Chapter 1: Embryology and Anatomy PDF

Chapter 1 Embryology and Anatomy Chapter Outline Embryology _ 3 Anatomy _ 3 Cornea _ 4 Sclera _ 4 Anterior Chamber 4 Lens _ 6 Uveal Tract _ 8 Posterior Chamber and Vitreous Humour 9 Retina _ 9 The Blood Supply of the Eye _ Clinical Anatomy of the Eye _ 11 13 EMBRYOLOGY The central ner...

Chapter 1 Embryology and Anatomy Chapter Outline Embryology _ 3 Anatomy _ 3 Cornea _ 4 Sclera _ 4 Anterior Chamber 4 Lens _ 6 Uveal Tract _ 8 Posterior Chamber and Vitreous Humour 9 Retina _ 9 The Blood Supply of the Eye _ Clinical Anatomy of the Eye _ 11 13 EMBRYOLOGY The central nervous system is developed from the neural groove which invaginates to form the neural tube running longitudinally down the dorsal surface of the embryo. At either side from the lateral aspect of the anterior portion of this structure, which is the precursor of the forebrain, a thickening appears at an early stage (the optic plate) which then grows outwards as a diverticulum towards the surface to form the primary optic vesicle (Fig. 1.1A and B). From this pair of diverticula from the sides of the forebrain and the mesodermal and ectodermal structures in contact with it, the two eyes develop. After it meets the surface ectoderm, the primary optic vesicle invaginates from below (the optic cup), the line of invagination remaining open for some time as the embry- onic fissure (Fig. 1.1C). The inner layer of the cup forms the main structure of the retina, the nerve fibres from which eventually grow backwards towards the brain. Its outer layer remains as a single layer of pigment epithe- lium; between the two lies a narrow space representing the original optic vesicle; and from its anterior border devel- ops parts of the ciliary body and iris (Fig. 1.1E). At the point where the neural ectoderm meets the surface ecto- derm, the latter thickens to form the lens plate, invaginates to form the lens vesicle (Fig. 1.1C) and then separates to form the lens (Fig. 1.1D). The hyaloid artery enters the optic cup through the embryonic fissure and grows forward to meet the lens, bringing temporary nourishment to the developing structures before it eventually atrophies and disappears; as it does so, its place is taken by a clear jelly (the vitreous) largely secreted by the surrounding neural ectoderm. While these ectodermal events are taking place, the mesoderm surrounding the optic cup differentiates to form the coats of the eye and the orbital structures; that between the lens and the surface ectoderm becomes hollowed to form the anterior chamber, lined by mesodermal condensa- tions which form the anterior layers of the iris, the angle of the anterior chamber and the main structures of the cornea; while the surface ectoderm remains as the corneal and con- junctival epithelium. In the surrounding region, folds grow over in front of the cornea, unite and separate again to form the lids (Fig. 1.1E and F). Summary of ocular embryogenesis is given in Table 1.1. In summary, the eye is essentially formed from both ectoderm and mesoderm. The ectoderm is of two types: (i) The neural ectoderm derived from the neural tube and (ii) The surface ectoderm on the side of the head (Table 1.2). ANATOMY The wall of the globe is composed of a dense, imperfectly elastic supporting tissue—the transparent cornea and the opaque sclera (Fig. 1.2). The anterior part of the sclera is covered by a mucous membrane, the conjunctiva, which is reflected from its surface onto the lids. Inside the eye, posteriorly the sclera is lined by the uveal tract and retina and the globe are broadly divided into the anterior segment and posterior segment by the lens. The iris divides the anterior segment into an anterior 3 https://t.me/MedicalBooksStore4 SECTION | I Anatomy and Physiology a a A bb B b bc C D l Stroma (substantia propria) l Dua’s layer (pre-Descemet’s layer) l Descemet’s membrane l Endothelium. Transparency of Cornea Transparency of the cornea is related to the regularity of the stromal components. The stromal collagen fibrils are of regular diameter, arranged as a lattice with an interfibrillar spacing of less than a wavelength of light so that tangential rows of fibres act as a diffraction grating resulting in destructive interference of scattered rays. The primary mechanism controlling stromal hydration is a function of the corneal endothelium which actively pumps out the electrolytes and water flows out passively. The endothe- lium is examined by a specular microscope at a magnifica- tion of 5003. Endothelial cells become less in number with age and the residual individual cells may enlarge to compensate. Blood Supply and Innervation The cornea is avascular with no blood vessels with the excep- tion of minute arcades, extending about 1 mm into the cornea at the limbus. It is dependent for its nourishment upon diffu- sion of tissue fluid from the vessels at its periphery and the aqueous humour. The cornea is very richly supplied with unmyelinated nerve fibres derived from the trigeminal nerve. E F FIGURE 1.1 The development of the eye. In each case the solid black is the neural ectoderm, the hatched layer is the surface ectoderm and its derivatives, the dotted area is the mesoderm: a, cavity of the forebrain; b, cavity of the optic vesicle; c, cavity of the optic cup (or secondary optic vesicle) formed by invagination. (A) Transverse section through the anterior part of the forebrain and optic vesicles of a 4 mm human embryo. (B) The primary optic vesicle. (C) The formation of the optic cup by invag- ination at the embryonic fissure and invagination of the surface epithelium. (D) The optic cup and lens vesicle. (E) The formation