglion found deep in the orbit. From that ganglion, postganglionic fibres run as the short ciliary nerves to the eyeball to innervate the constrictor muscle of the pupil and the weak ciliary muscle. Postganglionic sympathetic fibres reach the orbit in the plexus around the ophthalmic artery and join the long ciliary nerves (branches of the ophthalmic nerve (CN V (1)). The long ciliary nerves run to the eyeball and supply the dilator muscle of the pupil and provide sensory fibres to the cornea.
Blood Supply of the Eye and Associated Structures In man the ophthalmic artery arises within the skull from the internal carotid artery, and travels to the orbit with the optic nerve via the optic foramen. In the orbit it runs close to the optic nerve giving off the long and short posterior ciliary arteries that pierce the sclera and supply the choroid. As the long posterior ciliary arteries run forwards they are joined by the anterior ciliary arteries, which arise from the arteries supplying the extraocular muscles, forming an arterial circle at the outer edge of the iris. The ophthalmic artery also gives off the central artery of the retina, which pierces the sheath of the optic nerve and finds its way deep into the nerve to appear on the surface of the retina in the middle of the optic disc. In the rat, the ophthalmic artery arises from the palatine branch of the pterygopalatine artery, itself a branch of the internal carotid, taking a circuitous route through the foramen orbitorotundum to the posterior corner of the orbit-temporal fossa. The palatine branch of the pterygopalatine artery also gives rise to the lacrimal artery in the rat, which in man is a branch of the ophthalmic artery. Greene (1935) stated that the lacrimal artery supplies the intra-orbital lacrimal gland; Hebel and Stromberg (1976) stated that both the intra- and extra-orbital lacrimal glands were supplied by branches of the facial artery. In both the rat and man the extraocular muscles are supplied by branches of the ophthalmic artery. The posterior ciliary
artery of the rat runs inside the sheath of the optic nerve, dividing into the long posterior ciliary arteries and the central artery of the retina. There are no short posterior ciliary arteries in the rat (Sugiyama et al., 1999). These authors made no mention of anterior ciliary arteries and refer to the posterior ciliary artery as a terminal artery (end artery). In the rat, the central artery of the retina supplies more of the layers of the retina than in man, where it supplies only the ganglion cells and their nerve fibres, the remaining layers being supplied from the choroidal vessels. In the rat branches of the central artery (five to eight) of the retina radiate like the spokes of wheel, running a straight course over the surface of the retina towards its periphery (Heywood, 1973). Retinal veins run alongside the arteries, appearing darker and wider than the arteries when examined with an ophthalmoscope. The yellow-pink/creamy optic disc seen in man is not apparent on examination of the rat eye. This has been attributed this to the shape of the cup of the head of the optic nerve, described as ‘narrow, funnel-shaped, and oblique to the disc surface’, which gives the disc a dark appearance (Cohen et al., 2003). Venous blood from the choroid drains to the several vorticeal veins that penetrate the sclera and join the anastomosing veins behind the eyeball. The vorticeal veins are so named because the many venous channels of the choroid appear to sweep up to these veins like a vortex. The veins of the retina run with the arteries, but leave at the optic nerve to join the ophthalmic veins. Venous blood from all the tissues of the orbit is collected by the superior and inferior ophthalmic veins, joining to form the ophthalmic vein, which drains to the cavernous and petrosal sinuses within the skull. Timm (1979) reported that the veins behind the eyeball of the rat form a plexus, but in the mouse there is a confluent venous sinus. Some drainage from the veins of the orbit to those of the pterygopalatine fossa also occurs. Because the albino rat has no melanin in the ‘pigmented layer’ of the retina or its choroid, the blood vessels of the choroid can be seen more clearly on ophthalmoscopy than in pigmented animals. It is important not to confuse melanin (absent in the albino rat) with the visual pigment of the rods and cones of the retina, this is present in both white rats and albino humans, without it neither could see.
Lymphatic Drainage of the Eye It has often been said that the eye lacks a system of lymphatic drainage, but more recent work has shown that this is not true, and that the drainage system of the aqueous humour might best be regarded as at least an auxiliary part of the lymphatic system (Nakao et al., 2012). Lymphatic drainage of the uveal tract has also been suggested.
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The extraocular structures, muscles and the intra-orbital lacrimal gland and the Harderian gland all appear to have a conventional lymphatic drainage system.
