eye

The Eye: The Photoreceptor System

Structure of the Eye

  • Eyes are highly developed photosensitive organs that analyze the form, intensity, and color of light reflected from objects, providing the sense of sight.
  • They are protected within the orbits of the skull, which also contain adipose cushions.
  • Each eyeball consists of a tough, fibrous globe that maintains its overall shape.
  • Internally, the eye contains:
    • Transparent tissues that refract light to focus the image.
    • A layer of photosensitive cells.
    • A system of neurons that collect, process, and transmit visual information to the brain.
  • The eye is composed of three concentric tunics or layers:
    • A tough external fibrous layer: sclera and transparent cornea.
    • A middle vascular layer (uvea): choroid, ciliary body, and iris.
    • An inner sensory layer: retina, which communicates with the cerebrum through the posterior optic nerve.
  • Information about the external world is conveyed to the central nervous system (CNS) from sensory receptors.

Extraocular Muscles (EOM)

  • Responsible for controlling the movements of the eyeball and upper eyelid.
  • Also known as extrinsic eye muscles, distinguishing them from intrinsic eye muscles which control iris movement.
  • Three antagonistic pairs of muscles control eye movements:
    • Lateral and medial rectus muscles.
    • Superior and inferior rectus muscles.
    • Superior and inferior oblique muscles.

Surface of the Eye

  • The surface of the eye and the inner surface of the eyelids are covered with a clear membrane called the conjunctiva.
  • Layers of the tear film keep the front of the eye lubricated.
  • Tear film layers:
    1. Inner mucus layer: keeps the tear fastened to the eye.
    2. Watery middle layer: hydrates the eye, repels bacteria, and protects the cornea.
    3. Outer oily layer: keeps the tear surface smooth for clear vision and prevents evaporation of other layers.

Coats of the Eye

  • The eye comprises three coats, enclosing the aqueous humor, lens, and vitreous body.
  • Outermost coat: cornea and sclera.
  • Middle coat: main blood supply, consisting of the choroid, ciliary body, and iris (from back to front).

Development of the Eye

  • Optic Cup and Lens Vesicle:
    • Developing eyes appear in the 22-day embryo as optic grooves on the sides of the forebrain.
    • With the closure of the neural tube, these grooves form optic vesicles.
    • Optic vesicle invaginates and forms the optic cup. This invagination inferiorly forms the choroid fissure, which allows the hyaloid artery to reach the inner chamber of the eye.
    • During the seventh week, the lips of the choroid fissure fuse and become the future pupil.

Embryonic Eye Formation

  • Tissues of the eye are derived from neuroectoderm, surface ectoderm, and mesoderm.
  • By the 22nd day of development, eyes are evident as shallow grooves (optic sulci or grooves) in the neural folds at the cranial end of the human embryo.
  • As the neural tube closes, the paired grooves form outpocketings called placodes.
  • Invagination of the optic vesicle results in the formation of a double-layered optic cup.
    • Inner layer becomes the neural retina.
    • Outer layer becomes the RPE (retinal pigmented epithelium).
  • Mesenchyme surrounding the optic cup gives rise to the sclera.
  • Invagination of the central region of each lens placode results in the formation of lens vesicles.
  • By the fifth week, the lens vesicle loses contact with the surface ectoderm and lies in the mouth of the optic cup.
  • The retina also forms from the optic vesicle, which evaginates from the neuroectodermal diencephalic vesicle. The connection between the optic and diencephalic vesicles becomes the optic nerve.
  • The front surface of the optic cup becomes the neural retina, and the back surface becomes the retina's pigmented epithelium.
  • Mesenchyme extending into the invagination of the optic cup acquires a gelatinous consistency and becomes the vitreous component of the eye.
  • The lens vesicle is held in place by the free margins of the optic cup and the surrounding mesenchyme.
  • At the outer surface of the optic cup, the mesenchymal shell differentiates into the vascular choroid coat and the fibrous components of the sclera and cornea.
  • Posterior to the lens, the vascular choroid coat forms the ciliary body, ciliary muscle, and ciliary processes.
  • Anterior to the lens, the choroid coat forms the stroma of the iris.
  • Around the rim of the optic cup, the inner and outer layers form the posterior epithelium of the ciliary body and iris.
  • The sphincter and dilator pupillae muscles develop from the posterior epithelium.
  • At birth, the iris is light blue in fair-skinned people because pigment is usually not present.
  • The dilator muscle and sphincter pupillary muscles develop during the sixth month as derivatives of the neuroectoderm of the outer layer of the optic cup.