of the ciliary region and iris, the anterior chamber, the hyaloid artery and the lid folds. The lens is formed from the posterior cells of the lens vesicle. (F) The complete eye. chamber bounded by the cornea anteriorly and posterior chamber bounded by the lens posteriorly. The cavity contains a clear watery fluid called aqueous humour. In the posterior segment, the posterior chamber lies behind the lens, has a cavity filled with a transparent gel-like substance called the vitreous humour and is bounded by the ciliary body and retina. Sclera The sclera is the ‘white’ supporting wall of the eyeball and is continuous with the clear cornea. It is a dense white tis- sue, thickest in the area around the optic nerve. The outer surface of the sclera is covered by the conjunctiva, beneath which is a layer of loose connective tissue called episclera and the innermost layer of the sclera consists of elastic fibres called the lamina fusca. Lining the inner aspect of the sclera are two structures—the highly vascular uveal tract concerned chiefly with the nutrition of the eye, and within this a nervous layer, the true visual nerve endings concerned with the reception and transformation of light stimuli, called the retina. Cornea The cornea is the transparent front part of the eye which resembles a ‘watchglass’ and consists of different layers and regions: l Epithelium l Bowman’s membrane Anterior Chamber The anterior chamber is a space filled with fluid, the aque- ous humour; it is bounded in front by the cornea, behind by the iris and the part of the anterior surface of the lens which is exposed in the pupil. Its peripheral recess is known as the angle of the anterior chamber, bounded posteriorly by the root of the iris and the ciliary body and anteriorly by the corneosclera (Fig. 1.3). In the inner layers of the sclera at this part there is a circular venous sinus, sometimes broken up into more than one lumen, called the canal of https://t.me/MedicalBooksStoreChapter | 1 Embryology and Anatomy 5 TABLE 1.1 Summary of Ocular Embryogenesis Period After Conception Major Milestone 3 weeks Optic groove appears 4th week Optic pit develops into optic vesicle Lens plate forms Embryonic fissure develops Fig. 1.1A–D 1st month Lens pit and then lens vesicle forms Hyaloid vessels develop 1½ months Closure of embryonic fissure Differentiation of retinal pigment epithelium Proliferation of neural retinal cells Appearance of eyelid folds and nasolacrimal duct 7th week Formation of embryonic nucleus of the lens Sclera begins to form Migration of waves of neural crest First wave: formation of corneal and trabecular endothelium Second wave: formation of corneal stroma Third wave: formation of iris stroma Fig. 1.1E 3rd month Differentiation of precursors of rods and cones Anterior chamber appears Fetal nucleus starts to develop Sclera condenses Eyelid folds lengthen and fuse 4th month Formation of retinal vasculature begins Hyaloid vessels begin to regress Formation of physiological optic disc cup and lamina cribrosa Canal of Schlemm appears Bowman’s membrane develops Formation of major arterial circle and sphincter muscle of iris 5th month Photoreceptors differentiate Eyelid separation begins 6th month Differentiation of dilator pupillae muscle Nasolacrimal system becomes patent Cones differentiate Fig. 1.1F 7th month Rods differentiate Myelination of optic nerve begins Posterior movement of anterior chamber angle Retinal vessels start reaching nasal periphery 8th month Completion of anterior chamber angle formation, hyaloid vessels disappear 9th month Retinal vessels reach temporal periphery, pupillary membrane disappears After birth Macular region of the retina develops further https://t.me/MedicalBooksStore6 SECTION | I Anatomy and Physiology TABLE 1.2 Primordial Tissue and its Derivatives Precursor Derivatives Neural ectoderm Smooth muscle of the iris Optic vesicle and cup Iris epithelium Ciliary epithelium Part of the vitreous Retina Retinal pigment epithelium Fibres of the optic nerve Surface ectoderm Conjunctival epithelium Corneal epithelium Lacrimal glands Tarsal glands Lens Mesoderm Extraocular muscles Corneal stroma Sclera Iris Vascular endothelium of eye and orbit Choroid Part of the vitreous Neural crest* Corneal stroma, keratocytes and endothelium Sclera Trabecular meshwork endothelium Iris stroma Ciliary muscles Choroidal stroma Part of the vitreous Uveal and conjunctival melanocytes Meningeal sheaths of the optic nerve Ciliary ganglion Schwann cells of the nerve sheaths Orbital bones Orbital connective tissue Connective tissue sheath and muscular layer of the ocular and orbital blood vessels *During the folding of the neural tube, a ridge of cells comprising the neural crest develops from the tips of the converging edges and migrates to the dorsolateral aspect of the tube. Neural crest cells from this region subsequently migrate and give rise to various structures within the eye and the orbit. The structures are listed from anterior to posterior. Schlemm, which is of great importance for the drainage of the aqueous humour. At the periphery of the angle between the canal of Schlemm and the recess of the anterior cham- ber there lies a loosely constructed meshwork of tissues, the trabecular meshwork. This has a triangular shape, the apex arising from the termination of Descemet’s membrane and the subjacent fibres of the corneal stroma and its base merg- ing into the tissues of the ciliary body and the root of the iris. It is made up of circumferentially disposed flattened bands, each perforated by numerous oval stomata through which tortuous passages exist between the anterior chamber and Schlemm’s