REFERENCES Bassett, E.A., Wallace, V.A., 2012. Cell fate determination in the vertebrate retina. Trends Neurosci. 35 (9), 565 573. Braekevelt, C.R., Hollenberg, M.J., 1970. Development of the retinal pigment epithelium, choriocapillaris and Bruch’s membrane in the albino rat. Exptl. Eye Res. 9, 124 131. Bron, A.J., Tripathi, R.C., Tripathi, B.J., 1997. Wolff’s Anatomy of the Eye and Orbit, eighth ed. Chapman and Hall Medical, London. Campbell, F.W., 1957. The depth of field of the human eye. Opt. Acta (Lond.) 4, 157 164. Cavallotti, C., Artico, M., Pescosolido, N., Feher, J., 2001. Age-related changes in rat retina. Jpn. J. Ophthalmol. 45, 68 75. Cohen, B.E., Pearch, A.C., Jokelainen, P.T., Bohr, D.F., 2003. Optic disc imaging in conscious rats and mice. Invest Ophthalmol. Vis. Sci. 44 (1), 160 163. Davson, H., 1972. The Physiology of the Eye, third ed. Churchill Livingstone, Edinburgh and London (the 3rd edition contains a detailed account of ophthalmic optics: this was omitted in the 5th edition to make space for an extended account of the neurophysiology of vision.). Davson, H., 1990. The Physiology of the Eye, fifth revised ed. Palgrave Macmillan. Fawcett, D.W., 1994. A Textbook of Histology, twelth ed. Chapman & Hall, New York and London. Fernald, R., 2000. Evolution of eyes. Curr. Opin. Neurobiol. 10, 444 450. Fernald, R.D., 2004. Evolving eyes. Int. J. Dev. Biol. 48, 701 705. Gehring, W.J., 2004. Historical perspective on the development and evolution of eyes and photoreceptors. Int. J. Dev. Biol. 48, 707 717. Glickstein, M., Millodot, M., 1970. Retinoscopy and eye size. Science 168 (3931), 605 606. Grant, W.M., Joel, S., Schuman, M.D., 1993. In: Charles, C. (Ed.), Toxicology of the Eye, fouth ed. Thomas Pub Ltd. Greene, E.C., 1935. Anatomy of the Rat. Transactions of the American Philosophical Society, New Series: Volume XXVII, first ed. Hafner Publishing Company, New York and London (reprinted 1963). Gregory, R.L., 1966. Eye and Brain, the Psychology of Seeing. World University Library, Weidenfeld and Nicolson, London. Hebel, R., Stromberg, M.W., 1976. Anatomy of the Laboratory Rat. The Williams & Wilkins Company, Baltimore, MD. Heywood, R., 1973. Some clinical observations on the eyes of Sprague Dawley rats. Lab. Anim. 7, 19 27.
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Ju, C., Zhang, K., Wu, X., 2012. Derivation of corneal endothelial cell-like cells from rat neural crest cells in vitro. PLoS One 7 (7), https://doi.org/10.1371/journal. e42378. Available from: pone.0042378. Kuszal, J.R., Zoltoski, R.K., Tiedemann, C.E., 2004. Development of lens sutures. Int. J. Dev. Biol. 48, 889 902. Masland, R.H., 2001. The fundamental plan of the retina. Nat. Neurosci. 4 (9), 877 886. Masland, R.H., Raviola, E., 2000. Confronting complexity: strategies for understanding the microcircuitry of the retina. Ann. Rev. Neurosci. 23, 249 284. Meng, Q., Mongan, M., Carreira, V., Kurita, H., Liu, C.-Y., Kao, W.W.-Y., et al., 2014. Eyelid closure in embryogenesis is required for ocular adnexa development. Invest Ophthalmol. Vis. Sci. 55 (11), 7652 7661. Nakao, S., Hafezi-Moghadam, A., Ishibashi, T., 2012. Lymphatics and lymphangiogenesis in the eye. J. Ophthalmol. (Article ID 783163, 11 pages) https://doi.org/10.1155/2012/783163. Pirenne, M.H., 1967. Vision and the Eye, second ed. Chapman and Hall Ltd., London. Powers, M.K., Green, D.G., 1978. Single retinal ganglion cell responses in the dark-reared rat: grating acuity, contrast sensitivity and defocusing. Vis. Res. 18, 1533 1539. Sivak, J.G., Dovrat, A., 1983. Aging and the optical quality of the rat crystalline lens. J. Invest. Ophthalmol. Vis. Sci. 24 (9), 1162 1166. Sugiyama, K., Gu, Z.-B., Kawase, C., Yamamoto, T., Kitazawa, Y., 1999. Optic nerve and peripapillary choroidal microvasculature of the rat eye. Invest. Ophthalmol. Vis. Sci. 40 (13), 3084 3090. Tansley, K., 1965. Vision in Vertebrates. Chapman and Hall and Science Paperbacks, London. Timm, K.I., 1979. Orbital venous anatomy of the rat. Lab. Anim. Sci. 29 (5), 636 638. Walling, B.E., Marit, G.B., 2016. The eye and Harderian gland (Chapter 12) In: Parker, G.A., Picut, C.A. (Eds.), Atlas of Histology of the Juvenile Rat. Academic Press, Elsevier, Oxford, pp. 373 377. Walls, G.L., 1963. The Vertebrate Eye and Its Adaptive Radiation. Hafner Publishing Company, New York and London (originally published 1942). Zieske, J.D., 2004. Corneal development associated with eyelid opening. Int. J. Dev. Biol. 48, 903 911.
FURTHER READING Coulombre, A.J., 1965. The eye (Chapter 9) In: DeHaan, R.L., Ursprung, H. (Eds.), Organogenesis. Holt, Rinehart and Winston, New York, Chicago, San Francisco, Toronto and London, pp. 219 252.
The Eye The mammalian eye is a subject of intense research, and ophthalmic or visual physiology is now a well-defined and distinct subject to which textbooks and journals are devoted. In the short account provided here we have been selective; the histology of the retina is not covered in much detail, nor have we described the topographical anatomy of the intra-orbital nerves, or attempted a synopsis of the evolution of the vertebrate eye. For reviews of the evolution of the vertebrate eye and its many variations, we suggest Fernald (2000, 2004) and Gehring (2004). Readers with access to a good library may wish to consult the classic work, the Vertebrate Eye and Its Adaptive Radiation (Walls, 1963). Davson’s famous book Physiology of the Eye (1990) provides an outstanding, but rather dated account of visual physiology. Excellent accounts of the retina are available in textbooks of human histology, but beware the pitfalls of transferring the histological features of man to the rat. Books by Tansley (1965), Gregory (1966) and Pirenne (1967), though more than 50 years old, are still useful, and contain much of interest on structure, function and comparative physiology. There are also many fine up to date textbooks of human and veterinary ophthalmology. Grant’s Toxicology of the Eye (1993) is a large compendium of the effects of compounds on the eye, but does not deal with structure and function.
