Eye Anatomy and Physiology PDF

Eye Anatomy / Histology and Physiology DR. NABIH CHAMAS OPHTHALMOLOGIST BEIRUT ARAB UNIVERSITY Eyelids  The eyelids, or palpebrae, are folds of skin and tissue that, when closed, cover the globe.  The eyelids have four major functions:...

Eye Anatomy / Histology and Physiology DR. NABIH CHAMAS OPHTHALMOLOGIST BEIRUT ARAB UNIVERSITY Eyelids  The eyelids, or palpebrae, are folds of skin and tissue that, when closed, cover the globe.  The eyelids have four major functions: During blinking or eyelid closure, the eyelids help to push tears toward the medial canthus (the inner corner of (1) cover the globe for protection, the eye) where there is a small opening called puncta, from puncta tears reach nasal cavity (2) move the tears toward drainage at the medial canthus on closure, (3) spread the tear film over the anterior surface of the eye on opening, and During blinking tears spread over the anterior surface of the eye to keep it moist (4) contain structures that produce the tear film. On closure the upper eyelid moves down to cover the cornea, whereas the lower eyelid rises only slightly. When the eyes are closed gently, the eyelids should cover the entire globe The tear film is made up of 3 layers and covers the cornea Lipid layer by meibomian glands Aqueous layer by lacrimal glands Mucus layer by goblet cells  CLINICAL COMMENT: LAGOPHTHALMOS  Lagophthalmos refers to incomplete closure of the eyelids  Its cause may be physiologic, mechanical (e.g., scarring), or paralytic. ex Bell's Palsy ex due to surgery or trauma  Lagophthalmos is most evident during sleep, when drying of the inferior cornea may result. inability to fully close the eye irritation and dryness damage of corneal surface punctate keratitis  Scratchy, irritated eyes are evident on awakening, and punctate keratitis can occur. inferior eyelid is more exposed so more affected dryness and irritation cause damage of cornea and punctate keratitis  the inferior cornea will show varying degrees of epithelial disruption, manifested as punctate staining with fluorescein dye. under light damage is seen as green spots inability to fully close the eye irritation and dryness damage of corneal surface punctate keratitis  PALPEBRAL FISSURE  The palpebral fissure is the area between the open eyelids. The lacrimal caruncle is a small, pink mass of modified skin at the medial  Generally the upper lid just covers the superior limbus when one’s canthus, just next to the plica. It is covered with epithelium containing goblet cells eyes are open and looking straight ahead. as well it contains sweat glands to keep tear film stable  The lower lid position is more variable, usually lying within 1 mm of the inferior limbus.  The upper and lower eyelids meet at the corners of the palpebral fissure in the lateral and medial canthi.  The lateral canthus is located approximately 5 to 7mm medial to the bony orbital margin.  The medial canthus is at the medial orbital margin but is separated from the globe by a reservoir for the pooling of tears, the lacrimal lake.  The floor of the lacrimal lake is the plica semilunaris. This narrow, crescent-shaped fold of conjunctiva, located in the medial canthus, allows for lateral movement of the eye without stretching the bulbar conjunctiva. The lacrimal lake is a small pool of tears found at the medial canthus  The caruncle is a small, pink mass of modified skin located just medial to the plica. It is covered with epithelium that contains goblet cells and fine hairs and their associated sweat and sebaceous glands. The plica semilunaris is a crescent-shaped fold of conjunctiva at the medial canthus it allows for lateral movement of the eye Eyelid margin  The eyelid margin rests against the globe and contains the eyelashes and the pores of the meibomian glands.  The cilia (eyelashes) are arranged at the lid margin in a double or triple row, with approximately 150 in the upper eyelid and 75 in the lower lid.  The lashes curl upward on the upper and downward on the lower lid.  Replacement lashes grow to full size in approximately 10 weeks, and each lash is replaced approximately every 5 months.  The eyelashes are richly supplied with nerves  protective response (blink).  CLINICAL COMMENT: ABNORMALITIES AFFECTING THE CILIA  Various epithelial diseases can cause Madarosis (loss of  eyelashes) Madarosis due to trauma, autoimmune, alopecia, infection  or Trichiasis (misdirected growth of eyelashes), in which the eyelashes grow toward rather than away from the palpebral fissure.  Contact with the cornea can cause irritation and painful abrasions and can lead to ulceration. Trichiasis can cause direct contact with the cornea and conjunctiva, leading to: Irritation, Pain, Corneal abrasions (scratches on the corneal surface) and Ulceration of the cornea, if left untreated and scarring  The problem lashes can be removed by epilation  The pores of the meibomian glands are located posterior to the cilia and the transition from skin to conjunctiva, the mucocutaneous junction, occurs just posterior to these openings.  A groove called the gray line runs along the eyelid margin between the cilia insertions and the pores of the Meibomian glands.  lacrimal papilla, a small elevation containing the lacrimal punctum, the opening that carries the tears into the nasolacrimal drainage system. Usually, no cilia or meibomian pores are found medial to the punctum. CLINICAL COMMENT: EPICANTHUS Epicanthus is a skin fold at the nasal canthus. It’s common in newborns and may make it seem like the child has strabismus, but a cover test can confirm that the eyes are aligned. It usually disappears as the child’s facial structure develops.  Epicanthus is a vertical fold of skin at the nasal canthus, arising in the medial area of the upper eyelid and terminating in the nasal canthal area.  It is common in the newborn and may cause the appearance of esotropia  Parents of an infant with epicanthus might worry that the child’s eyes are crossed; however, a cover test will identify a true esotropia.  As the bridge of the nose develops, epicanthus gradually disappears Do cover test to rule out strabismus Orbicularis Oculi Muscle Orbicularis oculi is for eye closure  The striated fibers of the orbicularis oculi muscle are located below the subcutaneous connective tissue LR6 SO4 all other muscles CN3 layer and encircle the palpebral fissure from the EXCEPT ORBICULARIS OCULI CN7 eyelid margin to overlap onto the orbital margin.  The muscle can be divided into two regions, palpebral and orbital.  Orbicularis Action:  The orbicularis oculi muscle is innervated by cranial nerve VII (the facial nerve).  Contraction of the palpebral portion closes the eyelid gently, and the palpebral orbicularis is the muscle of action in an involuntary blink and a voluntary wink; relaxation of the levator muscle follows.  