canal. The extracellular spaces contain both a coarse framework (collagen and elastic components) and a fine framework (mucopolysaccharides) of extracellular materials, which form the probable site of greatest resis- tance to the flow of aqueous. The endothelial cells of Schlemm’s canal are connected to each other by junctions which are not ‘tight’ but this intercellular pathway accounts for only 1% of the aqueous drainage. The major outflow pathway appears to be a series of transendothelial pores, which are usually found in out- pouchings of the endothelium called ‘giant vacuoles’. The anterior chamber is about 2.5 mm deep in the centre in a normal adult; it is shallower in very young children and in old people. Lens The lens is a biconvex mass of peculiarly differentiated epithelium. It has three main parts the outer capsule lined by the epithelium and the lens fibres and is developed from an invagination of the surface ectoderm of the fetus, so that what was originally the surface of the epithelium comes to lie in the centre of the lens, the peripheral cells corresponding to the basal cells of the epidermis. Just as the epidermis grows by the proliferation of the basal cells, the old superficial cells being cast off, so the lens grows by the proliferation of the peripheral cells. The old cells, how- ever, cannot be cast off, but undergo changes (sclerosis) analogous to that of the stratum granulosum of the epider- mis, and become massed together in the centre or nucleus.; moreover, the newly formed cells elongate into fibres. The lens fibres have a complicated architectural form, being arranged in zones in which the fibres growing from oppo- site directions meet in sutures. Without going into details, it is important to bear in mind that the central nucleus of the lens consists of the oldest cells and the periphery or cortex the youngest (Fig. 1.4). The fibres of the lens are split into regions depending on the age of origin. The central denser zone is the nucleus surrounded by the cortex. The oldest and innermost is the central embryonic nucleus (formed 6–12 weeks of embry- onic life) in which the lens fibres meet around Y-shaped sutures. Outside this embryonic nucleus, successive nuclear zones are laid down as development proceeds, called, depending on the period of formation, the fetal nucleus (3–8 months of fetal life), the infantile nucleus (last month of intrauterine life till puberty), the adult nucleus (corre- sponding to the lens in early adult life), and finally and most peripherally, the cortex consisting of the youngest fibres. In https://t.me/MedicalBooksStoreChapter | 1 Embryology and Anatomy 7 Ora serrata Posterior chamber Extraocular muscle Canal of Schlemm Retinal vessels Lens Iris Fovea centralis Pupil Dura of optic nerve sheath Cornea Vitreous body Cerebrospinal fluid Anterior chamber Ciliary processes and suspensory ligament of lens Canal of Schlemm Ciliary body Central retinal artery and vein Sclera Optic nerve (axons of retinal ganglion cells) Choroid Pigmented epithelium (outermost layer of retina) Photosensitive retina (innermost 9 layers) Extraocular muscle FIGURE 1.2 Review USMLE Step 3. St. Louis: Mosby; 2007. pp. 28–44) General anatomy of the eyeball, including its tunics and chambers. (From David Rolston, Craig Nielsen. Chapter 2: Ophthalmology. Rapid Corneal epithelium Bowman’s membrane Trabecular meshwork Cornea Descemet’s membrane Anterior chamber Limbus Iris crypts Iris Scleral spur Canal of Schlemm Posterior chamber Sclera Zonule of Zinn Lens Ciliary body Ciliary muscle Ciliary processes Pars plana of the ciliary body Pars plicata of the ciliary body Vitreous humour FIGURE 1.3 The region of the angle of the anterior chamber. this part of the lens also the fibres meet along the sutures with a general stellate arrangement. The mass of epithe- lium which constitutes the lens is surrounded by a hyaline membrane, the lens capsule, which is thicker over the anterior than over the posterior surface and is thinnest at the posterior pole; the thickest basement membrane in the body it is a cuticular deposit secreted by the epithelial cells having on the outside a thin membrane, the zonular lamella. The lens in fetal life is almost spherical; it gradually becomes flattened so as to assume a biconvex shape. It is held in place by the suspensory ligament or zonule of Zinn. This is not a complete membrane, but consists of bundles of strands which pass from the surface of the cili- ary body to the capsule where they join with the zonular lamella. The strands pass in various directions so that the bundles often cross one another. Thus, the most posterior arise from the pars plana of the ciliary body almost as far https://t.me/MedicalBooksStore8 SECTION | I Anatomy and Physiology FIGURE 1.4 The structure of the lens in an adult 40 years of age, as shown in the optical beam of the slit-lamp: 1, anterior capsule; 2, cortex; 3, adult nucleus; 4, infantile nucleus; 5, fetal nucleus; and 6, embryonic nucleus (see Fig. 18.9). back as the ora serrata; these lie in contact with the ciliary body for a considerable distance and then curve towards the equator of the lens to be inserted into the capsule slightly anterior to the equator. A second group of bundles springs from the summits and sides of the ciliary processes, i.e. far forwards, and passes backwards to be inserted into the lens capsule slightly posterior to the equator. A third group passes from the summits of the processes almost directly inwards to be inserted at the equator. Uveal Tract The uveal tract consists of three parts, of which the two posterior, the choroid and ciliary body, line the sclera while the