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DIFFERENCES BETWEEN THE RAT AND HUMAN EYE Clearly the rat eye is substantially smaller than the human eye (5.5 mm in diameter compared with 24 mm), and by way of introduction we have provided a list of differences between the eyes of man and rat. 1. The lens of the rat eye is close to spherical in shape, and unlike the lens of the human eye, cannot be brought to a shorter focal length by contraction of the ciliary muscle which is poorly developed in the rat, so the rat lacks the capacity of accommodation. 2. The depth of focus (depth of field is a better term) of the eye is defined by the minimal distance from the
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eye at which objects can be focussed without changes to the focal length of the optical system. In man objects can be brought in from infinity to 2.3 m without loss of focus if the diameter of the pupil is held at 2 mm (Campbell, 1957). A smaller pupil increases the depth of focus by reducing spherical aberration introduced by the periphery of the lens, so the rat has a greater depth of focus than man: from infinity to 7 cm (Powers and Green, 1978). This large depth of focus offsets the lack of accommodation in the rat. The lens of the rat eye is close to spherical, that of man is lenticular (meaning like a lentil or lens) and biconvex. The rat has poor visual acuity compared with man. In the rat the minimal angle subtended at the retina by distinguishable points is about 1 degree of arc, in man it is about 30 seconds of arc (0.0083 degrees of arc). The rat is hypermetropic (long sighted) compared with man. If one were thinking in terms of human ophthalmic optics then the rat would be described as 7 dioptres hypermetropic (Glickstein and Millodot, 1970). The power of a lens expressed in dioptres is the reciprocal of its focal length in metres. As a nocturnal animal, the rat has a higher ratio of rods to cones in its retina than man; 99% of the receptors in the rat retina are rods. The rat lacks a fovea, the small central area of the human retina that is occupied only by cones and where the retina is thinned. This accounts for the high visual acuity of man. Man has good colour vision based on cones that are sensitive to red, green and blue light, but in the rat, only cones sensitive to green and blue/ultraviolet light are found. Rod development depends in part on dihydroxyphenylalanine (DOPA), a compound not produced in albino animals, so as a result albino rats have poorly developed rods. Binocular stereoscopic vision is dependent on the overlap of visual fields. The laterally placed eyes of the rat have less overlap of fields then frontally placed human eyes.
Anatomy and Histology of the Laboratory Rat in Toxicology and Biomedical Research. DOI: https://doi.org/10.1016/B978-0-12-811837-5.00022-8 © 2019 Elsevier Inc. All rights reserved.
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All this means that the rat sees less well than man. Compared with man, the image produced by the rat eye is blurred with poor colour definition. Of course, the rat is a macrosmatic mammal and relies on its sense of smell to a large extent, and using its whiskers to provide it with information on close objects (see Chapter 10: Nasal Cavity).
DEVELOPMENT OF THE EYE Development of the eyes begins with the evagination of the right and left optic vesicles, which arise from the ventral aspect of the forebrain. The optic vesicles and the stalks that connect the vesicles to the brain are hollow. As the vesicles grow, they come into contact with the ectoderm of the lateral aspect of the head. Interaction between neuroectoderm and surface ectoderm induces formation of the lens placode from which the lens of the eye will develop. The developing lens sinks below the surface and invaginates the optic vesicle, which assumes the shape of a cup. The cup has an outer layer that will give rise to the pigment epithelium of the retina and an inner layer that gives rise to all the other elements of the retina. Mesenchyme (mesoderm) condenses around the developing eyeball and forms the choroid layer and the sclera. The collagen of the sclera is produced by fibroblasts, so although the sclera and the cornea are essentially continuous, they are produced by cells of different embryological origins. The choroid provides the bulk of the ciliary body, and is covered by two layers of epithelium derived from the retina that continue over the ciliary body and form the posterior epithelium of the iris. The remainder of the iris and its anterior surface are thought to be formed from mesenchyme. In man the ciliary body contains the smooth muscle cells that allow accommodation; in the rat these are poorly developed, the rat has minimal powers of accommodation. The iris contains the constrictor and dilator muscles of the pupil, also thought to be derived from mesenchyme. The epithelium of the anterior surface of the cornea is derived from surface ectoderm, and the posterior epithelium (often called endothelium), Descemet’s membrane and the cells that produce the collagen fibres of the corneal stroma from neural crest tissue that infiltrates between the developing lens and the anterior surface of the developing cornea. The collagen fibres of the corneal stroma are produced by keratocytes, a cell similar to the fibroblast, but not to be confused with the keratin producing keratinocytes, although both words have the same derivation the Greek, kerato, meaning horny. With the exception of the microglial cells and endothelium of the capillaries and other blood vessels, the retina develops from the retinal progenitor cells of the optic vesicle.