Spontaneous involuntary blinking renews the precorneal tear film.  A reflex blink is protective and may be elicited by a number of stimuli—a loud noise; corneal, conjunctival, or cilial touch; or the sudden approach of an object.  When the orbital portion of the orbicularis contracts, the eye is closed tightly.  The antagonist to the palpebral portion of the orbicularis is the levator muscle. The antagonist to the orbital portion is the frontalis muscle.  if CN3 is affected ptosis occur LPS muscle innervated by CN3 Superior Palpebral for elevation of eye Levator Muscle  The superior palpebral levator muscle, the retractor of the upper eyelid, is located within the orbit above the globe and extends into the upper lid.  It originates on the lesser wing of the sphenoid bone above and in front of the optic foramen, and its sheath blends with the sheath of the superior rectus muscle.  As the levator approaches the eyelid from its posterior origin at the orbital apex, a ligament, the superior transverse ligament (Whitnall’s ligament) may act as a fulcrum,changing the anteroposterior direction of the levator to superoinferior Tarsal Muscle (of Müller)  The superior tarsal muscle (muscle of Müller) is composed of smooth muscle and originates on the posteroinferior aspect of the levator muscle.  The superior tarsal muscle inserts on the superior edge of the tarsal plate.  Contraction of Müller’s muscle can provide 2 mm of additional lid elevation. muller ana LPS both for eye  A similar smooth muscle, the inferior tarsal muscle, is elevation Muller is innervated by found in the lower eyelid. Sympathetic CN3  It arises from the inferior rectus muscle sheath and inserts into the lower conjunctiva and lower border of he tarsal.  Both tarsal muscles are innervated by sympathetic fibers that widen the palpebral fissure when activated. CLINICAL COMMENT: PTOSIS muller and LPS affected severe ptosis muller alone less severe ptosis or incomplete  Ptosis is a condition in which the upper eyelid droops or sags. It can be caused by weakness or paralysis of the levator or Müller’s muscle.  If Müller’s muscle alone is affected, a less noticeable form of ptosis occurs (incomplete ptosis) than when the levator is involved.  An individual with ptosis might attempt to raise the lid by using the frontalis muscle, which results in elevation of the eyebrow and wrinkling of the forehead.  part of tear film secretes sebum for the lipid layer Glands of the Lids pores are located on lids posterior to cilia  The meibomian glands (tarsal glands) are sebaceous glands embedded in the tarsal plate.  These long, multilobed glands resemble a large bunch of grapes and are arranged vertically such that their openings are located in a row along the lid margin posterior to the cilia.  Approximately 30 to 40 Meibomian glands are found in the upper lid and 20 to 30 in the lower lid.  On eyelid eversion the vertical rows of the meibomian glands can sometimes be seen as yellow streaks through the palpebral conjunctiva.  These glands secrete the outer lipid layer of the tear film.  The sebaceous glands of Zeis secrete sebum into the hair follicle of the cilia, coating the eyelash shaft to keep it from becoming brittle.  The glands of Moll are modified sweat glands located near the lid margin. Their ducts empty into the hair follicle, into the Zeis gland duct, or directly onto the lid margin.  Tarsal Plates 1-Support for Eyelid Shape 2-Protection of the eye 3-Contain Meibomian glands (part of tear film) 4-Facilitate eye movement  The tarsal plates are composed of dense connective tissue.  The collagen fibrils of this tissue are of uniform size and run both vertically and horizontally to surround the meibomian glands. Conjunctiva  The palpebral conjunctiva is composed of two layers, a stratified epithelial layer and a connective tissue stromal layer, the submucosa.  The epithelial layer of the conjunctiva is continuous with the skin epithelium at the mucocutaneous junction of the lid margin.  At the mucocutaneous junction, the epithelial layer is approximately five cells thick and is the location of stem cells that repopulate the palpebral conjunctival epithelium.  The palpebral conjunctival epithelium of the upper lid is two or three cells thick, whereas over much of the lower lid, the epithelium is three or four cells thick.  This stratified columnar epithelium continues throughout the fornices into the bulbar conjunctiva, where it changes to a stratified squamous layer near the limbus, becoming continuous with the corneal epithelium. Palpebral epithelium is made of 2 layers: 1-Epithelium continuous with the skin epithelium at the mucocutaneous junction and epithelium contains stem cells important for regeneration and repair epithelium in the upper eyelid is 2-3 cells thick, while in the lower eyelid 3-4 cells thick. it is Stratified columnar 2-Connective tissue for support in bulbar conjunctiva epithelium become stratified squamous near the limbus (the boundary between the cornea and the sclera).  Goblet cells located in palpebral conjunctiva Lacrimal glands in the orbit area Meibomian glands located in the tarsal plate in eyelids (30-40 in upper eyelid 20-30 lower eyelid) limbus is where sclera and cornea meets thin area area of transition between columnar to squamous epithelium Goblet cells, which produce the innermost mucous layer of the tear film, are scattered throughout the stratified columnar conjunctival epithelium Sclera  The sclera forms the posterior five sixths of the connective tissue coat of the globe.  The sclera maintains the shape of the globe, offering resistance to internal and external forces, and provides an attachment for the extraocular muscle insertions.  The thickness of the sclera varies from 1 mm at the posterior pole to 0.3 mm jus behind the rectus muscle insertion  Scleral innervation: Sensory innervation is supplied to the posterior sclera by branches of the short ciliary nerves; the remainder ofthe sclera is served by branches of the long ciliary nerves SCLERAL FORAMINA AND CANALS lamina cribrosa at posterior sclera is where optic nerve exits the eyeball contains cribriform holes through which the nerve fibers pass  The sclera contains a number of foramina and canals.  The anterior scleral foramen is the area occupied by the cornea.  The optic nerve passes through the posterior scleral foramen, which is bridged by a network of scleral tissue called the lamina cribrosa.  