Uvea

  • The uvea forms the pigmented vascularized tunic of the eye and is divided into three regions:
    1. The choroid.
    2. The ciliary body.
    3. The iris.
  • It consists of two layers:
    1. An outer pigmented layer (pars pigmentosa).
    2. An inner retinal layer (pars nervosa or optica).
  • The cornea is transparent.
  • The sclera is opaque.

Sclera

  • The conjunctiva covers the external surface.
  • There are three indistinct layers, described from superficial to deep:
    • The episclera: a thin, loose, collagenous connective tissue.
    • The stroma: a thick layer of dense, collagenous connective tissue made of interlacing type I collagen fibers alternating with networks of elastic fibers.
    • The suprachoroid lamina: a thin connective tissue layer containing fibroblasts and melanocytes.
  • The sclera-corneal junction is called the limbus. Here, the deep surface of the suprachoroid lamina is covered by the scleral endothelium, a simple squamous epithelium.
  • The limbus is the location of corneal stem cells.
  • The sclera is nearly avascular, although it does have some visible blood vessels that pierce the surface to reach the retina below.
  • The sclera is divided into three rather ill-defined layers:
    • The episcleral layer (episclera): the external layer, is the loose connective tissue adjacent to the periorbital fat.
    • The substantia propria (sclera proper, also called Tenon’s capsule): the investing fascia of the eye and is composed of a dense network of thick collagen fibers.
    • The suprachoroid lamina (lamina fusca): the inner aspect of the sclera, is located adjacent to the choroid and contains thinner collagen fibers and elastic fibers as well as fibroblasts, melanocytes, macrophages, and other connective tissue cells.
  • The limbus is the zone of transition of the epithelium of the conjunctiva with that of the cornea.
  • Functions:
    1. Protects the inner structures of the eye.
    2. Maintains the shape and consistency of the eyeball.
  • The sclera is dense connective tissue made of mainly type 1 collagen fibers, oriented in different directions.
  • The sclera has four layers, from the outside to the inside:
    • Episclera
    • Stroma
    • Lamina fusca
  • The lamina cribrosa (LC) is a mesh-like structure at the optic nerve head that surrounds and supports the retinal ganglion cell axons as they form the optic nerve.
  • A healthy optic nerve has approximately 1.2 million nerve fibers.
  • These nerve fibers pass through a sieve-like portion of the posterior sclera, called the lamina cribrosa, before exiting the eye.