Differentiation, migration and maturation of retinal cells are closely controlled by regulatory genes. In the mouse, development of the cones, ganglion cells, horizontal cells and amacrine cells begins at gestational day (GD) 11. Development of the cones and horizontal cells is complete at birth, with ganglion and horizontal cells continuing to develop for a few days after birth. Rods, bipolar cells and Mu¨ller cells begin to develop between GD 12 and 16 and continue to do so after birth until the eyes open. The axons of the ganglion cells grow into the stalk of the optic vesicle and form the optic nerve (Bassett and Wallace, 2012). The development of the eye of the rat has not been documented in such detail but it is thought to be similar, beginning on GD 11, with the eyelids forming at about GD 17 and fusing shortly thereafter, with the result that rats are born with their eyes closed, only opening at around day 12 of postnatal life. The period of eye-closure is important for the normal development of the rat eye (Meng et al., 2014). The eye of the rat is much less well developed at birth than the human eye, and development continues for around 3 weeks after birth. Corneal development continues for a week or so after the eyes have opened, with the anterior epithelium of the cornea increasing in thickness from two to six layers of cells. At birth the lens of the eye is biconvex, and development into the almost spherical lens of the adult rat occurs after birth. The blood vessels that run through the hyaloid canal of the vitreous to the lens disappear between 7 and 22 days of postnatal life; in man they have disappeared by birth. There are two sets of hyaloid vessels in the rat, a central group of three to five arteries running to the posterior pole of the lens, and a peripheral set that runs over the surface of the retina to reach the equator of the lens (Heywood, 1973). The development of the eye is substantially more complicated than described above and for a detailed account we suggest Walling and Marit (2016), who have provided a short account of prenatal development, and a more detailed account of postnatal development. Publications by Bassett and Wallace (2012), Kuszal et al. (2004), Zieske (2004), Meng et al. (2014), Braekevelt and Hollenberg (1970) and Ju et al. (2012) should also be consulted for further details.
THE GENERAL ANATOMY OF THE RAT EYE Although the rat eye is similar in general structure to other mammalian eyes, it differs from man in that it is not completely surrounded by bone, so is held in place by the Harderian and intra-orbital lacrimal glands and the extraocular muscles, with the lateral aspect of the eye being relatively unprotected. The rat eye bulges forwards from the face, in human terms the rat is exophthalmic. The bulging of the eyes is increased during chewing by contraction of
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that part of the masseter muscle that runs through the orbit (see Chapter 6: The Musculature of the Rat). The optic nerve runs from the eye to the optic foramen.
Conventional Axes and Planes of Reference An easy way to remember the axes and planes of reference of the eye is to imagine the planet earth lying on its side. The poles are thus anterior and posterior, and the equator runs vertically around the globe. The optic nerve is not quite in line with the centre of the cornea, it leaves below and medial to the point where the optical axis, running horizontally from the centre of the cornea to the retina, meets the back of the eyeball. An equatorial section divides that the globe into anterior and posterior halves is not much use for histological work, but if the vitreous is removed from the anterior half the posterior surface of the lens and the surrounding ciliary processes can be seen. Planes passing through the axis are described as meridians. For histological and pathological purposes, sections that are taken in either the horizontal or vertical meridian will show approximately the same appearance as the eye is close to symmetrical around the optical axis. Fig. 22.1 shows a diagram of a section taken through the vertical meridian.
The Eyeball The wall of the eyeball comprises three layers, a tough outer sclera, a vascular middle layer called the uveal tract
(more often called the choroid membrane or the choroid), and the retina. The sclera is made up of collagen and some elastic fibres, and is continuous with the cornea at the front of the eye, although the radius of curvature of the cornea is smaller than that of the sclera. Human anatomists use the analogy of a watch glass sitting on a cricket ball to represent the eye. At the back of the eye, the dura mater that surrounds the optic nerve blends into the sclera, and where the optic nerve penetrates the sclera, bundles of Type IV collagen fibres are arranged as a lattice through which the interstices of the bundles of nerve fibres run. The nuclei of the fibroblasts that produce the collagen and elastic fibres of the sclera can be seen pressed between these bundles of collagen fibres. The uveal tract (or vascular tunic) can be divided into the choroid, which underlies the retina, the ciliary process and the iris, which forms the diaphragm that defines the pupil of the eye. The light-receptive retina occupies all the inner surface of the eye posterior to the lens. The anterior portion runs forwards over the ciliary body and its processes onto the posterior surface of the iris. Here the retina is thin and does not function as a photoreceptor. The interior of the eyeball can be divided into three parts. The anterior chamber lies between the cornea and the iris and has a volume of around 15 µL. The posterior chamber lies between the posterior surface of the iris, the lens and the suspensory ligaments that link the lens with the ciliary processes. The anterior and posterior chambers contain aqueous humour, which is produced by the ciliary
Lining of eyelid Vitreous humour
Posterior chamber Anterior chamber
both filled with aqueous humour
Sclera Choroid Retina Lens Conjunctiva Cornea Blind spot Pupil Optic nerve Sheath
Iris Suspensory ligament Ora serrata
FIGURE 22.1 Eyeball (vertical section).
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Ciliary body (double ciliary processes)
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processes of the posterior chamber and percolates between the iris and the lens into the anterior chamber where it is absorbed at the irido-corneal angle. The vitreous body (the vitreous) fills the space between the lens, its suspensory ligaments and the retina. The vitreous is a gel with a volume of about 35 µL (Hebel and Stromberg, 1976). Substances found in the aqueous humour can diffuse into the vitreous, but beyond that little is known of its turnover (Davson, 1972). Note that the vitreous does NOT occupy the posterior chamber. One of the most important things to grasp about the structure of the eye is that the iris and ciliary body are circular structures. Similarly, the suspensory ligaments of the lens form a circle around the periphery of the lens. The canal of Schlemm, see below, runs in a circle around the irido-corneal angle.