The lamina cribrosa is the weakest area of the outer connective tissue. Lamina cribrosa pores seen in Glaumatous optic disc In conditions like glaucoma, the lamina cribrosa can become deformed or damaged due to elevated intraocular pressure. This deformation can lead to optic nerve head damage optic nerve head cupping seen in glaucoma and widening of the pores (become visible) Limbus thin transitional zone  The limbus, located at the corneoscleral junction  is a band approximately 1.5 to 2 mm wide that encircles the periphery of the cornea.  The limbus is the transitional zone between cornea and conjunctiva and between cornea and sclera.  Some layers of the cornea continue into the limbal area, and others terminate Iris/Pupil iris is for light entry regulation and for eye color determination  The iris is a thin, circular structure located anterior to the lens.  The center aperture, the pupil.  Pupil size regulates retinal illumination.  The pupil diameter can vary from 1 mm to 9 mm depending on lighting conditions.  The pupil is very small (miotic) in brightly light conditions and fairly large (mydriatic) in dim illumination.  The average diameter of the iris is 12 mm, and its thickness varies. It is thickest in the region of the collarette, a circular ridge approximately 1.5 mm from the pupillary margin. During daytime, the pupil constricts (miosis) to limit the amount of light entering the eye With less light entering, the central retina (specifically the fovea, located at the center of the macula) is the primary area of focus. This part of the retina is responsible for sharp central vision  at night the pupil dilates, more light enters the eye, which is necessary for activating the rods in the retina rods are not as sensitive to detail or color but are excellent at detecting motion and shapes in low light  This slightly raised ridge was the attachment site for the fetal pupillary membrane during embryologic development.  The collarette divides the iris into the pupillary zone, which encircles the pupil, and the ciliary zone, which extends from the collarette to the iris root  The color of these two zones often differs.  the iris root, approximately 0.5 mm thick, is the thinnest part of the iris and joins the iris to the anterior aspect of the ciliary body The collarette is a visible, ring-like structure on the iris that separates the pupillary zone from the ciliary zone  Pupil and pupillary ruff are at a, and iris root is at b. collarette  Pupillary portion of iris is at c, and ciliary portion is at d.  Collarette (e) and minor arterial circle of the iris lie at the junction of these two anterior border layer portions.  Cellular anterior border layer (f) is distinct from loosely arranged stromal tissue (g).  Sphincter muscle lies in the stroma (h).  Posterior iris shows posterior (i) and anterior (j) epithelium; the latter forms the dilator muscle.  Anterior chamber angle (k).  Trabecular meshwork (l) and canal of Schlemm (m).  Ciliary body and its muscle are posterior to iris (n). root of iris  The iris divides the anterior segment of the globe into anterior and posterior chambers  The pupil allows the aqueous humor to flow from the posterior into the anterior chamber with no resistance. CLINICAL COMMENT: BLUNT TRAUMA iridodialysis is detachment of the iris due to trauma Double vision in 1 eye /the affected eye (mono-ocular diplopia)  With blunt trauma to the eye or head, the thin root may tear away from the ciliary body, creating a condition called iridodialysis, which can result in damaged blood vessels and nerves.  Blood may hemorrhage into either the anterior or the posterior chamber, or both, and nerve damage may cause sector paralysis of the iris muscles. HISTOLOGIC FEATURES OF IRIS  The iris can be divided into four layers:  (1) the anterior border layer  (2) stroma and sphincter muscle  (3) anterior epithelium and dilator muscle  (4) posterior epithelium 1- Anterior Border Layer  The surface layer of the iris, the anterior border layer, is a thin condensation of the stroma.  In fact, some do not consider this to be a separate layer.  It is composed of fibroblasts, pigmented melanocytes, and collagen fibrils.  The highly branching processes of the cells interweave to form a meshwork in which the fibroblasts are on the surface and the melanocytes are located below  The thickness of the melanocyte layer may vary throughout the iris, with accumulations of melanocytes forming elevated frecklelike masses, evident in the anterior border layer. The density and arrangement of the meshwork differ among irises and are contributing factors in iris color.  The collagen fibrils are arranged in radial columns that are seen easily as white fibers in light-colored irises.  The anterior border layer ends at the root 2- Iris Stroma and Sphincter Muscle  The connective tissue stroma is composed of pigmented and nonpigmented cells, collagen fibrils, and extensive ground substance.  The pigmented cells include melanocytes and clump cells, whereas the nonpigmented cells are fibroblasts, lymphocytes, macrophages, and mast cells.  Although melanocytes and fibroblasts have man branching processes, the cells are widely spaced in the stroma, so their branches do not form a meshwork.  Clump cells are large, round, darkly pigmented cells and usually are located in the pupillary portion of the stroma, often near the sphincter muscle Pupillary portion of the iris  Dense cellular anterior border layer (a) terminates at pigment ruff (b) in pupillary margin.  Sphincter muscle is at c.  The arcades (d) from the minor circle of iris extend toward pupil and through sphincter muscle.  Sphincter muscle and iris epithelium are close to each other at the pupillary margin.  Capillaries, nerves, melanocytes, and clump cells (e) are found within and around the muscles.  The three to five layers of dilator muscle (f) gradually diminish in number until they terminate behind midportion of sphincter muscle (arrow) CLINICAL COMMENT: IRIDECTOMY  In some cases of glaucoma, to facilitate the movement of aqueous from the posterior chamber to the anterior chamber, an iridectomy is performed.  In this surgical procedure a wedge-shaped, full- thickness section of tissue is removed from the iris. If the sphincter muscle is cut during this procedure, the ability of the muscle to contract is not lost.  Iridotomy, a similar procedure in which an opening is made in the iris without excising tissue, often is accomplished using a laser, and the muscle is usually not involved  The iris arteries are branches of a circular vessel, the major circle of the iris, located in the ciliary body near the iris root.  The iris vessels usually follow a radial course from the iris root to the pupil margin  the bundles of collagen fibrils encircling the vessels are continuous with the collagen network of the stroma and not part of the actual vessel wall. This fibril network anchors the vessels in place and protects them from kinking and compression during extensive iris movement in miosis and mydriasis  An incomplete circular vessel, the minor circle of the iris, is located in the iris stroma in the region of the collarette and is a remnant of embryologic development.  