Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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The deeper mesenchymal layer is transparent and extends from the ciliary border to the edge of the pupil. The superficial layer glides over it. This is why when the pupil dilates the edge of the pupil seems to be nearer to the collarette and the pupillary ruff disappears.
The pupil regulates the entry of light into the eye. It is pinpoint in bright sunlight and widely dilated in the dark. The diameter of the pupil can vary from 1.5 mm to 8 mm. In old people the size of the pupil becomes smaller due to fibrotic changes in the sphincter and atrophy of the dilator muscle. When mydriatics are used the pupil can dilate up to 9 mm.
The color of the iris is determined by melanocytes in the stroma and the anterior mesenchymal layer. Blue irises are thinly pigmented.
Microscopically the line of demarcation between the pupillary zone and the peripheral ciliary zone is marked by the collarette. This lies about two millimeters from the edge of the pupil. The iris is thickest at this region. An incomplete vascular circle called the circulus vasculosus iridis minor lies here. Remnants of the vascular connection to the tunica vasculosa lentis is sometimes present. It usually appears as thin fibrils attached to the collarette. Rarely even a thick sheet may be seen obscuring vision.
The anterior surface of the iris has a trabecular structure with pit like depressions. These are called Fuch’s crypts. The extension of the posterior pigmented epithelial layer onto the front surface is called the pupillary ruff. This is the anterior edge of the optic cup. If the extension is prominent it is called ectropion uveae. This often occurs due to abnormal traction by tumors or any other pathological conditions.
The posterior surface of the iris is smooth with faint radial and concentric folds. The crenated appearance of the pupillary edge is due to the Schwalbe’s contraction folds. The Schwalbe’s furrows start as narrow depressions 1.5 mm from the pupil and become broad and shallow towards the periphery. The circular furrows are formed by the variations in the thickness of the pigment epithelium and due to the arrangement of the stroma. The epithelial cells here have a high rate of proliferation and hence cysts are more common here. Besides furrows few pits are also seen.
Layers of Iris
a.Anterior border (limiting) layer.
b.Stroma and sphincter pupillae muscle.
c. Anterior epithelium and dilator muscle.
d.Posterior pigment epithelium.
Anterior Border Layer
This is a condensation of connective tissue and pigment cells. Fibroblasts form a continuous sheet of flat stellate cells and interlacing processes throughout
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the anterior surface. The melanocytes lie deep to the fibroblasts and parallel to the surface. Rarely iris processes an extension of the anterior border layer may extend up to the Schwalbe’s line.
Stroma: Contains highly vascular connective tissue, collagen fibers, fibroblasts, melanocytes and matrix. Nerve fibers, sphincter pupillae and myoepithelial cells of the dilator muscle are also present. The spaces between the collagen fibers are filled with mucopolysaccharides. There are no elastic fibers. Mast cells, macrophages and lymphocytes are also present in the stroma.
The sphincter pupillae muscle is situated in the pupillary zone of the iris. It forms a ring of smooth muscle fibers around the pupil. It is one millimeter wide. The muscle fiber bundles are separated by nerve fibers, blood vessels and connective tissue. When this muscle contracts the pupil will constrict. It is supplied by the postganglionic, parasympathetic fibers, which travel through the oculomotor nerve, branch to the inferior oblique and then the short ciliary nerves. Sympathetic innervation is also seen which probably has an inhibitory role to relax the sphincter in the dark. As the muscle fibers are attached to the surrounding tissues even after a sector iridectomy the remaining fibers can contract.
The blood vessels run radially and are sinuous to allow for the dilatation and contraction of the iris. The vessels are from the long ciliary arteries and the anterior ciliary arteries.
The dilator pupillae muscle is a thin layer of myoepithelium. It extends from the root of iris to the sphincter pupillae. It is derived from the anterior layer of the iris epithelium and is supplied by the postganglionic fibers of the superior cervical sympathetic ganglion via the long ciliary nerves. Here also parasympathetic innervation is also seen for inhibition. Both these muscles are derived from the external layers of the optic cup. The dilator and constrictor muscles fuse together near the edge of the pupil. In the periphery the dilator continues into the ciliary body from where the muscle originates. Thus when the muscle is stimulated it draws the pupillary edge towards its origin thereby dilating the pupil.
Epithelial layer: The two layers of epithelium the anterior and posterior are derived from the neuroectoderm of the optic vesicle. The cells are opposed apex to apex. This polarity of cells is maintained from embryogenesis. Between the two layers lies a potential space. When fluid gets into this space an iris cyst is formed. The anterior epithelial layer is closely associated with the muscular processes of dilator pupillae and is continuous with the outer pigmented layer of the ciliary epithelium.