anterior forms a free circular diaphragm, the iris. The plane of the iris is approximately coronal; the aperture of the diaphragm is the pupil. Situated behind the iris and in contact with the pupillary margin is the crystalline lens. Iris The iris is thinnest at its attachment to the ciliary body, so that if torn it tends to give way in this region (Fig. 1.3). It is composed of a stroma containing branched connective tis- sue cells, usually pigmented but largely unpigmented in blue irides, with a rich supply of blood vessels which run in a general radial direction. The tissue spaces communicate directly with the anterior chamber through crypts found mainly near the ciliary border; this allows the easy transfer of fluid between the iris and the anterior chamber. The stroma is covered on its posterior surface by two layers of pigmented epithelium, which developmentally are derived from the retina and are continuous with each other at the pupillary margin. The anterior layer consists of flattened cells and the posterior of cuboidal cells. From the epithelial cells of the former, two unstriped muscles are developed which control the movements of the pupil, the sphincter pupillae, a circular bundle running round the pupillary margin, and the dilator pupillae, arranged radially near the root of the iris. The anterior surface of the iris is covered with a single layer of endothelium, except at some minute depressions or crypts which are found mainly at the ciliary border; it usually atrophies in adult life. The iris is richly supplied by sensory nerve fibres derived from the trigeminal nerve. The sphincter pupillae is supplied by parasympathetic autonomous secretomotor nerve fibres derived from the oculomotor nerve, while the motor fibres of the dilator muscle are derived from the cervical sympathetic chain. Ciliary Body The ciliary body in anteroposterior section is shaped roughly like an isosceles triangle, with the base forwards. The iris is attached about the middle of the base, so that a small portion of the ciliary body enters into the posterior boundary of the anterior chamber at the angle (Fig. 1.3). The chief mass of the ciliary body is composed of unstriped muscle fibres, the ciliary muscle. This consists of three parts with a common origin in the ciliary tendon, a structure which runs circumferentially round the globe blending with the ‘spur’ of the sclera and related to the trabecular mesh work. The greater part of the muscle is composed of meridional fibres running anteroposteriorly on the inner aspect of the sclera to find a diffuse insertion into the suprachoroid. Most of the remaining fibres run obliquely in interdigitating V-shaped bundles so as to give the impression of running in a circle round the ciliary body, concentrically with the base of the iris. The third portion of the muscle is composed of a few tenuous iridic fibres arising most internally from the common origin and finding insertion in the root of the iris just anterior to the pigmentary epithelium in close relation to the dilator muscle. The inner surface of the ciliary body is divided into two regions; the anterior part is corrugated with a number of folds running in an anteroposterior direction while the posterior part is smooth. The anterior part is therefore, called the pars plicata; the posterior, the pars plana. About 70 plications are visible around the circumference macro- scopically, but if microscopic sections are examined, many smaller folds, the ciliary processes, will be seen between them. These contain no part of the ciliary muscle, but con- sist essentially of tufts of blood vessels, not unlike the glomeruli of the kidney. They are covered upon the inner surface by two layers of epithelium, which belong properly https://t.me/MedicalBooksStoreChapter | 1 Embryology and Anatomy 9 to the retina, and are continuous with similar layers in the iris; the outer layer, corresponding to the anterior in the iris, consists of flattened cells, the inner of cuboidal cells, but only the outer layer in the ciliary body is pigmented. The ciliary body extends backward as far as the ora ser- rata, at which point the retina proper begins abruptly; the transition from the ciliary body to the choroid, on the other hand, is gradual, although this line is conveniently accepted as the limit of the two structures. The ora serrata thus circles the globe, but is slightly more anterior on the nasal than on the temporal side. The ciliary body is richly supplied with sensory nerve fibres derived from the trigeminal nerve. The ciliary muscle is supplied with motor fibres from the oculomotor and sympathetic nerves. Choroid The choroid is an extremely vascular membrane in con- tact everywhere with the sclera, although not firmly adher- ent to it, so that there is a potential space between the two structures—the epichoroidal or suprachoroidal space. On the inner side, the choroid is covered by a thin elastic membrane, the lamina vitrea or membrane of Bruch. The blood vessels of the choroid increase in size from within outwards, so that immediately beneath the mem- brane of Bruch there is a capillary plexus of fenestrated vessels, the choriocapillaris. Upon this is the layer of medium-sized vessels, while most externally are the large vessels, the whole being held together by a stroma consisting of branched pigmented connective tissue cells. The choroid is supplied with sensory nerve fibres from the trigeminal as well as autonomic nerves, presumably of vasomotor function. Posteriorly, the vitreous body is attached to the margin of the optic disc and to the macula forming a ring around each structure and also to the larger blood