The iris capillaries are not fenestrated and form part of the blood-aqueous barrier.  The iris stroma is continuous with the stroma of the ciliary body.  The sphincter muscle lies within the stroma and is composed of smooth-muscle cells joined by tight junctions.  As its name implies, the sphincter is a circular muscle 0.75 to 1 mm wide, encircling the pupil and located in the pupillary zone of the stroma.  Contraction of the sphincter causes the pupil to constrict in miosis. The  muscle is innervated by the parasympathetic system. 3 - Anterior epithelium and dilator muscle  Posterior to the stroma are two layers of epithelium.  The first of these, the epithelial layer lying nearest to the stroma, is the anterior iris epithelium, which is composed of the unique myoepithelial cell.  The apical portion is pigmented cuboidal epithelium joined by tight junctions and desmosomes, whereas the basal portion is composed of elongated, contractile, smooth muscle processes  The muscle fibers extend into the stroma, forming three to five layers of dilator muscle joined by tight junctions Light micrograph of ciliary portion of iris: Dilator muscle is evident as pink band (arrow) anterior to pigmented epithelium  The dilator muscle is present from the iris root to a point in the stroma below the midpoint of the sphincter.  Near the termination of the dilator muscle, small projections insert into the stroma or, more accurately, into the sphincter  Because the fibers are arranged radially, contraction of the dilator muscle pulls the pupillary portion toward the root, thereby enlarging the pupil in mydriasis.  The dilator is sympathetically innervated.  The anterior epithelium continues to the pupillary margin as cuboidal epithelial cells, and the epithelium continues posteriorly as the pigmented epithelium of the ciliary body. 4 - Posterior Epithelium  The second epithelial layer posterior to the stroma is the posterior iris epithelium, a single layer of heavily pigmented, approximately columnar cells joined by tight junctions and desmosomes.  In the periphery the posterior iris epithelium begins to lose its pigment as it continues into the ciliary body as the nonpigmented epithelium.  A thin basement membrane covers the basal aspect of this cellular layer, lining the posterior chamber.  The anterior and posterior iris epithelial layers are positioned apex to apex, a result of events during embryologic development.  Apical microvilli extend from both surfaces, and desmosomes join the two apical surfaces.  The epithelial cells curl around from the posterior iris to the anterior surface at the pupillary  margin, forming the pigmented pupillary ruff which encircles the pupil; this normally has a serrated appearance (see Figure 3-6). CLINICAL COMMENT: PIGMENTARY DISPERSION SYNDROME (PDS)  In pigmentary dispersion syndrome, pigment Krukenberg spindle in PDS granules are shed from the posterior iris surface and are dispersed into the anterior chamber. red reflex affected  They can be deposited on the iris, lens, or corneal endothelium or in the trabecular meshwork, where they might compromise aqueous outflow (increase IOP and cause pigmentary Gluacoma) Krukenberg Spindle: A classic sign of PDS, this is the vertical spindle of pigment that accumulates on the corneal endothelium.  Significant pigment loss will be evident on transillumination of the iris when the red fundus Transillumination defects in PDS reflex shows through in the depigmented areas. in PDS, the pigment granules are released from the posterior iris epithelium and accumulate in the trabecular meshwork, which is part of the drainage system of the eye responsible for regulating intraocular pressure (IOP). less drainage of humor, so increased IOP and Pigmentary glaucoma occurs  Anterior iris epithelium has two morphologically distinct portions: 1 - Apical epithelial portion (a) 2 - Basal muscular portion (b)  Tonguelike muscular processes overlap, creating three to five layers.  Tight junctions (arrows), such as those in sphincter muscle, are found between dilator muscle cells  A basement membrane (c) surrounds the muscle processes.  Unmyelinated nerves and associated Schwann cells (d)  Posterior pigmented iris epithelium shows lateral interdigitations (g) and areas of infolding along its basal surface (h).  A typical basement membrane (i) is found on the basal side as well.  Numerous tight junctions and desmosomes occur along lateral and apical walls CLINICAL COMMENT: IRIS SYNECHIAE Due to inflammation adhesions occur between iris and lens (posterior synechiae) and iris and cornea anteriorly (anterior synechiae)  An iris synechia is an abnormal attachment between the iris surface and another structure.  In a posterior synechia the posterior iris surface is adherent to the anterior lens surface.  In an anterior synechia the anterior iris surface is adherent to the corneal endothelium or the trabecular meshwork. IRIS COLOR less melanin so less light absorption and more visible blood vessels Red reflex becomes pinkish ex Albinism  Iris color depends on the arrangement and density of connective tissue components in the anterior border layer and stroma, melanocyte density and pigment density within the melanocyte.  If the iris is heavily pigmented, the anterior surface appears brown and smooth, even velvety, whereas in a lighter iris, the collagen trabeculae are evident and the color ranges from grays to blues to greens depending on the density of pigment and collagen.  A freckle or a nevus is an area of hyperpigmentation, an accumulation of melanocytes, and frequently is seen in the anterior border layer.  In all colored irises, the two epithelial layers are heavily pigmented.  Only in the albino iris do the epithelial layers lack pigment.  CLINICAL COMMENT: HETEROCHROMIA  Heterochromia of the iris is a condition in which one iris differs in color from the other or portions of one iris differ in color from the rest of the iris.  This can be either congenital or a sign of uveal inflammation.  If congenital, a disruption of the sympathetic innervation may be suspected.  A history regarding iris coloration should be elicited. innervation is needed to produce melanin so if have disrupted Sympathetic Kate Bosworth (actress) has one blue innervation we may have heterochromia eye, and one eye that is partially hazel Cornea  The cornea is the principal refracting component of the eye.  The transparent cornea appears, from the front, to be oval.  