Cornea

  • The 5 layers of the cornea seen in a transverse section are the following:
    • Corneal epithelium
    • Bowman’s membrane (anterior basement membrane)
    • Corneal stroma
    • Descemet’s membrane (posterior basement membrane)
    • Corneal endothelium
  • The corneal epithelium is a nonkeratinized stratified squamous epithelium.
  • The basal layer of the limbus contains corneolimbal stem cells that generate and maintain the corneal epithelium.
  • The posterior surface of the cornea is covered by simple squamous epithelium that rests on Descemet’s membrane adjacent to the stroma.
  • The corneal epithelium rests firmly on the thick homogeneous Bowman’s membrane.
  • The stroma is completely avascular, and nutrients reach the keratocytes and epithelial cells by diffusion from the surrounding limbus and aqueous humor behind the cornea.
  • Myelinated nerves can be found in the stroma.
  • After crossing Bowman's layer, nerves become unmyelinated and extend toward the surface in the intercellular spaces of the corneal epithelium.
  • Corneal endothelium is permeable to air oxygen used for various oxidative reactions, in particular glutathione reduction and oxidation.
  • The glutathione pathway neutralizes excess active oxygen in the cornea.
  • About 30% of glucose is metabolized in the cornea by glycolysis.
  • The stroma is formed by collagen lamellae oriented at an angle to one another. Fibroblasts, surrounded by extracellular matrix, are present between lamellae. Blood vessels are not present.
  • Bowman’s membrane is a homogeneous-appearing layer on which the corneal epithelium rests.
  • Descemet’s membrane is an unusually thick basal lamina.
  • Bowman’s membrane imparts some strength to the cornea and acts as a barrier to the spread of infections.
  • Bowman's membranes does not regenerate. Therefore if damaged, an opaque scar forms that may impair vision. In addition, changes in Bowman’s membrane are associated with recurrent corneal erosions.
  • Unlike Bowman’s membrane, Descemet’s membrane readily regenerates after injury and thickens with age.
  • The uniform spacing of collagen fibrils and lamellae, as well as the orthogonal array of the lamellae (alternating layers at right angles), is responsible for the transparency of the cornea.
  • The corneal stroma constitutes 90% of the corneal thickness.
  • The corneal stroma, also called substantia propria, is composed of about 60 thin lamellae.
  • Each lamella consists of parallel bundles of collagen fibrils.
  • Located between lamellae are nearly complete sheets of slender, flattened fibroblasts.
  • The fibrils measure approximately 23nm23 nm in diameter and are as long as 1cm1 cm.
  • The collagen fibrils in each lamella are arranged at approximately right angles to those in the adjacent lamellae
  • Normally, the cornea contains no blood vessels or pigments.
  • During an inflammatory response involving the cornea, large numbers of neutrophilic leukocytes and lymphocytes migrate from blood vessels of the corneoscleral limbus and penetrate the stromal lamellae.
  • The corneal stroma is composed of stacks of lamella within which collagen fibers are oriented orthogonal to adjacent lamellae.

Vascular Coat (Uvea)

  • The uvea is the vascular layer in the middle, subdivided into the iris, ciliary body, and choroid.
  • At the corneoscleral junction (CSJ), or limbus, encircling the cornea, the posterior endothelium and its underlying Descemet’s membrane are replaced by a meshwork of irregular channels lined by endothelium and supported by trabeculae of connective tissue.
  • At the iridocorneal angle between limbus and iris aqueous humor moves from the anterior chamber into channels of this trabecular meshwork and is pumped by endothelial cells into the adjacent scleral venous sinus.