vessels. The primary vitreous is concentrated into the centre of the globe by the secondary vitreous and forms the canal of Cloquet which contains material less optically dense than the secondary vitreous. The body of the vitreous has a loose fibrous framework of collagenous fibres whereas its cortex is made up of collagen-like fibres and protein. Posterior Chamber and Vitreous Humour It will be noticed that there is somewhat a triangular space between the back of the iris and the anterior surface of the lens, having its apex at the point where the pupillary margin comes in contact with the lens; it is bounded on the outer side by the ciliary body. This is the posterior chamber and contains aqueous humour. Behind the lens is the large vitreous chamber, contain- ing the vitreous humour. This is a jelly-like material chemically of the nature of an inert gel containing a few cells and wandering leucocytes. As in other gels, the con- centration of the micellae on the surface gives rise to the appearance of a boundary membrane in sections—the so-called hyaloid membrane. The vitreous body is attached anteriorly to the posterior lens surface by the ligament of Weigert. In the region of the ora the vitreous cortex is firmly attached to the retina and pars plana and this attachment is referred to as the vitreous base. Retina The retina corresponds in extent to the choroid, which it lines, although the same embryological structure is contin- ued forward as a double layer of epithelium as far as the pupillary margin. If the two layers of epithelium are traced backwards, the anterior layer in the iris is found to be con- tinuous with the outer layer in the ciliary body, and this again is continued into the pigment epithelium of the retina as a single layer of hexagonal cells lying immediately adja- cent to the membrane of Bruch. Similarly, the posterior layer in the iris, although pigmented, passes into the inner unpigmented layer of the ciliary body, and this suddenly changes at the ora serrata into the highly complex visual retina. The retina consists of a number of layers (Fig. 1.5) formed by three strata of cells and their synapses. l The visual cells (lying externally) l A relay layer of bipolar cells (lying intermedially), and l A layer of ganglion cells (lying internally), the axons of which run into the central nervous system. Layers of Retina (Outer to Inner) 1. Rods and cones: Most externally, in contact with the pigment epithelium, is a neural epithelium, the rods and cones, which are the end-organs of vision (Fig. 1.6). The microanatomy of the rods and cones reveals the trans- ductive region (outer segment), a region for the mainte- nance of cellular homoeostasis (inner segment), a nuclear region (outer nuclear layers) and a transmissive region (the outer plexiform or synaptic layer). When the outer segments of the rods are sectioned parallel to their long axes, they are seen by the electron microscope to consist of a boundary or cell membrane, which encloses a stack of membrane systems. The discs in the rods of many species are continuously renewed throughout life. New discs are formed in the region of the inner segment and are progressively dis- placed towards the pigment epithelium. Rod discs have a limited life and are eventually lost to the pigment epithelium. https://t.me/MedicalBooksStore10 SECTION | I Anatomy and Physiology Retina Photoreceptor cell RPE cell Ch RPE Axoneme Transition zone Basal body Rootlet Outer segment Photoreceptor sensory cilium ONL Inner segment INL Cell body Nucleus GC AB FIGURE 1.5 Retina and photoreceptor cell structure. (A) Cross-section of human retina, showing retinal layers. (B) Drawing of rod photoreceptor cell, showing different portions of the cell. The photoreceptor sensory cilium is indicated. Ch, choroid; GC, ganglion cell layer; INL, inner nuclear layer; ONL, outer nuclear layer; RPE, retinal pigment epithelium. (From Leonard A Levin, Daniel M Albert. Chapter 74: Retinitis pigmentosa and related disorders. Ocular Disease: Mechanisms and Management. Edinburgh: Saunders; 2010. pp. 579– 589) A B C D FIGURE 1.6 Anatomical features of rods and cones revealed by elec- tron microscopy. (A) Cross section of the human retina (×2440) demon- strating rod and cone outer segments adjacent to the pigment epithelium. (B) Tangential section through the inner segments of the human photoreceptor layer (×3750). The large inner segments belong to cones, and the smaller inner segments are those of rods; note the large number of mitochondria in the inner segments. (C) Tangential section of human retina at the outer segment level, showing rod discs contained within the cell membrane (×14 110). (D) Rod outer segment showing discs contained within the cell. A phagosome within a pigment epithelial cell is on the upper right (rhesus monkey ×23 000). (From Stephen Ryan, Andrew Schachat, Charles Wilkinson, David Hinton, Charles Wilkinson, eds. Retina. 4th ed. Edinburgh: Elsevier; 2005) At the junction of the inner and outer segments, the cell body of both rods and cones constricts. The electron microscope reveals a connecting cilium which is always eccentric and provides the only link between the inner and outer segments. 