The posterior epithelium is in contact with the aqueous. The cells in this layer are larger than those in the anterior layer and are cuboidal in shape. They are full of melanin granules. This is continuous with the inner pigmented layer of the ciliary epithelium. The pigmentary layer is sometimes seen prominently
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projecting around the pupil. This is called ectropion uveae. But this is a misnomer as the posterior layers are derived from neuroectoderm.
Blood supply: The major arterial circle, which lies in the stroma of the ciliary body, is formed by the long posterior ciliary arteries and the anterior ciliary arteries. Radial arteries arise from the major arterial circle and converge towards the pupil in a spiral pattern. These form the radial ridges formed on the surface of the iris. The spiral pattern is needed as the pupil has to dilate and constrict. On reaching the collarette the arteries and veins anastomose with the minor arterial circle.
The veins follow the arteries and form the minor venous circle and then drain into the vortex veins. The vessels are non-fenestrated and the endothelial cells have tight junctions. So they are less permeable. There are few smooth muscle fibers but no elastic lamina.
Nerve supply: The iris has both sensory and autonomic nerve supply.
The long ciliary nerves, branches of the nasociliary division of the ophthalmic branch of the trigeminal nerve contains sensory fibers that ascend through the trigeminal nerve. The long ciliary nerves also contain postganglionic sympathetic fibers from the superior cervical ganglion. These fibers innervate the dilator pupillae and the blood vessels.
The short ciliary nerves arise from the ciliary ganglion and contain the postganglionic parasympathetic fibers that originate from the Edinger Westpal nucleus of the oculomotor nerve. It supplies the sphincter pupillae.
THE CILIARY BODY
The ciliary body is divided into two parts the pars plana and the pars plicata (Fig. 5.2).
Fig. 5.2: Ciliary body
Pars plicata has seventy radiating ridges called the ciliary processes with darker valleys in between them. Its width is two millimeters. The width of the whole ciliary body is 5.9 mm on the nasal side and 6.7 mm on the temporal
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side. The pars plana is 4 mm wide and is situated 3-4 mm from the limbus. During surgeries like vitrectomy this is the safest region to make the incision.
The ciliary body is triangular in shape the anterior face being the shortest. Part of this surface forms the angle of the anterior chamber. The iris arises from the middle of this surface. On the scleral side of the triangle lies the ciliary muscle. Suprachoroidal tissue lies in between the ciliary body and the sclera. The inner side of the triangle is formed by the ciliary processes and the suspensory ligaments. The equator of the lens is situated 0.5 mm from the ciliary body
Parts of the Ciliary Body
Suprachoroidal lamina consists of collagen fibers.
The next layer the ciliary muscle is triangular in shape. It has bundles of nonstriated fibers. The external fibers are longitudinal or meridional, the intermediate ones are oblique or radial and the internal fibers are circular. The internal circular fibers form the sphincter.
The longitudinal fibers are also called Brucke’s muscle. It takes origin from the scleral spur and the trabecular meshwork. This attachment is the main attachment to the sclera. The muscle fibers pass backwards into the suprachoroidal lamina to even beyond the equator. The fibers are slender spindles. Some of these fibers incline obliquely towards the inner side of the eye to form the oblique layer of the muscle. As the longitudinal muscle fibers are inserted into the scleral spur it has a pumping action on the canal of Schlemm. The pull of the muscle opens the canal. Thus, it facilitates drainage of aqueous.
The circular fibers are called the Muller’s muscle. This is situated more internally, close to the lens. Some of the radial and circular fibers also are attached to the scleral trabeculae. The tendinous fibers of origin form a ring. The long posterior ciliary and the anterior ciliary arteries pass through the muscle and supply the same. The anterior ciliary veins drain the blood into the choroidal vessels.
The three layers of ciliary muscle act as one component. Miotics do not have much effect in infants as the muscle tendon attachment is not well developed in them.
The stroma is continuous with the suprachoroidal lamina. In the radial portion of the ciliary body the stroma consists of the blood vessels, nerves and melanocytes. As age advances the stroma becomes sclerosed and hyaline degeneration also sets in.
The function of the ciliary body is to slacken the zonules. This results in decreased tension to the capsule of the lens so that it can bulge forwards and become more convex. When the circular fibers constrict its circumference becomes less and the zonules are relaxed. The longitudinal muscle also may play a part by drawing the choroid forward but this is very doubtful.