Limbus

  • Encircling the cornea is the limbus, a transitional area where the transparent cornea merges with the opaque sclera.
  • Here Bowman’s membrane ends and the surface epithelium becomes more stratified as the conjunctiva that covers the anterior part of the sclera (and lines the eyelids).
  • Also at the limbus Descemet᾿s membrane and its simple endothelium are replaced with a system of irregular endothelium-lined channels called the trabecular meshwork.
  • These penetrate the stroma at the corneoscleral junction and allow slow, continuous drainage of aqueous humor from the anterior chamber.
  • This fluid moves from these channels into the adjacent larger space of the scleral venous sinus, or canal of Schlemm ,which encircles the eye.
  • From this sinus aqueous humor drains into small blood vessels (veins) of the sclera.
  • Deep to the limbus, the choroid layer is thickened into the ciliary body.
  • The ciliary body is a ring of smooth muscle fibers arranged concentrically around the opening in which the lens is suspended.
  • The lens is suspended from the ciliary body by the ciliary processes and then by fibers of collagen, called suspensory fibers or zonules.
  • Together, the ciliary body and suspensory fibers control the shape of the lens.
  • The surface of the ciliary body is covered by extensions.
  • Small projections of this tissue form the ciliary processes.
  • The epithelium of the ciliary processes secretes the aqueous humor.
  • At the junction of the cornea and sclera, Bowman’s membrane ends abruptly.
  • Melanocytes are prominent in the choroid, especially in its outer region,the suprachoroidal lamina (SCL).
  • The choroid’s inner region, the choroidocapillary lamina (CCL), has a rich microvasculature that helps provide O2O_2 and nutrients to the adjacent retina.
  • Between the choroid and the retina is a thin layer of extracellular material known as Bruch’s membrane or layer (B).
  • In the stromal layer, endothelium-lined channels called the trabecular meshwork (or spaces of Fontana) merge to form the scleral venous sinus (canal of Schlemm).
  • The ciliary muscle occupies the bulk of the ciliary body.
  • Zonular fibers from the ciliary epithelium extend toward the lens.
  • Contraction of the ciliary muscle relaxes the tension exerted by the zonular fibers on the lens during accommodation
  • The ciliary processes project from the ciliary body.
  • They are lined by the cllary epithelium,which produces aqueous humor
  • The Inner layer of the epithelium is nonpigmented and faces the posterior chamber
  • The Iris has surfaces. The surface is lined by a epithelial electron microscope
  • Both epithelial layers are pigmented
  • The outer layer of the epithelium is pigmented and faces the stroma of the ciliary body.
  • Cells of this dual epithelium have extensive basolateral folds with Na+/K+−ATPaseNa^+/K^+-ATPase activity and are specialized for secretion of aqueous humor.
  • Fluid from the stromal microvasculature moves across this epithelium as aqueous humor, with an inorganic ion composition similar to that of plasma but almost no protein.
  • Aqueous humor is secreted by ciliary processes into the posterior chamber, flows through the pupil into the anterior chamber, and drains at the angle formed by the cornea and the iris into the channels of the trabecular meshwork and the scleral venous sinus, from which it enters venules of the sclera.
  • The surface epithelium of ciliary processes is a double layer of pigmented (PE) and nonpigmented epithelial (NE) low columnar or cuboidal cells.
  • The two layers are derived developmentally from the folded rim of the embryonic optic cup, so that the exposed surface of the nonpigmented layer is actually the basal surface of the cells.
  • No true basal lamina is present, but instead these cells produce the components that give rise to the fibers of the ciliary zonule in the embryo.
  • Beneath the double epithelium is a core of connective tissue with many small blood vessels (V).
  • Fluid from these vessels is pumped by the epithelial cells out of the ciliary processes as aqueous humor.
  • Dilator pupillae, consisting of myoepithelial cells, contains a-adrenergic receptors and is innervated by sympathetic nerve fibers.
  • Contraction of the dilator causes pupil dilation, or mydriasis.
  • Innervated by parasympathetic nerve fibers. Sphincter contraction reduces the diameter of the pupil, or miosis.
  • The ciliary epithelium is an extension of the retina beyond the ora serrata and covers the inner surface of the ciliary body.
  • Zonular fibers, normally associated with the cillary processes
  • The aqueous veins collect and transport the aqueous fluid to the episcleral veins.
  • When the iridocorneal angle is more narrow than usual, the thickening of the peripheral iris that occurs with dilation of the pupil can occlude the angle and obstruct drainage of aqueous humor at the trabecular Meshwork.
  • This can result in the rapid development of intraocular hypertension known as angle closure glaucoma, acute glaucoma, or closed (narrow) angle glaucoma.
  • This condition usually affects both eyes and causes blurred vision, eye pain, and headache.
  • Aqueous humor is produced continuously.
  • If its drainage from the anterior chamber is impeded, typically by obstruction of the trabecular meshwork or scleral venous sinus, intraocular pressure can increase, causing the condition called glaucoma.
  • The ora serrata is the most anterior extent of the retina.
  • It is the peripheral of the retina and lies approximately 5mm5 mm anterior to the equator of the eye.
  • The ora srrrata is the serrated junction between the choroid and the ciliary body.
  • This junction marks the transition from the simple, non-photosensitive area of the ciliary body to the complex, multi-layered, photosensitive region of the retina
  • The pigmented layer is continuous over choroid, ciliary body and iris while the nervous layer terminates just before the ciliary body.