The anterior horizontal diameter is 12 mm, and the anterior vertical diameter is 11 mm  The central corneal thickness is 0.53 mm, whereas the corneal periphery is 0.71 mm thick  Function: 1- transmission of light 2- refraction of light (2/3 of total refractive power of eye) 3- barrier against infection, foreign bodies 4- transparency due to avascularity, uniform collagen structure and deturgescence (relative dehydration Corneal layers: 5 layers (anterior to posterior): 1. Epithelium 2. Bowman’s layer, 3. Stroma 4. Descemet’s membrane 5. Endothelium (dehydrates the cornea; dysfunction leads to corneal edema). Newly described: a 6th layer, “Dua’s layer”, although it is debated if this is a truly unique and additional layer extensive sensory fiber network (V1 distribution); therefore, abrasions are very painful Epithelium is a stratified squamous epithelium The epithelium has a high regenerative capacity  Maintenance of the smooth corneal surface depends on replacement of the surface cells that constantly are being shed into the tear film.  Cell proliferation occurs in the basal layer, basal cells move up to become wing cells, and wing cells move up to become surface cells  Only the cells in contact with the basement membrane have the ability to divide; the cells that are displaced into the wing cell layers lose this ability.  Stem cells are located in a 0.5- to 1- mm–wide band around the corneal periphery are the source for renewal of the epithelial basal cell layer. Bowman’s Layer thin, acellular layer composed of collagen fibers , not a membrane  The second layer of the cornea is approximately 8 to 14 μm thick.  Bowman’s layer is a dense, fibrous sheet of interwoven collagen fibrils randomly arranged in a mucoprotein ground substance Stroma thickest layer of the cornea It maintains the shape of the cornea contains GAGs and Keratocytes  The middle layer of the cornea is approximately 500 μm thick, or about 90% of the total corneal thickness.  The stroma (substantia propria) is composed of collagen fibrils, keratocytes, and extracellular ground substance.  The collagen fibrils have a uniform 25- to 35-nm diameter and run parallel to one another, forming flat bundles called lamellae  Each contains uniformly straight collagen fibrils arranged with regular spacing -> transparency  Keratocytes (corneal fibroblasts) are flattened cells that lie between and occasionally within the lamellae  Ground substance fills the areas between fibrils, lamellae, and cells. It contains proteoglycans, macromolecules with a carbohydrate glycosaminoglycan (GAG) portion. Descemet’s Membrane  Descemet’s membrane is considered the basement membrane of the endothelium. Acellular layer  It is produced constantly and therefore Serves as a basement membrane for the endothelium thickens throughout life, such that it has doubled by age 40 years.  In children it is 5 μm thick and will increase to approximately 15 μm over a lifetime  Descemet’s membrane consists of two laminae. The anterior lamina, approximately 3 μm thick, exhibits a banded appearance and is a latticework of collagen fibrils secreted during embryonic development.  The posterior lamina is nonbanded and homogeneous; it is the portion secreted by the endothelium throughout life  A thickened area of collagenous connective tissue may be seen at the membrane’s termination in the limbus; this circular structure is called Schwalbe’s line (or ring).  one single layer non proliferative role in maintaining the corneal transparency by pumping fluid out of the stroma, Endothelium preventing the cornea from becoming swollen and opaque if endothelial cell count drops below a certain level (usually under 700-1000 cells, inability to pump cornea's fluid As a result, fluid accumulates in the corneal stroma, causing corneal edema  The innermost layer of the cornea, the endothelium, lies adjacent to the anterior chamber and is composed of a single layer of flattened cells. It normally is 5 μm thick.  The basal part of each cell rests on Descemet’s membrane, and the apical surface, from which microvilli extend, lines the anterior chamber.  Endothelial cells are polyhedral: five-sided and seven sided cells can be found in normal cornea, but 70% to 80% are hexagonal.  Endothelial cells do not divide and replicate  The very regular arrangement of these cells is described as the endothelial mosaic  The cell density (cells per unit area) of the endothelium decreases normally with aging because of cell disintegration; density ranges from 3000 to 4000 cells/mm2 in children to 1000 to 2000 cells/mm2 at age 80 years.  The minimum cell density necessary for adequate function is in the range of 400 to 700 cells/mm CLINICAL COMMENT: EFFECTS OF CONTACT LENSES less oxygenation and hypoxia lead to neovascularization in peripheral cornea (pannus)  Clinical studies indicate that epithelial thinning, stromal thinning, and a decreased number of keratocytes are associated with long-term extended wear of contact lenses.  Numerous studies show that contact lens wear can induce changes in the regularity of the endothelial mosaic. Lens biconvex located behind the iris oval not circle avascular for UVR absorption and refractive status of eye along with cornea  The crystalline lens is an avascular, transparent elliptical structure that aids in focusing light rays on the retina.  The lens is located within the posterior chamber, anterior to the vitreous chamber and posterior to the iris  The lens is suspended from the surrounding ciliary body by zonular fibers.  Ciliary muscle contraction can cause a change in lens shape, increasing the dioptric power of the eye. This mechanism that causes an increase in lens power is accommodation, which allows near objects to be focused on the retina.  The posterior lens surface is attached to the anterior vitreous face by the hyaloid capsular ligament, a circular ring adhesion.  Within this ring is a potential space, the retrolental space (of Berger), an area of nonadhesion between vitreous and lens  ZONULES (OF ZINN): The lens is attached to the ciliary body by a group of threadlike fibers, the zonules (of Zinn), or the suspensory ligament of the lens. LENS DIMENSIONS  The lens is biconvex, with the posterior surface having the steeper curve.  The anterior radius of curvature measures 8 to 14 μm, and the posterior radius of curvature measures 5 to 8 μm.  The centers of the anterior and posterior surfaces are called the poles, and the lens thickness is the distance from anterior to posterior pole.  The thickness of the unaccommodated lens is 3.5 to 5 mm and it increases 0.02 mm each year throughout life.  The lens diameter is the nasal-to-temporal measurement and in the infant is 6.5 mm; the diameter reaches 9 mm during the teenage years and does not change significantly from that.  The equator is the largest circumference of the lens at a location between the two poles.  