2. Retinal pigment epithelium: The pigment epithe- lium consists of a single layer of hexagonal cells lying between the photoreceptor outer segment and Bruch’s membrane. They assist the metabolism of the retina by transporting selected substances to the receptor cells. Products of metabolism are freely exchanged between the receptor cells and the pigment epithelium. The most striking inclusions in the pigment epithe- lium are the organelles responsible for its colour, the melanin granules. Most of the light which passes through the retina and is not absorbed by the photopig- ments in the photoreceptor outer segments is absorbed by these granules. Phagosomes are known to be dis- carded rod discs that have been engulfed by the pigment epithelium. The phagocytic capacity of the pigment epi- thelium is demonstrated in the response of the retina to injury as by laser irradiation, when the number of phagosomes in the underlying epithelial cells increases significantly. 3. Outer nuclear layer: The outer nuclear layer (the nuclei of the rods and cones). 4. Outer plexiform layer consisting of synapses. 5. Inner nuclear layer (the nuclei of the bipolar cells). 6. Inner plexiform layer (again synaptic). 7. Ganglion cell layer. 8. The nerve fibre layer composed of the axons of ganglion cells running centrally into the optic nerve. https://t.me/MedicalBooksStoreChapter | 1 Embryology and Anatomy 11 These special nervous constituents are bound together by neuroglia, the better developed vertical cells being called the fibres of Müller, which in addition to acting as a supportive framework, have a nutritive function. The struc- ture is completed by two limiting membranes, the outer perforated by the rods and cones, and the inner separating the retina from the vitreous. To excite the rods and cones, incident light has to tra- verse the tissues of the retina but this arrangement allows these visual elements to approximate the opaque pigmented layer to form a functional unit, and their source of nourish- ment is the choriocapillaris. At the posterior pole of the eye, which is situated about 3 mm to the temporal side of the optic disc, a specially dif- ferentiated spot is found in the retina, the fovea centrali, a depression or pit, where only cones are present in the neu- roepithelial layer and the other layers are almost completely absent. The fovea is the most sensitive part of the retina, and is surrounded by a small area, the macula lutea, or yel- low spot which, although not so sensitive, is more so than other parts of the retina. It is here that the nuclear layers become gradually thinned out, while parts of the plexiform layers are especially in evidence. The ganglion cells too, instead of consisting of a single row of cells, are heaped up into several layers. There are no blood vessels in the retina at the macula, so that its nourishment is entirely dependent upon the choroid. At the optic disc, the fibres of the nerve fibre layer pass into the optic nerve (see Chapter 17, Diseases of the Uveal Tract), the other layers of the retina stopping short abruptly at the edge of the aperture in the scleral canal. This is spanned by a transverse network of connective tissue fibres containing much elastic tissue, the lamina cribrosa, through the meshes of which the optic nerve fibres pass; on the posterior side they suddenly become surrounded by medul- lary sheaths. The fibres, the axons of the ganglion cells of the retina, are of course, afferent or centripetal fibres, but the optic nerve also contains a few efferent or centrifugal fibres. THE BLOOD SUPPLY OF THE EYE The arteries of the eye in man are all derived from the ophthalmic artery (Fig. 1.7A and B), which is a branch of the internal carotid artery. The ophthalmic artery has few anastomoses, so that on the arterial side the ocular circula- tion is an offshoot of the intracranial circulation. This does not apply in so marked a degree to the venous out- flow from the eye. In man, most of the blood passes to the cavernous sinus by way of the ophthalmic veins, but they anastomose freely in the orbit, the superior ophthalmic vein communicating with the angular vein at the root of the nose and the inferior ophthalmic vein with the pterygoid plexus. The retina is supplied by the central retinal artery, which enters the optic nerve on its lower surface, 15–20 mm behind the globe. The central artery divides on, or slightly posterior to, the surface of the disc into the main retinal trunks, which will be considered in detail later (Fig. 1.7A). The retinal arteries are end-arteries and have no anastomoses at the ora serrata. The only place where the retinal system anastomoses with any other is in the neighbourhood of the lamina cribrosa. The veins of the retina do not accurately follow the course of the arter- ies, but they behave similarly at the disc, uniting on, or slightly posterior to, its surface to form the central vein of the retina, which follows the course of the corresponding artery. The blood supply of the optic nerve head in the region of the lamina cribrosa is served by fine branches from the arterial circle of Zinn but mainly from the branches of the posterior ciliary arteries (Fig. 1.8). The central retinal artery makes no contribution to this region. The prelaminar region is supplied by centripetal branches from the peripap- illary choroidal vessels with some contribution from the vessels in the lamina cribrosa region. The central artery of the retina does not contribute to this region either. The sur- face layer of the optic disc contains the main retinal vessels and a large number of capillaries in addition to some small Vessels of the iris Ciliary processes Recurrent ciliary arteries Orbiculus ciliaris Vortex vein Circulus iridis major Anterior ciliary arteries Anterior ciliary veins Ciliary muscle Recurrent ciliary arteries Anterior ciliary arteries Long posterior ciliary arteries Vortex vein Choroid A FIGURE 1.7 B Short posterior ciliary arteries Optic nerve (A) The retinal circulation. (B) The choroidal circulation. https://t.me/12 SECTION | I Anatomy and Physiology Retinal vein