Iris

  • Colored structure surrounding the pupil.
  • Controls amount of light entering the eye
  • Controls the size of the pupil through the sphincter pupillae (shrinks pupil) and a diffuse dilator pupillae (enlarges pupil)
  • Spongy stroma with melanocytes: faces anterior chamber
  • Pigmented epithelium faces posterior chamber. This epithelium becomes the ciliary body laterally.
  • The anterior surface, exposed to aqueous humor in the anterior chamber, has no epithelium and consists only of a matted layer of interdigitating fibroblasts and melanocytes.
  • Cells of the external pigmented epithelium (PE) are very rich in melanin granules to protect the eye’s interior from an excess of light.
  • Cells of the other layer are myoepithelial, less heavily pigmented, and comprise the dilator pupillae muscle (DPM) that extends along most of the iris.
  • Near the pupil, fascicles of smooth muscle make up the sphincter pupillae muscle (SPM).
  • The underlying stroma (S) contains many melanocytes with varying amounts of melanin.
  • The deep stroma also is richly vascularized.
  • Bruch's membrane is a thin (2–4µm)(2–4 µm), acellular, five-layered extracellular matrix located between the retina and choroid.It extends anteriorly to the ora serrata, interrupted only by the optic nerve.

The Lens

  • The lens is a perfectly transparent biconvex structure held in place by a circular system of zonular fibers that attach it to the ciliary body and by close apposition to the posterior vitreous body.
  • Partly covering the anterior surface of the lens is an opaque pigmented extension of the middle layer called the iris , which surrounds a central opening, the pupil.
  • Located in the anterior portion of the eye, the iris and lens are bathed in clear aqueous humor that fills both the anterior chamber between the cornea and iris and the posterior chamber between the iris and lens.
  • Note that the embryonic lens is basically a "bubble" of epithelium, the lens vesicle, which invaginates and pinches off from the surface ectoderm.
  • (The formation of this vesicle is induced by the underlying optic vesicle, below.)
  • Cells forming the front layer of this vesicle mature into simple cuboidal epithelium, while the cells forming the back layer elongate into lens fibers
  • Histologically, the lens is bizarre.
  • It is an isolated island of epithelial tissue with an anterior layer that is simple cuboidal and a posterior layer consisting of extravagantly elongated cells, called lens fibers, that are packed with lens protein
  • The lens is a transparent, elastic tissue that focuses light on the retina.
  • Surrounding the entire lens is a thick, homogenous external lamina called the lens capsule (LC) composed of proteoglycans and type IV collagen surrounds the lens.
  • The anterior surface of the lens, beneath the capsule, is covered by a simple columnar lens epithelium (LE)
  • The epithelial cells attach basally to the surrounding lens capsule and their apical surfaces bind to the internal lens fibers.
  • Lens fibers are highly elongated, terminally differentiated cells that appear as thin, flattened structures
  • Developing from cells in the lens epithelium, lens fibers typically become 7-10- mm long, with cross-section dimensions of only 2by8μm2 by 8 μm.
  • The cytoplasm becomes filled with a group of proteins called crystallins, and the organelles and nuclei undergo autophagy
  • Together, the ciliary body and suspensory fibers control the shape of the lens.
  • When smooth muscle of the ciliary body is in a relaxed state, the elasticity of the eyeball (sclera) puts tension on the suspensory fibers, which in turn stretches the lens radially, thereby making it thinner.
  • In this condition, the lens brings distant objects into focus on the retina.
  • When smooth muscle of the ciliary body contracts, the opening in which the lens is suspended becomes (slightly) smaller and tension on the suspensory fibers is relaxed.
  • The lens is then allowed to adopt its "resting" shape, which is more round.
  • In this condition, the lens brings nearby objects into focus on the retina.
  • Note that a stretched lens, for distant focus, is maintained by a relaxed muscle. Hence eyes at rest are focussed for distance vision
  • Continuous changes in the shape of the lens keep images focused on the retina.
  • (a) The lens flattens for distant vision when the ciliary muscles are relaxed and the shape of the ciliary body holds the ciliary zonule taut.
  • (b) To see closer objects, the ciliary muscle fibers contract, changing the shape of the ciliary body, relaxing tension on the ciliary zonule, and allowing the lens to assume the more rounded shape.
  • Accommodation defines the process by which the lens becomes rounder to focus the image of a nearby object on the retina and flattens when the image of a distant object is focused on the retina.
  • Three components contribute to the accommodation process:
    1. The ciliary muscle.
    2. The ciliary body.
    3. The suspensory ligaments, inserted at the equatorial region of the lens capsule.