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According to Fincham when the eye is at rest the lens capsule is stretched by the zonule and the lens is in its normal form. When the ciliary muscle contracts the zonules are relaxed and the capsule compresses the lens matter at the equator. This makes the lens to bulge forward and the lens reaches its accommodative state. So normally the lens is in its passive state.
Nerve supply: The short ciliary nerves from the ciliary ganglion form a rich plexus in the muscle. Proprioceptive and sympathetic nerve endings are not demonstrated clearly in this tissue.
The ciliary processes are the most vascular part of the eyeball as they are formed mostly by blood vessels. This is a continuation of the choroid without the choriocapillaries. Each ciliary process is a ridge 2 mm long and 0.5 mm high. Anterior end is broader and is called the head of the process. The ridges are white in color whereas the valleys are pigmented and dark. The vessels are mostly veins that run parallel to each other. They run backwards to become continuous with the choroid.
The basal lamina is a continuation of the same lamina of the choroid. But at ora serrata the lamina splits into two and has connective tissue between the two layers. The inner layer is a continuation of the basement membrane of the pigment epithelium and forms the basement membrane of the deeper layer of pigmented epithelial cells.
The stroma has vessels and some elastic fibers. The vessels are mostly capillaries and veins. Loose connective tissue separates the ciliary muscle from the anterior chamber. The major arterial circle of the iris is situated in the stroma of the ciliary body.
The Epithelium
The basal lamina has two layers of cells. The outer layer consists of pigment cells and is the forward continuation of the pigment layer of the retina. These cells are smaller in size and do not have pigment processes. But the cells are more pigmented and hence the ciliary body appears darker. The inner layer is equivalent to all the layers of retina except the pigment layer. It is a single layer of nonpigmented cells. The cells are adherent to the pigment epithelium.
The capillaries in the ciliary body are thin walled and fenestrated. The junctional zone between the epithelia may get opened up. The mitochondriae in the ciliary epithelium are more in the apical region. These cells seem to be secretory in nature.
The internal limiting membrane is on the inner side of the non-pigmented epithelium and is the continuation of the internal limiting membrane of the retina.
The ciliary epithelium develops from the neurectoderm during the 12th week. The ciliary muscles develop from the mesoderm. The meridional fibers develop first followed by the circular and radial fibers. The circular muscle continues to develop up to one year after birth.
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The Choroid
The choroid is the most posterior part of the uveal tract. It is homologous to the pia-arachnoid. It supplies blood to the outer part of the retina. It extends from the optic nervse (to which it is firmly attached) to the ora serrata. It is thicker posteriorly especially in the macular region. The choroid is also attached at the points where the vessels and nerves enter it. The choroid is formed by three layers, the layer of vessels in between two laminae.
The suprachoroidal lamina has a grill pattern. This is more evident when fluid gets collected in between choroid and the sclera, i.e. the suprachoroidal space. Since the two laminae are more adherent in the posterior region and the suprachoroidal lamina is attached to the sclera, choroidal detachment occurs only anteriorly. Un-striped muscle fibers are also seen in this layer. This gives the rough appearance to this layer when it is stripped off from the sclera.
The long and short posterior ciliary arteries and nerves pass through this space.
The blood vessels in the choroid are arranged in three layers. The external layer of larger vessels (Haller’s), internal medium sized (Sattler’s) layer and the inner layer of choriocapillaries. The arteries are deep posteriorly. But in the anterior region they lie superficial to the veins. Near the macula and where the veins join the vortex veins the vessels are larger in size. No valves are seen in these veins.
The stroma of the choroid contains loose collagenous tissue with elastin and reticulin fibers. The melanocytes are present in this layer. The pigment granules in these cells are of the same size in all the cells in a particular individual. It is also smaller and lighter in color compared to the granules in the retinal pigmentary epithelium. Pigmentation of the choroid and its density are of importance when photocoagulation is considered.
Macrophages, lymphocytes, mast cells and plasma cells are also present in the stroma.
The layer of choriocapillaries end at ora serrata whereas the larger vessels continue in to the ciliary body. This layer is not pigmented. The capillaries in the macular region form a dense network and have a wider bore. The choriocapillaries in general are thin walled with fenestration. The pericytes in the vessels are present only on the outer wall. The capillaries have a lobular pattern. This is better seen with fluorescein angiogram. Each lobule is supplied by a central arteriole. Blood flow in the choroid is so high that the oxygen level in the venous blood is only 2-3% less than that of the arteries.
Bruch’s membrane: This membrane is two microns thick and is closely adherent to the pigment epithelium. Near the optic disc the fibrils become circular and merges with the connective tissue of the optic nerve. Though it has been called a membrane this PAS positive tissue has five parts.