The Anterior Parts of the Eye The Conjunctiva The surface of the cornea is covered by a nonkeratinised, stratified squamous epithelium that is continuous with the conjunctival epithelium covering the sclera (bulbar conjunctiva) and the inner surfaces of the eyelids (palpebral conjunctiva), blending into the skin at the edges of the eyelids. The bulbar and palpebral parts of the conjunctiva meet at the fornices of the conjunctival sac (Fornix is Latin for an arch). The conjunctival sac sometimes causes confusion. If you shut your eye the sac is a closed space limited by the cornea, sclera and the posterior or inner surfaces of the eyelids. The conjunctiva is more than an epithelium, it includes a subepithelial lamina propria that contains many blood and lymphatic vessels and many nerve fibres. In cases of conjunctivitis, blood vessels dilate and the inner surfaces of the eyelids and the bulbar conjunctivae become reddened. Although no capillaries enter the cornea, severe damage to the cornea may lead to capillaries growing into it from the conjunctiva. Sensory nerve fibres running in the conjunctiva mean the cornea is exquisitely sensitive to touch. Secretions from the lacrimal and Harderian glands flow into the lateral part of the conjunctival sac, and are drained by the two lacrimal ducts at the medial ends of the upper and lower eyelids. These join to form the lacrimal sac from where the fluid drains via the nasolacrimal duct to the nasal cavity.
The Cornea The rat cornea is about 250 µm in thickness, somewhat thicker than the sclera (Fig. 22.2). The surface epithelium is continuous with that of the bulbar conjunctiva and rests
FIGURE 22.2 The cornea.
on a basement membrane. In man, but not in the rat, this basement membrane is supported by a well-defined layer of fine collage fibres, Bowman’s membrane. Basal cells of the epithelium divide in response to injury and areas of denudation are covered by sliding of epithelial cells. This sliding of epithelial cells is an interesting phenomenon as these cells interdigitate with one another and are connected to one another by desmosomes. The bulk of the corneal stroma is made up of bundles of collagen fibres produced by fibroblasts and arranged as lamellae or sheets. Nuclei can be seen in the bundles of fibres in adjacent lamellae running at 90 degrees to one another. The most important functional characteristic of the cornea is that it is transparent. Transparency is maintained by the actions of endothelium of the posterior surface of the cornea that controls the amount of fluid that lies between the bundles of fibres; if the cornea becomes oedematous then blurring of vision occurs. These endothelial cells lie on a thickened basement membrane known as Descemet’s membrane, which is present in both man and the rat. These cells pump ions from the interstitium of the cornea and water follows. The water and ions are subsequently secreted into the aqueous humour of the anterior chamber.
The Choroid A number of names and eponyms have been given to the layers that can be defined in the choroid. In essence, there is an outer layer containing large blood vessels, a middle layer containing fenestrated capillaries and an inner membrane. The inner membrane is known as Bruch’s membrane, a combination of the basement membrane of the pigment epithelium of the retina and the endothelial cells of the capillaries of the choroid with an intervening layer of fine collagen and elastic fibres. It is frequently described as vitreous or glassy because of its glistening
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appearance in histological sections. Bruch’s membrane has been subdivided into five layers that can be defined at electron microscopy (Fawcett, 1994). Many melanocytes are found in the choroid of pigmented animals but not in the albino rat.
The Ciliary Body The bulk of the ciliary body (Fig. 22.3) is made up from connective tissue containing a wealth of fenestrated capillaries. In histological sections the ciliary body looks like a wedge, but is, in fact, a ridge that runs round the inner surface of the eyeball. Extending from the surface of the ring are about 100 finger-like ciliary processes reaching from the apex of the ring towards the lens. From these ‘fingers’, fine collagenous filaments run to the capsule of the lens to make up its suspensory ligament. Ciliary processes are less well developed in the rat than they are in man, and not much smooth muscle is found in the ciliary body of the rat, accounting for its poor powers of accommodation. The epithelium that covers the ciliary body and processes is both interesting and unique. Two layers of cuboidal cells, each layer resting on a basement membrane, are arranged with their apices facing each other across a narrow gap. Aqueous humour is secreted by this double layer of epithelium. This unique arrangement has an embryological basis, the outer layer being derived from the outer wall of the original invaginated cup, and the inner layer from the inner wall of the cup, and the narrow gap between the apical surfaces of the two layers represents the remains of the original optic vesicle. The outer layer is thus no more than a forward extension of the pigment epithelium of the retina and the inner layer a forward extension of the retina itself. However, the 10 layers of the retina have been scaled down to a single
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layer of cuboidal cells and a basement membrane that probably represents the inner limiting membrane of the retina proper. Note that this basement membrane is in contact with the aqueous humour of the posterior chamber. The apices of the cells are characterised by microvilli and their basal surfaces by marked infolding of the basal cell membrane. In pigmented animals, the cells of the outer layer are laden with melanin granules. Similar granules appear in the cells of the inner layer as it runs forwards from the ciliary processes to the posterior surface of the iris, but again only in pigmented animals.