Eyelids

  • Eyelids are pliable structures containing skin, muscle, and conjunctiva that protect the eyes.
  • The skin is loose and elastic, lacks fat, and has only very small hair follicles and fine hair, except at the distal edge, where large follicles with eyelashes are present.
  • Associated with the follicles of eyelashes are sebaceous glands and modified apocrine sweat glands
  • Associated with these hair follicles are small sebaceous glands and modified sweat glands.
  • The tarsal glands (TG) (aka Meibomian glands), with acini secreting into long central ducts (D) that empty at the free edge of the eyelids
  • Oils in the sebum produced by these tarsal glands, also called Meibomian glands, form a surface layer on the tear film, reducing its rate of evaporation, and help lubricate the ocular surface.
  • The eyelid is a pliable tissue with skin (S) covering its external surface and smooth conjunctiva (C) lining its inner surface.
  • At the outer rim of the eyelid are a series of large hair follicles (F) for the eyelashes.
  • Internally eyelids contain fascicles of striated muscle (M) comprising the orbicularis oculi muscle and closer to the conjunctiva a thick plate of fibroelas connective tissue called the tarsus (T).
  • This tarsal plate provid structural support for the eyelid and surrounds a series of larg sebaceous glands, the tarsal glands (TG) (aka Meibomian glands),

Vitreous Body

  • The vitreous humor is a transparent, colorless, gel-like substance located in the posterior chamber of the eye.
  • It helps maintain the round shape of the eye and can also help with vision clarity and shock absorbance.
  • With aging, the vitreous humor undergoes vitreous degeneration, acquiring a thinner liquid consistency.
  • The only cells in the vitreous body are a small mesenchymal population near the membrane called hyalocytes, which synthesize the hyaluronate and collagen, and a few macrophages
  • It consists of transparent, gellike connective tissue that is 99% water (vitreous humor), with collagen fibrils and hyaluronate, contained within an external lamina called the vitreous membrane

Tear Glands (Lacrimal Glands)

  • The tear glands located above each eyeball, called the lacrimal glands, continuously supply tear fluid that's wiped across the surface of your eye each time you blink your eyelids.
  • Excess fluid drains through the tear ducts into the nose
  • The lacrimal gland is a tubuloacinar gland located in the groove of the frontal bone, which is involved in synthesis and secretion of major tear proteins and other aqueous components of the trilayered tear film.
  • The lacrimal gland is a serous, compound tubuloacinar gland that resembles that parotid gland.
  • It secretes lacrimal fluid (tears) via ducts that open into the conjunctival sac at the lateral portion of the superior conjunctival fornix.
  • Tears are composed of water and lysozyme, an antibacterial agent.
  • The lacrimal gland is subdivided into lobules by connective tissue septa (Se).
  • Tears are produced by the acini (Ac) and enter the ducts (D), which them to the surface of the eyeball.
  • The lacrimal canaliculi drain the lacrimal fluid away from the surface of the eye.
  • They are lined by stratified, non-keratinized squamous epithelium.
  • The lacrimal sac is the dilated portion of the duct system.
  • It is lined by pseudostratified ciliated columnar epithelium and contains numerous goblet cells.