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a.basal lamina of the retinal pigment epithelium
b.inner collagenous zone
c. band of elastic fibers
d.outer collagenous zone
e.basal lamina of the outer layer of choriocapillaries,
The basal lamina is highly permeable to small molecules like fluorescein. Defects can develop in myopia, pseudoxanthoma elasticum, trauma, inflammations etc. These defects cause neovascularization.
Development: The choroid starts developing from the anterior part of the optic cup and then proceeds posteriorly. The cells of choroidal stroma and melanocytes develop from the neural crest cells. The blood vessels develop from the mesenchymal cells. The choroidal capillaries develop after the fourth week. The junctional complexes develop before the fenestrations in the endothelium. The vessels are initially supplied by the internal carotid and then by the ophthalmic artery. The superior and inferior orbital venous plexuses drain into the developing cavernous sinus. The three layers of the choroidal vasculature are seen by the 12th week. The anterior ciliary and the long posterior ciliary vessels develop by the 16th week.
6 Lymphatic
Drainage of the Eye
The eyelids have true lymphatics, which drain into the parotid lymph node and the submandibular lymph nodes. Otherwise the aqueous is considered the lymph of the eye.
Anteriorly the aqueous goes into the posterior chamber then through the pupil into the anterior chamber. From there it passes through the trabecular meshwork into the sinus venosus sclerae and then the aqueous veins. The crypts of Fuchs on the surface of the iris also help in drainage. The aqueous passes through the ciliary veins and suprachoroidal lymph space and perivascular lymphatics to the Tenon’s space.
Posteriorly the lymph passes backward through the slit like spaces of the suspensory ligament into the canal of Petit around the equator of the lens. From here it reaches the Berger’s space and through the hyaloid space into the optic disc.
Anterior chamber associated immune deviation: This reaction is supposed to be the basis for the immunological privilege of the eye. When antigens like viruses, allogenic spleen cells or tumor cells are introduced into the anterior chamber the delayed type of hypersensitivity, which should occur, is downregulated, while antibody and cytotoxic T cell responses are enhanced. Down regulation of hypersensitivity is caused by the presence of immunosuppressive cytokines in the anterior chamber. These cells suppress the reaction of T cells. These antigen-presenting cells do not induce hypersensitivity but induce cytotoxic T cell response. The down-regulation of hypersensitivity prevents tissue damage caused by cell mediated immune responses. Transforming growth factor beta is the most important inhibitory factor found in the aqueous.
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Retina
The retina is part of the central nervous system and is made of 10 layers of tissues. There are three types of tissues namely neural, glial and vascular (Fig. 7.1).
Fig. 7.1: Layers of retina
The pigment epithelium, which is close to the choroid, is made of a single layer of approximately 5 million cells extending from the ora serrata to the optic disc. The cells are hexagonal in shape. They are uniformly tall and narrow in the posterior pole, and more pigmented in the region of the macula but wider and shorter in the retinal periphery. Near the ora they assume a cuboidal configuration and continue as the pigment epithelium of the ciliary body. At the optic nerve the pigment epithelium falls short of basal lamina of choroid and the terminal depigmented cells heap up to form a ring at the edge of the optic disc.
The fine mottling of the cells is due to irregular distribution of pigments and this gives the fundus a granular appearance. Absence of specialized adhesion molecules (laminin and fibronectin) in the interphotoreceptor matrix and lack
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of junctional complexes between apical microvilli of RPE cells and outer segments of photoreceptors makes the sensory retina prone to detachment in pathological conditions. The size of melanin granules varies between uveal and retinal pigment epithelium.
The pigment epithelium is needed for
a.vitaminA metabolism
b.maintenance of outer blood retinal barrier
c. phagocytosis of outer segments of the photoreceptors
d.absorption of light
e.heat exchange
f.formation of basal lamina
g.production of mucopolysaccharide matrix in the outer segment
h.Active transport of materials in and out of the cells.
Disruption of interaction between the RPE and photoreceptors causes retinal
degeneration. Drugs like hydroxy chloroquine binds with the melanin granules in the retinal pigment epithelium and interferes with maintenance of photoreceptors (Fig. 7.2).
Fig. 7.2: Rods and cones
RODS AND CONES LAYER
The rods and cones that are the photoreceptor cells are highly specialized cells. They constitute the neuroepithelium. These cells absorb light by the visual pigments. Light alters the molecules of these pigments thereby producing nerve impulses. Thus, they transform physical energy into nerve impulses. The rods and cones have inner and outer segments. The transition between