The Iris The choroid continues forwards from the ciliary body to form the iris. Like the ciliary body, the posterior surface of the iris is covered by a double layer of epithelial cells that is heavily pigmented in non-albino animals. The constrictor muscle of the iris is arranged in a ring around the inner edge that defines the pupil; the dilator muscles are placed peripherally and arranged radially. The constrictor muscle is under parasympathetic control, and the dilators under sympathetic control, although a good deal of dilation of the pupil results from a reduction of parasympathetic input to the constrictor muscle. The bulk of the iridial stroma comprises a loose vascular connective tissue in which two arterial circles, one near the margin of the pupil, and one at the periphery of the iris, can be seen. The anterior surface of the iris is described by some authors as being covered by endothelium that is continuous with the posterior surface of the cornea, and by others as comprising a surface made up of fibroblasts and connective tissue fibres.
The Irido-Corneal Angle This angle, which can be seen in histological sections, is a cross section of a narrow groove spanned by trabeculae of connective tissue, and runs round the inner surface of the eye. Aqueous humour percolates between the trabeculae and enters the canal of Schlemm (seen in Fig. 22.3 as several small spaces) that runs around the eyeball at this location. Fluid flows from the canal into the unique aqueous veins of the sclera (we are now beyond the periphery of the cornea) and thence to the veins that drain the sclera (Fig. 22.3).
The Lens
FIGURE 22.3 The ciliary body and the canal of Schlemm.
The lens is enclosed by the thickened (11 18 µm) basement membrane of the epithelial cells that cover the anterior surface of the lens: the lens capsule. The lens forms as a vesicle, the cells of the posterior wall of the vesicle lengthen and form long curved ribbons. These cells lose their nuclei and become the lens fibres, becoming arranged as a series of intricately spiralling lamellae
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FIGURE 22.4 The lens in the area of the bow.
(Kuszal et al., 2004). The lumen of the vesicle is lost, and the adult lens comprises the lens fibres and an anterior epithelium. At the equator or bow of the lens (an upright equator in a plane parallel to the equator of the eye itself), nuclei can be seen turning in from the anterior layer (Fig. 22.4). Sivak and Dovrat (1983) have shown that the optical properties of the rat lens change with age. They noted that the lens of the rat was over corrected for spherical aberration, indeed it showed negative spherical aberration that increased with age. This has been regarded as an adaption for nocturnal living (Walls, 1963).
The Vitreous The transparent vitreous is a gel made up of a liquid phase comprising mostly hyaluronic acid, and a solid phase made up of fine collagen fibres. Hyalocytes produce both phases, but are difficult to detect in histological sections, although occasional nuclei are found at the periphery of the vitreous. In young rats the remains of embryonic blood vessels can be seen, these having reached the lens via the hyaloid canal during development.
The Retina The structural complexity of the structures described earlier is as nothing compared with that of the retina (Fig. 22.5). Detailed accounts of the structure of individual types of cells may be found in textbooks of histology and in Bron et al. (1997). The retina comprises the pigment epithelium that develops from the outer wall of the invaginated optic cup, and the many layers of cells that develop from the inner wall (these may become separated and form a ‘detached retina’). As the rat retina ages, the thickness and the area of the internal surface of the eyeball covered by retina decreases. It appears that ganglion cells are more affected by age than other cells of the retina,
FIGURE 22.5 The retina.
and are reduced in number by 20% 25% (Cavallotti et al., 2001). Most accounts divide the retina into no less than 10 layers. These are set out, for reference, in Table 22.1. The albino rat has no pigment in the ‘pigmented layer’ of the retina, but the rods and cones of the albino rat do contain visual pigment. The time-honoured 10 layers of the retina do scant justice to the complexity of the tissue. Many accounts refer to: The vertical system of neurons: photoreceptors bipolar cells ganglion cells. The horizontal system of neurons: horizontal cells and amacrine cells that connect across the junctions of the vertical system and sharpen the coding of the retinal image. The supporting cells: those of Mu¨ller than run vertically through the retina and the astrocytes and microglial cells that occur amidst the other cells. The details of the cytoarchitecture of the retina inspire a feeling of awe. Masland (2001) reported 55 distinct cell types, including 29 types of amacrine cells and 10 15 types of ganglion cells. In 2000, Masland and Raviola asked whether it was necessary to treat so many types of cells as distinct entities and pleaded ‘surely nature is not malicious’! Upon detailed consideration they concluded that the distinctions were necessary. In addition to the 55 types of cells, there are at least 6 neurotransmitters, mainly amino acids, acting at conventional chemical synapses and gap junctions serving as electrical synapses. Reading recent accounts of the retina is likely to inspire feelings of elation or despair, depending on the reader’s familiarity with the field. It seems likely that when the cytoarchitecture is eventually fully understood and the ‘wiring diagram’ properly modelled, the nonspecialist will be unable to understand, let alone remember, these details.