Retina

  • The retina represents the innermost layer of the eye.
  • The retina, derived from the inner and outer layers of the optic cup, is the innermost of the three concentric layers of the eye
  • It consists of two basic layers:
    • The neural retina or retina proper is the inner layer that contains the photoreceptor cells.
    • The RPE is the outer layer that rests on and is firmly attached through the Bruch’s membrane to the choriocapillary layer of the choroid
  • Radial branches from blood vessels (arteries and veins), located on the retinal surface, are interconnected by capillary beds present in the inner layers of the retina.
  • Retinal capillary beds are lined by endothelial cells linked by tight junctions creating an internal blood-retinal barrier.
  • An external retinal barrier is formed by tight junctions linking the cells of the pigmented epithelium.

Retina Highlights

  • The retina derives from the neuroectoderm and represents an extension of the brain.
  • The retina is a stratified layer of nervous cells formed by two layers:
    • The outer retinal pigmented epithelium
    • The inner sensory retina.
  • The nonsensory retinal pigmented epithelium is a simple cuboidal epithelium with melanin granules.
  • The sensory retina spans from the margin of the optic disk posteriorly to the ciliary epithelium anteriorly.
  • The optic disk includes the optic papilla, formed by protruding nerve fibers passing from the retina into the optic nerve.
  • The optic papilla lacks photoreceptors and represents the blind spot of the retina.
  • The fovea centralis is the area of sharpest vision.

Retinal Pigment Epithelium (RPE)

  • The RPE is a single layer of cuboidal cells about 14μm14 μm wide and 10 to 14μm14 μm tall.
  • The cells rest on Bruch’s membrane of the choroid layer. The pigment cells are tallest in the fovea and adjacent regions, which accounts for the darker color of this region.
  • Adjacent RPE cells are connected by a junctional complex consisting of gap junctions and elaborate zonulae occludentes and adherentes.
  • This junctional complex is the site of the blood–retina barrier

RPE Functions

  • It absorbs light passing through the neural retina to prevent reflection and resultant glare.
  • It isolates the retinal cells from blood-borne substances.
  • It serves as a major component of the blood–retina barrier via tight junctions between RPE cells.
  • It participates in restoring photosensitivity to visual pigments that were dissociated in response to light. The metabolic apparatus for visual pigment resynthesis is present in the RPE cells.
  • It phagocytoses and disposes of membranous discs from the rods and cones of the retinal photoreceptor cells.
  • The site where the optic nerve joins the retina is called the optic disc or optic papilla .
  • Because the optic disc is devoid of photoreceptor cells, it is a blind spot in the visual field
  • The fovea centralis is a shallow depression located about 2.5mm2.5 mm lateral to the optic disc. .
  • It is the area of greatest visual acuity. The visual axis of the eye passes through the fovea.
  • A yellow-pigmented zone called the macula lutea surrounds the fovea
  • The retina is the thick layer of the eye immediately inside the choroid.

