Outer Coats of the Eye - Lecture Notes PDF

THE OUTER COATS OF THE EYE 1. THE SCLERA The term sclera is from the Greek word skleros which means hard. The sclera occupies the posterior 5/6 of the outer coat of the eye. It is composed of dense connective tissue and forms a tough protection for the interior eye, and al...

THE OUTER COATS OF THE EYE 1. THE SCLERA The term sclera is from the Greek word skleros which means hard. The sclera occupies the posterior 5/6 of the outer coat of the eye. It is composed of dense connective tissue and forms a tough protection for the interior eye, and allows for a firm adhesion of the extra-ocular muscles. The whiteness is due to reflection at the surfaces of many transparent fibres. In children and in certain pathological states, it is thin and looks bluish due to the underlying choroid becoming visible. Elderly people may have yellowing of the sclera from fatty deposits. The histologic structure of the sclera is similar to that of the cornea, but the transparency of the cornea is due to its relative deturgescence. Source: library.thinkquest.org/26111/media/sclera.gif The sclera is about 1mm thick behind and becomes thinner to the front (0.3mm) particularly where extra-ocular muscle tendons are inserted into the sclera. Externally, the sclera has an even and smooth surface, except where the extraocular muscles are inserted. The sclera imposes on the globe a roughly spherical shape, slightly flattened in its vertical axis. Across the posterior scleral foramen are bands of collagen and elastic tissue, forming the lamina cribrosa, between which pass the axon bundles of the optic nerve. The outer surface of the anterior sclera is covered by a thin layer of fine elastic tissue, the episclera, which contains numerous blood vessels which nourish the sclera. 1 Source: www.scienceclarified.com/.../uesc_05_img0242.jpg There are three groups of apertures in the sclera: i. at the back around the optic nerve, the posterior ciliary arteries (2 long and 10-12 short) and the posterior ciliary nerves ( 2 long and 6-10 short) ii. four medial perforations are found 3-4mm behind the equator transmitting the vortex veins. iii. Smaller perforations lie anteriorly between the limbus and the extra-ocular muscle insertions carrying the anterior ciliary arteries, anterior ciliary veins and the aqueous veins from Schlemm’s canal. Histology There are three types of scleral tissue: i. outermost, the episclera ii. centrally, the stroma iii. innermost, the lamina fusca, which also comes into direct contact with the uvea, and forms the outer layer of the suprachoroidal space. Pigment cells, similar to those in the uvea may be found and thus assists in preventing unwelcome penetration of light into the eye. The blood vessels and nerves travel forwards within this layer. 2 Blood Supply The blood supply is derived from the posterior short and long ciliary arteries. Within the sclera there are very small blood vessels and at the extra-ocular muscle attachments there is a very rich blood supply in the episcleral layer. There is a linear network of vessels around the limbus and in the deeper layers, there are limbal arcades. Supply here is from both long posterior and anterior ciliary arteries. Nerve Supply The nervous supply is from the posterior ciliary nerves, the shorter ones supplying the back of the globe, while the two long nerves supply the front. 2. THE CORNEA The cornea is the transparent 1/6th of the eyeball, and contributes 2/3rds of the ocular power (more than 40D). The refractive index is 1.376. In the older eye there are slight changes in form and power. The corneal thickness varies from thinnest at the apex at about 0.52mm(0.46-0.67mm) and thickest near the limbus about 0.67mm (0.65 to 1.1mm) which also results in the front curvature being different from that of the back surface. The corneal dimensions are about 12mm horizontally and 11mm vertically. Microcornea is when the greatest corneal diameter is less than 11 mm. Megalocornea is when the greatest corneal diameter is greater than 13 mm. The average radius of the front surface is 7.8mm and the back radii range from 6 to 7mm. The central zone (4-5mm in diameter) is spherical or toroidal. The vertical meridian tends to be steeper than the horizontal one, and this physiological astigmatism is compensated for by the inverse toricity on the back of the cornea, together with the ‘against-the-rule’ lenticular astigmatism. The peripheral zone, is much flatter. Source: www.eyelaser.co.za/graphics/corneal_image2.jpg 3 The cornea meets the sclera at a shallow groove, known as the corneal sulcus. Opaque scleral tissue overlies the limbus so the external corneal diameter is grater than the diameter of the visible iris. Corneal tissues: There are five corneal layers: source: www.missionforvisionusa.org/anatomy/uploaded_... The epithelium : This anterior most corneal layer is between 5 to 6 cells thick (approx 70m thick) and firmly attached to a basal membrane. The epithelial cells are continuous with that of the conjunctiva. The basal membrane can be easily detached from the Bowman’s layer. A single layer of basal cells lie attached to the basal membrane by means of hemidesmosomes. The lack of the hemidesmosomes is linked to recurrent corneal erosions. The cells are cubical in shape, and have a high metabolic rate as this is the layer which is primarily involved in regeneration. New cells are produced by mitosis, 4 and migrate to the more external layers. The turnover time is about 7 to 8 days for the whole process of renewal. Minor abrasions heal within a few hours. Repair takes place by surrounding cells migrating laterally, in a flattened form, sliding to cover over the denuded area. Above the basal cells is to be found the wing cells, called