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TABLE 22.1 Structure of the Retina Layer Number
Layer Name
Description
1
Pigmented epithelium
Cuboidal, heavily pigmented cells with processes that extend between the tips of the outer segments of the rods and cones
2
Bacillary layer
Outer segments of the rods and cones containing the stacks of membranous discs that contain the visual pigment
3
Outer limiting membrane
Not a true membrane but formed by the peripheral processes of the cells of Mu¨ller that support the rods and cones. These processes are held together by junctional complexes
4
Outer nuclear layer
Nuclei of the rods and cones. The segments of the rods and cones containing the nuclei are connected to the outer segments containing the photoreceptors by modified, intracellular, cilia
5
Outer plexiform layer
Synapses between the photoreceptors (rods and cones) and the bipolar cells and horizontal cells
6
Inner nuclear layer
Nuclei of the bipolar cells
7
Inner plexiform layer
Synapses between the bipolar cells and the ganglion cells and the amacrine cells
8
Ganglion cell layer
Nuclei of the ganglion cells
9
Nerve fibre layer
Unmyelinated axons of the ganglion cells running to the optic disc when they form the nerve fibre bundles of the optic nerve
10
Inner limiting membrane
Basal foot processes of the Mu¨ller cells linked by junctional complexes
The Blood Retinal Barrier The retina can be regarded as a displaced part of the brain, and like the brain, is defended by a barrier that allows only selected substances to reach its neurones. The barrier comprises the pigment epithelium (unpigmented in the albino rat), the epithelium of the ciliary processes, and the tight junctions between the endothelial cells of the retinal capillaries. Damage to the blood-retinal barrier can be caused by a number of toxic substances and by light. Phototoxic damage promotes the formation of free radicals that damage lipidrich structures such as cell membranes and the membranous stacks that make up the photoreceptors of the rods and cones. The pigment epithelium is much more than merely a dark coat that prevents reflection of light, playing an important role in ‘nibbling away’ at the distal ends of the outer segments of rods and cones, and removing ‘worn out’ photoreceptor membrane. New photoreceptor discs are synthesised continuously by the rods and cones with a rod turning over its complement of photoreceptors every 10 days (Fawcett, 1994). The pigment epithelium also cycles visual pigment cycles to and from the rods and stores the Vitamin A that is an essential component of the pigment.
The Extraocular Muscles The rat has all the extraocular muscles found in man, plus the retractor bulbis muscle, which lies beneath the eyeball
and pulls it back into the orbit. The four rectus muscles (lateral, medial, superior and inferior) arise from just below the optic foramen, and are inserted into the sclera of the eyeball just behind the equator. The lateral (or external) and medial (or internal) muscles turn the eye laterally and medially; the superior rectus turns the eye upwards and a little inwards, the inferior rectus turns the eye downwards, and again, a little inwards. The superior oblique muscle runs a rather unusual course. From the orbital aspect of the frontal bone it forms a tendon that runs through a tiny cartilaginous or bony hook attached to the bone, before swinging back to the eyeball as a second muscle belly that is inserted into the sclera. It turns the eye downwards and a little outwards. The inferior oblique runs from medial to lateral beneath the eyeball and turns the eye upwards, and again, a little outwards. Acting in concert the combination of their actions permits movement of the eyeball in all directions. The splendidly named levator palpebrae superioris muscle that lifts the upper eyelid lies in the roof of the orbit and is inserted into the connective tissue of the upper lid. The extraocular muscles are supplied by different cranial nerves that enter the orbit through the foramen orbitorotundum (see Chapter 21(3): The Autonomic Nervous System). The nerve supply of the individual muscles is as follows: Superior oblique by the trochlear nerve, CN IV (trochlea means pulley).
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Lateral rectus by the abducens nerve, CN VI (so named because it supplies the muscle that abducts the eyeball). Superior and inferior recti, the medial rectus, levator palpebrae superioris and inferior oblique by the oculomotor nerve, CN III.
Autonomic Supply of the Eyeball and Associated Structures Preganglionic parasympathetic fibres travel from the Edinger Westphal nuclei of the midbrain to the orbit via CN III, the oculomotor nerve. Here they leave the oculomotor nerve and run to the ciliary ganglion found deep in the orbit. From that ganglion, postganglionic fibres run as the short ciliary nerves to the eyeball to innervate the constrictor muscle of the pupil and the weak ciliary muscle. Postganglionic sympathetic fibres reach the orbit in the plexus around the ophthalmic artery and join the long ciliary nerves (branches of the ophthalmic nerve (CN V (1)). The long ciliary nerves run to the eyeball and supply the dilator muscle of the pupil and provide sensory fibres to the cornea.