The refractive power of the unaccommodated lens is approximately 20 diopter (D)  The power of the lens increases in accommodation with the maximum accommodative amplitude; 14 D, reached between ages 8 and 12 years.  Accommodation decreases with age, approaching zero after 50 years LENS CAPSULE  The lens capsule is a transparent envelope that surrounds the entire lens. The eye lens is wrapped in a transparent, elastic capsule. Small, elastic fibers called zonules suspend the lens from  Its thickness varies with location, being thinnest at the poles and equator and the ciliary body above and below it thickest in an annular area around the anterior pole. Ciliary muscles help adjust the shape of the lens. When they contract, the zonules actually relax, allowing the lens  The capsule is a basement membrane and with time becomes the thickest in to become rounder. This is how you focus on something the body. close up  The capsule consists primarily of collagen fibers; it contains no elastic fibers but is highly elastic because of the lamellar arrangement of the fibers.  It encloses all lens components and helps to mold the shape of the lens.  The capsule would prefer to take a more spherical shape, but this tendency is counteracted by the pull from the zonular fibers.  The zonular fibers insert into the capsule, merging with it in an area from the equator to near both poles.  This outer superficial zone of the capsule is called the zonular lamella  The lens capsule provides some barrier function preventing large molecules, such as albumin and hemoglobin, from entering the lens.  The anterior lens capsule is produced by the anterior epithelium and thickens with age.  The posterior lens capsule may receive some contribution from the basal membrane of lens fibers, but the thickness of the posterior capsule changes minimally throughout life.  LENS EPITHELIUM  Adjacent to the anterior lens capsule is a layer of cuboidal epithelium, the anterior lens epithelium  These cells secrete the anterior capsule throughout life and are the site of metabolic transport mechanisms.  As noted, no posterior epithelium is present because it was used during embryologic development to form the primary lens fibers.  The basal aspect of the epithelial cell is adjacent to the capsule, and the apical portion is oriented inward toward the center of the lens.  The lateral membranes of the epithelial cells are joined by desmosomes and gap junctions.  The band of cells in the pre-equatorial region that lies just anterior to the equator is called the germinal zone, the location Light micrograph of anterior lens epithelium of cell mitosis. As cell division continues throughout life, each and capsule. newly formed cell elongates; the basal aspect stretches toward Honeycomb appearance of lens fibers above the posterior pole and the apical aspect toward the anterior epithelium. pole.  Eventually, as it loses all cellular organelles, the elongated cell becomes a lens fiber.  The lateral membranes have numerous and elaborate interdigitations along the fiber length that take various shapes, such as ball-and- socket and tongue-in- groove junctions, and allow for sliding between fibers  The fibers also are joined by desmosomes  LENS FIBERS  Lens fiber production continues throughout life, with the new lens fibers being laid down outer to the older fibers; growth results in concentric layers of secondary lens fibers.  The structure of the lens is similar to an onion; each layer of fibers approximates a layer of an onion,  Section through the equator of the lens shows that the fibers cut in cross section are hexagonal in shape and arranged in concentric rings (Figure 5-6).  Lens fiber length is approximately 8 to 10 nm  Lens fiber cytoplasm contains a high concentration of proteins, known as crystallins, which account for approximately 40% of the net weight of the fiber.  The distribution and concentration of crystallins contribute to the gradient refractive index.  The crystallin concentration varies from approximately 15% in the cortex to 70% in the nucleus.  he anterior lens epithelium is a layer of cuboidal cells beneath the lens capsule, involved in capsule secretion and metabolic transport. The  Adult lens, showing nuclear zones, germinal zone near the equator is where cell epithelium, and capsule. division occurs, and newly formed cells elongate and differentiate into lens fibers. These fibers,  Thickness of lens capsule in various joined by desmosomes and complex zones is shown interdigitating junctions, form the concentric layers of the lens. The lack of posterior epithelium in the adult lens reflects its embryological development, where the posterior  UVR below 300 nm cornea  ULTRAVIOLET RADIATION AND THE LENS: 300-400 nm lens above 400 nm retina  The cornea absorbs wavelengths below 300 nm  the lens absorbs wavelengths between 300 and 400 nm, and wavelengths greater than 400 nm are transmitted to the retina.  The lens absorbs almost all UV light to which it is exposed, and any resulting unstable free radicals cause molecular changes.  The first active tissue of the lens that encounters UV radiation is the lens epithelium, which is susceptible to damage from free radicals.  Morphologic changes apparent in the epithelial layer may lead to irreversible changes throughout the lens, although the mechanism by which this progresses is not defined.  Ciliary body  When viewed from the front of the eye, the ciliary body is a ring-shaped structure. Its width is approximately 5.9 mm on the nasal side and 6.7 mm on the temporal side.  The posterior area of the ciliary body, which terminates at the ora serrata, appears fairly flat, but the anterior ciliary body contains numerous folds or processes that extend into the posterior chamber  In sagittal section the ciliary body has a triangular shape, the base of which is located anteriorly; one corner of the base lies at the scleral spur, the iris root extends from the approximate center of the base, and a portion of the base borders the anterior chamber  The outer side of the triangle lies against the sclera, and the inner side lines the posterior chamber and a small portion of the vitreous cavity  The apex is located at the ora serrata.  Ciliary Epithelium: Outer pigmented epithelium: Similar to the retinal pigment epithelium,  The ciliary body can be divided into two parts: this layer is involved in the blood-aqueous barrier. Inner non-pigmented epithelium: This layer produces aqueous humor  The pars plicata and the pars plana. and participates in its transport into the anterior chamber.  The pars plicata is the wider, anterior portion containing the ciliary processes.  