Retina: Arteries and Veins

  • The central retinal artery and vein pass through the optic nerve and enter the eye at the optic disc where they divide to form smaller lateral branches in the retina’s nerve fiber layer.
  • Capillaries extend as deep as the inner nuclear layer. (Nutrients and O2O_2 for the outer retinal layers diffuse from capillaries in the choroid.)
  • The fovea (F) is a small specialized area of the retina where cell bodies and axons of the ganglionic and inner layer are largely dispersed peripherally, thinning this retinal area and allowing light to hit the cones with very little light scattering.
  • The fovea contains no rods and a greatly increased density of cones, causing the layer with their cell bodies to be slightly thicker here than elsewhere.
  • The exchange between the retina and the choroid occurs mostly at the choriocapillaris
  • Capillaries are characteristically large in diameter; avascular septae or pillars separate them.
  • Perhaps one of the most distinguishing features of choriocapillaris vessels is their fenestration; pores line their inner endothelium.
  • The choriocapillaris attaches internally to Bruch’s membrane, which forms a complex five-layered structure between the choroid and the retinal pigment epithelium.
  • Because choriocapillaris vessels are fenestrated, they are highly permeable to many low-molecular weight substances such as glucose. This helps maintain a high concentration at the interface between the choroid and the retina and facilitates transport to and across the RPE.
  • The choriocapillaris also participates in maintaining an adequate oncotic pressure in the extravascular choroidal tissue, which is key to ensuring proper fluid movement and drainage.
  • These processes include the supply of passive compounds, such as oxygen, to the photoreceptors, the maintenance of fluid movement in the back of the eye and the clearance of various retinal metabolic wastes.
  • So the RPE forms a tight-junction epithelium located between the blood flow of the choroid and the outer segments of the photoreceptors.
  • There are tight junction between the lateral surfaces of the hexagonal epithelial sheet In this way the RPE forms a part of the blood/retina barrier.
  • The barrier function by a tight epithelium implies an efficient isolation of the inner retina from systemic influences at the choroidal side.
  • This epithelial transport serves to supply nutrients to the photoreceptors (transport from the blood to the retinal side), control the ion homeostasis in the subretinal space and to eliminate water and metabolites from retinal tissue (transport from the retinal to the blood side).
  • The apical ends of the cells extend processes and sheath-like projections that surround the tips of the photoreceptors.
  • Melanin granules are numerous in these extensions and in the apical cytoplasm .
  • This cellular region also contains numerous phagocytic vacuoles and secondary lysosomes, peroxisomes, and abundant smooth ER (SER) specialized for retinal (vitamin A) isomerization.
  • The site where the optic nerve leaves the eyeball is called the optic disc (OD).
  • It is characteristically marked by a depression, evident here.
  • Receptor cells are not present at the optic disc, and because it is not sensitive to light stimulation, it is sometimes referred to as the blind spot.
  • The fibers that give rise to the optic nerve originate in the retina, more specifically, in the ganglion cell layer.
  • They traverse the sclera through a number of openings (arrows) to form the optic nerve (ON).
  • The region of the sclera that contains these openings is called the lamina cribrosa (LC) or cribriform plate.
  • The optic nerve contains a central artery and vein (not seen here) that also traverse the lamina cribrosa.
  • Branches of these blood vessels (BV) supply the inner portion of the retina
  • In addition to the photoreceptors, there are four other cell types in the retina.
  • The photoreceptors synapse on bipolar cells, and the bipolar cells synapse on the ganglion cells.
  • Horizontal and amacrine cells allow for communication laterally between the neurons.

Organization of the Retina

Ten layers of the retina:

  1. Internal limiting membrane
  2. Ganglion cell axons
  3. Ganglion cell bodies
  4. Inner synaptic layer
  5. Inner nuclear layer (interneurons)
  6. Outer synaptic layer
  7. Outer nuclear layer (photoreceptor cell bodies)
  8. Outer limiting membrane
  9. Rod and cone photoreceptor outer segments
  10. Retinal pigment epithelium
  • There are two types of photoreceptors distributed unevenly across the retina: rods and cones.
  • Rods are very sensitive cells specialized for night vision.
  • In bright light conditions the response of the rods is saturated and cones, faster but less sensitive photoreceptors, mediate day vision
  • The rod:cone ratio is lowest in the foveal region and higher in the periphery.
  • There are three types of cones, each one of them responding best to different wavelengths (short, middle, and long).
  • Their combined responses generate color vision.
  • The outer segment is a modified primary cilium, photosensitive and shaped like a short rod; the inner segment contains glycogen, mitochondria, and polyribosomes for the cell’s biosynthetic activity
  • The rod-shaped segment consists mainly of 600 to 1000 flattened membranous discs stacked like coins and surrounded by the plasma membrane
  • Proteins on the cytoplasmic surface of each disc include abundant rhodopsin (or visual purple)

Cone Cells

  • Less numerous and less light-sensitive than rods
  • produce color vision in adequately bright light The human retina contains on average 92 million rod cells.
  • They are extremely sensitive to light, responding to a single photon, and allow some vision even with light low levels, such as at dusk or nighttime
  • Membranous discs are continuous with the cell membrane
  • Horizontal cells are the laterally interconnecting neurons having cell bodies in the inner nuclear layer of the retina of vertebrate eyes.
  • They help integrate and regulate the input from multiple photoreceptor cells.
  • Among their functions,