Blood Supply of the Eye and Associated Structures In man the ophthalmic artery arises within the skull from the internal carotid artery, and travels to the orbit with the optic nerve via the optic foramen. In the orbit it runs close to the optic nerve giving off the long and short posterior ciliary arteries that pierce the sclera and supply the choroid. As the long posterior ciliary arteries run forwards they are joined by the anterior ciliary arteries, which arise from the arteries supplying the extraocular muscles, forming an arterial circle at the outer edge of the iris. The ophthalmic artery also gives off the central artery of the retina, which pierces the sheath of the optic nerve and finds its way deep into the nerve to appear on the surface of the retina in the middle of the optic disc. In the rat, the ophthalmic artery arises from the palatine branch of the pterygopalatine artery, itself a branch of the internal carotid, taking a circuitous route through the foramen orbitorotundum to the posterior corner of the orbit-temporal fossa. The palatine branch of the pterygopalatine artery also gives rise to the lacrimal artery in the rat, which in man is a branch of the ophthalmic artery. Greene (1935) stated that the lacrimal artery supplies the intra-orbital lacrimal gland; Hebel and Stromberg (1976) stated that both the intra- and extra-orbital lacrimal glands were supplied by branches of the facial artery. In both the rat and man the extraocular muscles are supplied by branches of the ophthalmic artery. The posterior ciliary
artery of the rat runs inside the sheath of the optic nerve, dividing into the long posterior ciliary arteries and the central artery of the retina. There are no short posterior ciliary arteries in the rat (Sugiyama et al., 1999). These authors made no mention of anterior ciliary arteries and refer to the posterior ciliary artery as a terminal artery (end artery). In the rat, the central artery of the retina supplies more of the layers of the retina than in man, where it supplies only the ganglion cells and their nerve fibres, the remaining layers being supplied from the choroidal vessels. In the rat branches of the central artery (five to eight) of the retina radiate like the spokes of wheel, running a straight course over the surface of the retina towards its periphery (Heywood, 1973). Retinal veins run alongside the arteries, appearing darker and wider than the arteries when examined with an ophthalmoscope. The yellow-pink/creamy optic disc seen in man is not apparent on examination of the rat eye. This has been attributed this to the shape of the cup of the head of the optic nerve, described as ‘narrow, funnel-shaped, and oblique to the disc surface’, which gives the disc a dark appearance (Cohen et al., 2003). Venous blood from the choroid drains to the several vorticeal veins that penetrate the sclera and join the anastomosing veins behind the eyeball. The vorticeal veins are so named because the many venous channels of the choroid appear to sweep up to these veins like a vortex. The veins of the retina run with the arteries, but leave at the optic nerve to join the ophthalmic veins. Venous blood from all the tissues of the orbit is collected by the superior and inferior ophthalmic veins, joining to form the ophthalmic vein, which drains to the cavernous and petrosal sinuses within the skull. Timm (1979) reported that the veins behind the eyeball of the rat form a plexus, but in the mouse there is a confluent venous sinus. Some drainage from the veins of the orbit to those of the pterygopalatine fossa also occurs. Because the albino rat has no melanin in the ‘pigmented layer’ of the retina or its choroid, the blood vessels of the choroid can be seen more clearly on ophthalmoscopy than in pigmented animals. It is important not to confuse melanin (absent in the albino rat) with the visual pigment of the rods and cones of the retina, this is present in both white rats and albino humans, without it neither could see.
Lymphatic Drainage of the Eye It has often been said that the eye lacks a system of lymphatic drainage, but more recent work has shown that this is not true, and that the drainage system of the aqueous humour might best be regarded as at least an auxiliary part of the lymphatic system (Nakao et al., 2012). Lymphatic drainage of the uveal tract has also been suggested.
The Eye Chapter | 22
The extraocular structures, muscles and the intra-orbital lacrimal gland and the Harderian gland all appear to have a conventional lymphatic drainage system.
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Ju, C., Zhang, K., Wu, X., 2012. Derivation of corneal endothelial cell-like cells from rat neural crest cells in vitro. PLoS One 7 (7), https://doi.org/10.1371/journal. e42378. Available from: pone.0042378. Kuszal, J.R., Zoltoski, R.K., Tiedemann, C.E., 2004. Development of lens sutures. Int. J. Dev. Biol. 48, 889 902. Masland, R.H., 2001. The fundamental plan of the retina. Nat. Neurosci. 4 (9), 877 886. Masland, R.H., Raviola, E., 2000. Confronting complexity: strategies for understanding the microcircuitry of the retina. Ann. Rev. Neurosci. 23, 249 284. Meng, Q., Mongan, M., Carreira, V., Kurita, H., Liu, C.-Y., Kao, W.W.-Y., et al., 2014. Eyelid closure in embryogenesis is required for ocular adnexa development. Invest Ophthalmol. Vis. Sci. 55 (11), 7652 7661. Nakao, S., Hafezi-Moghadam, A., Ishibashi, T., 2012. Lymphatics and lymphangiogenesis in the eye. J. Ophthalmol. (Article ID 783163, 11 pages) https://doi.org/10.1155/2012/783163. Pirenne, M.H., 1967. Vision and the Eye, second ed. Chapman and Hall Ltd., London. Powers, M.K., Green, D.G., 1978. Single retinal ganglion cell responses in the dark-reared rat: grating acuity, contrast sensitivity and defocusing. Vis. Res. 18, 1533 1539. Sivak, J.G., Dovrat, A., 1983. Aging and the optical quality of the rat crystalline lens. J. Invest. Ophthalmol. Vis. Sci. 24 (9), 1162 1166. Sugiyama, K., Gu, Z.-B., Kawase, C., Yamamoto, T., Kitazawa, Y., 1999. Optic nerve and peripapillary choroidal microvasculature of the rat eye. Invest. Ophthalmol. Vis. Sci. 40 (13), 3084 3090. Tansley, K., 1965. Vision in Vertebrates. Chapman and Hall and Science Paperbacks, London. Timm, K.I., 1979. Orbital venous anatomy of the rat. Lab. Anim. Sci. 29 (5), 636 638. Walling, B.E., Marit, G.B., 2016. The eye and Harderian gland (Chapter 12) In: Parker, G.A., Picut, C.A. (Eds.), Atlas of Histology of the Juvenile Rat. Academic Press, Elsevier, Oxford, pp. 373 377. Walls, G.L., 1963. The Vertebrate Eye and Its Adaptive Radiation. Hafner Publishing Company, New York and London (originally published 1942). Zieske, J.D., 2004. Corneal development associated with eyelid opening. Int. J. Dev. Biol. 48, 903 911.
FURTHER READING Coulombre, A.J., 1965. The eye (Chapter 9) In: DeHaan, R.L., Ursprung, H. (Eds.), Organogenesis. Holt, Rinehart and Winston, New York, Chicago, San Francisco, Toronto and London, pp. 219 252.