Retinal artery A BCD Posterior ciliary artery Optic Disc PR LC Cilio-retinal artery Posterior ciliary artery FIGURE 1.8 Blood supply of the optic nerve. Region marked: A, represents the surface of the disc and peripapillary nerve fibre layer; B, portion anterior to the lamina cribrosa; C, portion related to the lamina cribrosa; D, portion behind the lamina cribrosa; LC, lamina cribrosa; and PR, prelaminar. _ (Reproduced with kind permission from Hayreh SS. Arch Ophthalmology 1977; 95:1560.) vessels. The capillaries on the surface of the disc are derived from branches of the retinal arterioles. In this part of the disc, vessels of choroidal origin derived from the adjacent prelaminar part of the disc may be seen usually in the temporal sector of the disc and one of them may enlarge to form a cilioretinal artery. The capillaries on the surface of the disc are continuous with the capillaries of the peri- papillary retina. These capillaries are mainly venous and drain into the central retinal vein. In the retrolaminar part of the optic nerve, blood is supplied by the intraneural centrifugal branches of the central artery of the retina with centripetal contributions from the pial branches of the choroidal arteries, circle of Zinn, central artery of the retina and the ophthalmic artery. Venous drainage of the optic disc is mainly carried out by the central retinal vein. The prelaminar region also drains into the choroidal veins. There is no venous channel corresponding to the circle of Zinn. The central retinal vein communicates with the choroidal circulation in the prelam- inar region. The uveal tract is supplied by the ciliary arteries, which are divided into three groups—the short posterior, the long posterior and the anterior (Figs 1.7B and 1.9). The short posterior ciliary arteries, about 20 in number, pierce the sclera in a ring around the optic nerve, running perpendicu- larly through the sclera, to which fine branches are given off. The long posterior ciliary arteries, two in number, pierce the sclera slightly farther away from the nerve in the horizontal meridian, one on the nasal, the other on the tem- poral side. They traverse the sclera very obliquely, running in it for a distance of 4 mm. Both these groups are derived from the ophthalmic artery, while the anterior ciliary arter- ies are derived from the muscular branches of the ophthal- mic artery to the four recti. They pierce the sclera 5 or 6 mm behind the limbus, or corneoscleral junction, giving off twigs to the conjunctiva, the sclera and the anterior part of the uveal tract. The ciliary veins also form three groups—the short posterior ciliary, the vortex veins and the anterior ciliary. The short posterior ciliary veins are relatively unimportant; they do not receive any blood from the choroid; but only from the sclera. The vortex veins or venae vorticosae are the most important, consisting usually of four large trunks which open into the ophthalmic veins. They enter the sclera slightly behind the equator of the globe, two above and two below, and pass very obliquely through this tissue. The anterior ciliary veins are smaller than the correspond- ing arteries, since they receive blood from only the outer part of the ciliary muscle. Of these ciliary vessels, the short posterior ciliary arter- ies supply the whole of the choroid, being reinforced ante- riorly by anastomoses with recurrent branches from the ciliary body. The ciliary body and iris are supplied by the long posterior and anterior ciliary arteries. The blood from https://t.me/MedicalBooksStoreChapter | 1 Embryology and Anatomy 13 the whole of the uveal tract, with the exception of the outer part of the ciliary muscle, normally leaves the eye by the vortex veins only. The two long posterior ciliary arteries pass forward be- tween the choroid and the sclera, without dividing, as far as the posterior part of the ciliary body. Here each divides into two branches (Fig. 1.9); they run forward in the ciliary muscle, and at its anterior part bend round in a circular direction, anastomosing with each other and thus forming the circulus arteriosus iridis major. This is situated in the ciliary body at the base of the iris; from it the ciliary pro- cesses and iris are supplied. Other branches from the major arterial circle run radially through the iris, dividing dendriti- cally and ending in loops at the pupillary margin. A circular anastomosis takes place a little outside the pupillary margin, the circulus arteriosus iridis minor. The tributaries of the vortex veins, which receive the whole of the blood from the choroid and iris, are arranged radially, the radii being bent, so as to give a whorled appearance—hence their name. The veins of the iris are collected into radial bundles which pass backwards through the ciliary body, receiving tributaries from the ciliary pro- cesses. Thus reinforced, they form an immense number of veins running backwards parallel to each other through the smooth part of the ciliary body. After reaching the choroid they converge to form the large anterior tributaries of the vortex veins. The veins from the outer part of the ciliary body, on the other hand, pass forward and unite with others to form a plexus (the ciliary venous plexus) which drains into the anterior ciliary veins and the episcleral veins. These vessels communicate directly with the canal of Schlemm which is intimately connected with the anterior chamber by means of numerous tortuous channels through the loose tissue of the trabecular meshwork. From this canal the efferent channels form a complex system (Fig. 1.10); some of them drain into efferent cili- ary veins in the sclera while