Approximately 70 to 80 ciliary processes extend into the posterior chamber, and the regions between them are called valleys (of Kuhnt). The pars plicata is the  A ciliary process measures approximately 2 mm in length, 0.5 mm in anterior portion of the ciliary width, and 1 mm in height, but there are significant variations in all body, located closer to the iris measurements. contains folds for humor secretion The pars plana is the flatter region of the ciliary body. It extends from less in surgeries  the posterior of the pars plicata to the ora serrata, which is the transition he pars plana is the posterior between ciliary body and choroid. portion of the ciliary body no folds  The ora serrata has a serrated pattern, the forward-pointing apices of important in vitreous body which are called teeth or dentate processes production Commonly used for posterior eye surgeries  Inner aspect of the ciliary body shows:  pars plicata (a) and pars plana (b).  Ora serrata is at c, and posterior to it, retina exhibits cystoid degeneration (d).  Bays (e) and dentate processes of Ora (f)  linear ridges or striae (g) project forward from dentate processes across pars plana to enter valleys between ciliary processes.  Zonular fibers arise from pars plana beginning 1.5 mm from ora serrata. These fibers curve forward from sides of dentate ridges into ciliary valleys, then from valleys to lens capsule.  Ciliary processes vary in size and shape and often are separated from one another by lesser processes.  Radial furrows (h) and circular furrows (i) of peripheral iris are shown. CILIARY MUSCLE The ciliary body contains the ciliary muscle, which is responsible for the accommodation of the lens. The muscle has three distinct parts: Longitudinal fibers Radial fibers Circular fibers  The ciliary muscle is composed of smooth muscle fibers oriented in longitudinal, radial, and circular directions.  The longitudinal muscle fibers (of Brücke) lie adjacent to the supraciliaris and parallel to the sclera.  Each muscle bundle resembles a long narrow V the base of which is at the scleral spur, whereas the apex is in the choroid. The tendon of origin attaches the muscle fibers to the scleral spur and to adjacent trabecular meshwork sheets.  Below the longitudinal muscle fibers, the radial fibers form wider, shorter interdigitating Vs that originate at the scleral spur and insert into the connective tissue near the base of the ciliary processes.  This layer is a transition from the longitudinally oriented fibers to the circular fibers.  The innermost region of ciliary muscle, (Müller’s) annular muscle, is formed of circular muscle bundles with a sphincter-type action. These fibers are located near the major circle of the iris.  Ciliary body, including the ciliary muscle and its components.  Trabecular meshwork (a)  Schlemm’s canal (b)  External collectors channels (c)  Scleral spur (d)  Three components of ciliary muscle are shown separately, viewed from the outside:  Section 1 shows longitudinal ciliary muscle.  Section 2 longitudinal ciliary muscle has been dissected away to show radial ciliary muscle.  Section 3, only the innermost circular ciliary muscle is shown.  Ciliary muscle originates in ciliary tendon, which includes scleral spur (d) and adjacent connective tissue.  CILIARY EPITHELIUM  Two layers of epithelium, positioned apex to apex, cover the ciliary body and line the posterior chamber and part of the vitreous chamber.  Both epithelial layers contain cellular components characteristic of cells actively involved in secretion.  The outer layer (i.e., the one next to the stroma) is pigmented and cuboidal, and the cells are joined by desmosomes and gap junctions.  Anteriorly, the pigmented ciliary epithelium is continuous with the anterior iris epithelium  Posteriorly, it is continuous with the retinal pigment epithelium (RPE)  A basement membrane attaches the pigment epithelium to the stroma.  The basement membrane of the pigmented ciliary epithelium is continuous anteriorly with the basement membrane of the anterior iris epithelium and posteriorly with the inner basement membrane portion of Bruch’s membrane of the choroid.  The inner epithelial layer (i.e., the layer lining the posterior chamber) is non-pigmented and is composed of columnar cells in the pars plana and cuboidal cells in the pars plicata.  The non-pigmented ciliary epithelium is continuous anteriorly with the posterior iris epithelium  It continues posteriorly at the ora serrata, where it undergoes significant transformation, becoming transition of neural retina pigmented iris epithelium (a) into  The lateral walls of the cells contain extensive nonpigmented ciliary epithelium interdigitations and are joined, near their apices, by (b). desmosomes, gap junctions, and zonula occludens, which form one site of the blood-aqueous barrier.  The metabolically active non-pigmented epithelial cells are involved in active secretion of aqueous humor components and serve as a diffusion barrier between blood and aqueous.  The two epithelial layers are positioned apex to apex  The basement membrane covering the non- pigmented ciliary epithelium, the internal limiting membrane of the ciliary body, lines the posterior chamber, is continuous with the internal limiting membrane of the retina, and is the attachment site for the zonular fibers as well as the fibers of the vitreous base. Retina  Layers of the retina:  1, Retinal pigment epithelial layer (RPE)  2, photoreceptor layer  3, external limiting membrane;  4, outer nuclear layer;  5, outer plexiform layer;  6, inner nuclear layer;  7,inner plexiform layer;  8, ganglion cell layer;  9, nerve fiber layer;  10, internal limiting membrane.  At top is inner portion of choroid with choriocapillaris (arrow). RETINAL PIGMENT EPITHELIUM  The retinal pigment epithelium (RPE), the outermost retinal layer, is a single cell layer and consists of pigmented hexagonal cells.  These cells are columnar in the area of the posterior pole, and more densely pigmented in the macular area.  The cells become larger and more cuboidal as the layer nears the ora serrata, where the transition to the pigmented epithelium of the ciliary body.  The RPE cells contain numerous melanosomes, pigment granules that extend from the apical area into the middle portion of the cell and somewhat obscure the nucleus, which is located in the basal region.  In the retina, melanin is densest in the RPE cells located in the macula and at the equator  Other pigmented bodies, lipofuscin granules, contain degradation products of phagocytosis and increase in number with age. Thick sheaths (a) of RPE enclose external portions of rod outer segments (b).  The apical portion of an RPE cell consists of microvilli that extend Numerous fingerlike villous processes (c) are into the layer of photoreceptors, enveloping the specialized outer found between photoreceptors and contain pigment granules (d). segment tips Apical portion of RPE layer of cells at bottom contains numerous pigment granules (e) CLINICAL COMMENT: RETINAL DETACHMENT (RD) do photocoagulation to prevent further detachement  When a retinal detachment occurs, the separation usually lies between the RPE cells and the photoreceptors because no intercellular junctions join these cells.  