so because they have wing-like extensions. They are arranged in 2-3 rows. The superficial layer of squamous cells contain very flat polygonal cells which provide the smooth surface of the cornea. These cells, however, do have microvilli, consisting of glycoproteins, which probably contribute to the stability of the tear film as they assist in movement and adhesion of the external epithelium layers. When microvilli are lost during contact lens wear, they increase the dry eye symptoms. The microvilli also interdigitate and are difficult to separate and thus enhance barrier function. As the squamous cells have the ability to regenerate, the epithelium doesn’t scar as a result of inflammation. After a lifespan of a few days, the epithelium is shed into the tears. Source: www.ooglasertrefpunt.nl/images Local anaesthetics retard healing as they slow down metabolic activity. Fluorescein stains the underlying Bowman’s layer green after injury, thus showing where the epithelium has been lost. Normally the epithelium prevents the movement of water into the cornea, thus breaks allow water into the stroma. 5 Bowman’s layer: This anterior limiting lamina is about 12m thick. It is made up of irregularly arranged collagen fibrils, much more delicate than those in the stroma and is resistant to infection. This layer is a superficial layer of the stroma and thus cannot be detached from the stroma. It is thought of a being a condensed layer of protein or acellular collagenous material and is thus not a true basement membrane. Damage to the Bowman’s layer will lead to a residual opaque scar. This layer is perforated by unmyelinated nerve fibres arising from the stroma to supply the epithelium. Stroma: Also known as the substantia propria and occupies about 90% of the total corneal thickness (0.5mm across). It is avascular, and made up of many regularly orientated lamellae which consist of collagen fibres that are arranged in parallel bundles which cross each other at various angles by remain roughly parallel to the corneal surfaces. This gives the cornea the appearance of layers when viewed in transverse section. The lamellae lie within a ground substance of hydrated proteoglycans in association with the keratocytes that produce the collagen and ground substance. The neighbouring cells are joined by long projections, and phagocytosis has been associated with the cells. The matrix of the stroma is made of mucopolysaccharides, which are highly hydrophilic. The stroma is unable to regenerate and thus injury at this level is likely to result in opacities. Descemet’s membrane: This is a posterior basal lamina, which is thin (8-10m thick) and elastic, and can be easily separated from the stroma. It is better defined than Bowman’s membrane. It can regenerate, and has good resistance against infective agents and acts as a basal membrane for the corneal endothelium from which it is secreted. It terminates as Schwalbe’s line which represents the anterior limit of the anterior chamber angle. In the ageing eye, it may produce posterior surface degenerate, wart-like elevations called Hassal-Henle bodies. These bodies represent collagen excrescences of Descemet’s membrane, project into the anterior chamber and are formed by stressed or abnormal endothelium cells that bulge forward towards the epithelium. They can lead to bullous keratopathy. They are similar in appearance to guttata (pitted orange peel appearance of the endothelium) but are peripheral and not central unlike guttata. Endothelium: This innermost corneal layer, comprises about a single layer of about 400 000 hexagonal cells, which reduces with age. Intraocular surgery, use of contact lenses can sometimes cause a similar reduction (polymegathism = lower cell density). The aging cells, however, become larger, thus compensating for the scarcity. There are many intracellular organelles indicating a high metabolic activity in this layer. As this layer is unable to regenerate, severe injury can lead to blindness. Endothelial repair is limited to enlargement and sliding of existing cells, with little capacity for cell division. Failure of 6 endothelial function leads to corneal edema. Endothelial guttata are cells that bulge out due to poor cell function. Blood supply of the cornea The cornea is avascular, but derives nourishment from the ciliary arteries via the conjunctival vessels. Trauma or disruption of metabolism tends to cause corneal vascularization, with radial intrusion of new vessels into the corneal tissues. Following healing, these new vessels empty, and become ghost vessels and can persist for a long time and may refill at the slightest provocation. Nerve supply of the cornea The ophthalmic division of the trigeminal nerve supplies the cornea via the long and short posterior ciliary nerves. Nerve fibres pierce the middle 1/3rd of the stroma, migrate through Bowman’s layer and terminates as free nerve endings among the epithelial cells. The 70-80 nerves that radiate into the cornea, lose their myelin sheath at the limbus which contributes to the transparency of the cornea. Sometimes, the fibres may be seen as grey threads within the cornea. The corneal nerves form plexuses at different levels in the anterior stroma, with a lot of overlap and about 70-80 fibres reaching 2/3rds cornea. Nerve fibres from the stromal plexuses traverse Bowman’s layer to form a subepithelial plexus from which branches move into the epithelium reaching the squamous layer as bare nerve endings. The exquisite sensitivity of the cornea is due to the bare never endings at the surface and the amount of anastomosis (indicating summation). The nerve supply is greatest centrally and diminishes towards the limbus, thus the cornea is most sensitive near the apex. Corneal sensitivity does