others traverse the sclera and only join the venous system in the subconjunctival tissues (aqueous veins). The marginal loops of the cornea and the conjunctival vessels are branches of the anterior ciliary vessels (Fig. 1.9). CLINICAL ANATOMY OF THE EYE On clinical examination, the parts of the external surface of the eye appear as shown in Fig. 1.11A. The appearance of the anterior segment from the cornea to the lens is as shown in Fig. 1.11B, and the posterior segment behind the lens as shown in Fig. 1.11C. Scleral plexus Limbal vessels Aqueous v ein Canal of Schlemm Circulus iridis major Radial vessels of iris Circulus iridis minor Ciliary efferent vein Ciliary venous plexus Anterior conjunctival artery Posterior conjunctival artery Anterior ciliary artery Anterior ciliary vein Branch of vortex veins from ciliary system Ciliary plexus Recurrent choroidal veins Choriocapillaris Vortex vein Episcleral artery Branch of short posterior ciliary artery to optic nerve Long posterior ciliary artery Short posterior ciliary artery Anastomosis of choroidal vessels with those of the optic nerve Vessels to the outer sheath of the optic nerve Vessels to the inner sheath of the optic nerve Central retinal artery FIGURE 1.9 The ciliary circulation. https://t.me/MedicalBooksStore14 SECTION | I Anatomy and Physiology C AV SC ACV T IP I EV CB S VP FIGURE 1.10 The exit channels of the aqueous humour in man: C, cor- nea; S, sclera; I, iris; and CB, ciliary body. The primitive drainage chan- nels of lower animals are seen in VP, the ciliary venous plexus, draining by EV, the ciliary efferent veins into ACV, and the anterior ciliary veins. Superimposed on this is the drainage system peculiar to primates, repre- sented by T, the trabeculae; SC, the canal of Schlemm; IP, the intrascleral plexus; and AV, an aqueous vein emptying into the anterior ciliary veins. 6 7 8 9 10 11 FIGURE 1.11(C) The normal fundus. The disc or optic nerve head is approximately 1.5 mm in diameter and the centre of the fovea or macula is located about 2 disc diameters temporal to it. 1 2 3 4 5 FIGURE 1.11(A) The eyelids and anterior aspect of the eyeball. 1. Pupil. 2. Plica semilunaris. 3. Lacrimal caruncle. 4. Medial canthus. 5. Conjunctiva. 6. Upper eyelid. 7. Eyelashes. 8. Lateral canthus. 9. Lid mar- gin. 10. Iris. 11. Lower eyelid. (From Susan Standring. Chapter 39: The orbit and accessory visual apparatus. In: Susan Standring, Neil R Borley, Patricia Collins, et al., editors. Gray's Anatomy: The Anatomical Basis of Clinical Practice, 40th ed. Edinburgh: Churchill Livingstone; 2009. pp.655–674) FIGURE 1.11(B) Optical section of the normal eye, as seen with the slit-lamp. The light (arrowed) comes from the left and, in the beam of the slit-lamp, the sections of the cornea and lens are clearly seen. Summary The human eye and its adnexal structures develop from the neuroectoderm of the neural groove and the adjoining sur- face ectoderm, mesoderm and cells of neural crest origin. Though the development takes place by a predetermined sequence of events, local interactions and trophic influ- ences affect the chain of interrelated processes which take place both simultaneously and sequentially. Teratogenic influences like intrauterine infections, noxious stimuli, maternal intake of drugs or alcohol, exposure to radiation, etc. can affect the normal course of development leading to abnormalities and congenital deformities. The milestones in embryological development are not absolute and are more representative of a time period than an actual finite time. Any disruption in that period will have an effect on the structures forming at that particular phase of development. As expected, abnormal developmental influences have a more severe impact if they occur early when the system is more immature and more prone to major developmental defects. The eye is a complex anatomical structure consisting of delicate tissues. It is a sense organ which is designed to cap- ture and focus light to form a retinal image which is trans- lated into electrical signals and transmitted to the central nervous system via the optic nerve. The eye is protected from the environment by the eyelids, lashes and the orbital wall. The extraocular muscles function in a synchronized fashion to stabilize the globes, enable binocular vision and the full functional field of vision by allowing the full range of ocular movements. The blood supply of the eye and orbit is derived from the ophthalmic artery. Overall, the eye is a unique organ of the body as most of its anatomical structures are available for direct observation using appropriate optical devices such as the slit lamp and ophthalmoscope. https://t.me/MedicalBooksStoreChapter | 1 Embryology and Anatomy 15 SUGGESTED READING 1. Beauchamp GR, Knapper PA. Role of the neural crest in anterior segment development and disease. J Pediatr Ophthalmol Strabismus 1984;21:209–14. 2. Duke-Elder S, Cook C. Normal and abnormal development. Embryol- ogy. In: Duke-Elder S (Ed.). System of Ophthalmology. Vol. III. London: CV Mosby, 1963:23–4. 3. Henry Gray. Gray’s Anatomy, 38th ed. Edinburgh: Churchill Living- stone, 1989. 4. Noden DM. Periocular mesenchyme. Neural crest and mesodermal interactions. In: Jakobiec FA (Ed.). Ocular Anatomy, Embryology and Teratology. Philadelphia: Harper & Row, 1982:97–119. 5. Ozaincs V, Jakobiec F. Prenatal development of the eye and its adnexa. In: Jakobiec FA (Ed.). Ocular Anatomy, Embryology and Teratology. Philadelphia: Harper & Row, 1982:11–96. 6. Snell RS, Lemp MA. Clinical Anatomy of the Eye. Hong Kong: Blackwell Scientific, 1989. 7. Wolff E. Anatomy of the Eye and Orbit, 7th ed. London: HK Lewi