The RPE cells remain attached to the choroid and cannot be separated from it easily.  Retinal detachment separates the photoreceptors from their blood supply, and if the layers are not repositioned quickly, the affected area of photoreceptor cells will necrose.  An argon laser often is used to photocoagulate the edges of the detachment,producing scar tissue. This photocoagulation prevents the detachment from enlarging and facilitates the repositioning of the photoreceptors. PHOTORECEPTOR CELLS  Photoreceptor cells, the rods and cones, are special sense cells containing photopigments that absorb photons of light.  Rods are more active in dim illumination, and cones are active in well light conditions.  Rods and cones are composed of several parts: starting nearest the RPE: (1) the outer segment, containing the visual pigment molecules for the conversion of light into a neural signal (2) A connecting stalk, the cilium (3) The inner segment, containing the metabolic apparatus; (4) The outer fiber (5) The cell body (6) The inner fiber, which ends in a synaptic terminal. RETINAL NERVE FIBER LAYER (RNFL)  The retinal nerve fiber layer (RNFL) consists of the ganglion cell axons.  Their course runs parallel to the retinal surface; the fibers proceed to the optic disc and exit the eye through the lamina cribrosa as the optic nerve.  The fibers generally are unmyelinated within the retina.  The RNFL is thickest at the margins of the optic disc, where all the fibers accumulate.  The group of fibers that radiate to the disc from the macular area is called the papillomacular bundle.  This important grouping of fibers carries the information that determines visual acuity.  The retinal vessels, including the superficial capillary network, are located primarily in the RNFL but may lie partly in the ganglion cell layer. CLINICAL COMMENT: FUNDUS VIEW OF THE INTERNAL LIMITING MEMBRANE  Reflections from the internal limiting membrane produce the retinal sheen seen with the ophthalmoscope.  In younger persons this membrane gives off many reflections and appears glistening; the sheen is less evident in older individuals.  Macula Lutea macula is a normal depression in retina  The macula lutea appears as a darkened region in the central retina and may seem to have a yellow hue because of the xanthophyll pigments lutein and zeaxanthin.  These pigments have been found throughout the retina, with the greatest concentration in the plexiform layers of the macula.  The newborn has little if any of these pigments, but they gradually accumulate from dietary sources.  These pigments apparently act as filters to reduce chromatic aberration but may also have an antioxidant effect, suggesting a protective role against UV radiation damage  The macula lutea is approximately 5.5 mm in diameter; its center is approximately 3.5 mm lateral to the edge of the disc and approximately 1 mm inferior to the center of the disc.  The pigment epithelial cells are taller and contain more pigment than cells elsewhere in the retina, which contributes to the darkness of this area.  However, the intensity of the pigment varies greatly from person to person  Fovea: The shallow depression in the center of the macular region is the fovea  This depression is formed because the retinal cells are displaced, leaving only photoreceptors in the center.  The fovea has horizontal diameter of approximately 1.5 mm.  The curved wall of the depression is known as the Clivus, which gradually slopes to the floor, the foveola.  The fovea has the highest concentration of cones in the retina; estimates vary from 199,000 to 300,000 cones per square millimeter. Layers present in the center of the foveal area are RPE, photoreceptor layer, external limiting membrane, outer nuclear layer, outer plexiform layer (Henle’s fiber layer)  Within the fovea is a capillary- free zone (FAZ) 0.4 to 0.5 mm in diameter.  The lack of blood vessels in this region allows light to pass unobstructed into the photoreceptor’s outer segment CLINICAL COMMENT: METAMORPHOPSIA Metamorphopsia is a visual distortion where straight lines or objects appear wavy, bent, or otherwise misaligned. due to abnormality in macula ex edema Amsler Grid Test: This is a simple diagnostic tool  The axis of the photoreceptor outer segment is oriented to accomplish capture of incident light rays.  If a disruption occurs so that the outer segment is no longer oriented toward the exit pupil, vision may be altered.  With macular edema, the orientation of the photoreceptors is changed, and metamorphopsia can often be elicited with an Amsler grid testing  in glaucoma we have cupping Optic Disc cup-to-disc ratio is a key measure of the optic nerve's health. Increased cup-to-disc ratio and optic disc changes are potential indicators of glaucoma and optic neuropathy.  The optic disc, or optic nerve head, is the site where ganglion cell axons accumulate and exit the eye.  The horizontal diameter of the disc is approximately 1.7 mm and the vertical diameter approximately 1.9 mm.  The number of nerve fibers appears to be positively correlated with the size of the optic nerve head; larger discs have relatively more fibers than smaller discs.  Smaller discs may demonstrate optic nerve head crowding.  Fiber number decreases with age.  The optic disc lacks all retinal elements except the nerve fiber layer and an internal limiting membrane.  It is paler than the surrounding retina because there is no RPE.  The pale-yellow or salmon color of the optic disc is a combination of the scleral lamina cribrosa and the capillary network.  In some individuals the openings of the lamina cribrosa may be visible through the transparent nerve fibers  CLINICAL COMMENT: PAPILLEDEMA edema with high ICP is called papilledema edema without ICP is called Optic nerve swelling  Papilledema is edema of the optic disc secondary to an increase in intracranial pressure (ICP).  As ICP increases, pressure within the meningeal sheaths around the optic nerve slows axoplasmic flow in the ganglion fibers, causing fluid to accumulate within the fibers so that they swell.  This accumulation of fluid is seen at the disc as an elevation of the nerve head with blurring of the disc margins  This condition is or will become bilateral. The central retinal vein may also be compromised, with hemorrhages becoming evident in the nerve fiber layer in the vicinity of the disc.  Edema of the optic disc from any other cause is referred to as simply “edema of the optic disc.” Thank You