decrease in increasing age. There is no difference between the sexes and the two eyes are normally similar. Blue eyes tend to be more sensitive than brown eyes, and lower sensitivity has also been found in the morning and then increasing through the day. Hormonal variations, eg during menstrual cycle, can also change corneal sensitivity. Surgery, eg. cataract extractions, refractive surgery, and the use of hard contact lenses can further reduce sensitivity. Damaged corneal nerves require 9 months to regenerate and the absence of innervation can decrease the adhesion of the epithelial cells and cause corneal decompensation. Corneal metabolism The epithelium and endothelium have relatively higher metabolic rates than the stroma. Metabolism in the cornea is essential to maintain temperature, renewal of cells and their contents and transport processes in the cornea. Corneal tissues require carbohydrates, aminoacids, oxygen, minerals and vitamins. As the cornea is avascular, it derives its nourishment from the limbal arcades, aqueous humour and tears. It derives oxygen and food from the limbal capillaries. Oxygen is also 7 derived from the tears and the aqueous. When the lids are closed, the oxygen is derived from the tarsal conjunctiva Glucose is the most important energy-producing carbohydrate supplied to the cornea minimally from the tears, but mostly from the aqueous humour. It is initially broken down by glycolysis (Embden-Meyerhof route) and then reduced to carbon dioxide and water through the Kreb’s cycle. Amino acids, which is necessary to the corneal production of enzymes and renewal of tissues is derived from the tears minimally, but mostly again from the aqueous. The corneal epithelium has very little permeability to amino acids and glucose. Corneal transparency The cornea is very transparent and reflects less than 1% of the incident light. According to a theory put forward by Maurice (1957) the following conditions are necessary for good stromal transparency: i. constant thickness of fibrils arranged in parallel patterns ii. constant separation between the fibrils iii. separation must be less than the wavelength of light. Goldmann modified this theory saying that a difference in refractive index will not be significant if this distance is less than 200nm or half a wavelength of light. Corneal transparency thus depends on the regular structure of the stroma. The healthy cornea has a constant balance between solids and water, in the proportion 22% solids to 78% water, which is called normal deturgescence. In addition, the cornea has a high content of mucopolysaccharides which are highly hydrophilic and thus has a tendency to absorb liquid into the cornea which can disturb the normal deturgescence. Thus the epithelium and endothelium resist this influx of water. The endothelium also serves as a pump to maintain the proper deturgescence. Water entering the stroma upsets the arrangement of the collagen fibres, which results in scattering and absorption of light, thus giving the cornea a milky appearance with lower transmission of light. Furthermore, when the cornea becomes oedematous, the cells become more separated and results in the oedematous appearance of the cornea e.g. Sattler’s veil. Subsequently, the individual will experience coloured haloes and reduced vision. Oedema results from inadequate oxygen supply to the cornea which then lowers endothelial activity due to a lowering of pH. The fall in pH is due to the build-up of lactate and carbonic acid within the cornea. In certain cases, this may be visible are dilated and engorged limbal arcades, and neovascularization. 8  3. THE LIMBUS The corneo-scleral junction is known as the ‘limbus’ and is between 1-2mm long between the sclera and the cornea. Clinical limbus = where iris and sclera meet (where white changes) Anatomical limbus = end of Bowman’s and Descemet’s membrane. Source: faculty.une.edu/com/abell/histo/limbus2.jpg At this region, the epithelial layer of the bulbar conjunctiva and cornea merge covering and filling radial folds in the stroma known as Vogt’s palisades which protrude into the cornea. The palisades serve as reservoirs for extra epithelial cells to migrate and often neovascularization can stem from here. The palisades contain blood vessels and lymphatics and are dentate conjunctival projections into the cornea. Source: www.bu.edu/histology/i/08005hoa.jpg 9 It is in this region that the epithelial basal cell thickness is increased and serves as an important source of renewal for the corneal epithelium. In this region the Bowman’s layer makes interconnections with the connective tissue of the conjunctiva and Tenon’s capsule, and fibres from the corneal stroma lose their well ordered arrangement when merging with the sclera. The Descemet’s membrane has connections with the connective tissue cells of the trabecular network. At the limbus, the endothelial layer continues laterally to cover the trabecular fibres with a single layer of cells. Schlemn’s canal encircles the limbal region. The limbus is an important landmark for surgical procedures for cataract extraction, glaucoma filtration surgery etc. Because the trabecular meshwork is encompassed in this region, the limbus has implications in the laser treatment for glaucoma. REFERENCES: 1. Spooner, JD. (1957) Ocular Anatomy. The Hatton Press Ltd: London. 2. Moses RA, Hart WM. (1987) Adler’s Physiology of the eye – clinical application. The CV Mosby Company: St Louis. 3. Miller SJH. (1990) Parson’s diseases of the eye (18th edition). Churchill Livingstone: Edinburgh. 4. Shauly Y, Miller B, Lichtig C, Modan M and Meyer E. (1992) Tenon’s capsule: ultrastructure of collagen fibrils in normals and infantile esotropia. Investigative Ophthalmology & Visual Science 33: 651-656. 10