Eye Anatomy PDF

31 Jeyad Alkhzouz Jeyad Alkhzouz Ghada Abu El Ghanam Eye - Highly developed photosensitive organs for analyzing the form, intensity, and color of light reflected from objects and provide a sense of sight. - Protected with the orbits of the skull which also contain adipose cushions...

31 Jeyad Alkhzouz Jeyad Alkhzouz Ghada Abu El Ghanam Eye - Highly developed photosensitive organs for analyzing the form, intensity, and color of light reflected from objects and provide a sense of sight. - Protected with the orbits of the skull which also contain adipose cushions, each eyeball consists of three concentric layers: 1. A tough external fibrous layer (maintains the overall shape) which consists of the: ▪ Sclera ▪ And the transparent cornea 2. A middle vascular layer that includes: ▪ The choroid ▪ Ciliary body ▪ And Iris 3. And an inner sensory layer, the retina, which communicates with the cerebrum through the posterior optic nerve - The lens is not part of these layers. It is a 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. - In an anterior portion of the eye, the Iris and lens are bathed in a clear aqueous humor that fills both: 1. The anterior chamber (between the cornea and iris) and 2. The posterior chamber (between the iris and lens) - This aqueous humor flows through the pupil and connects these two chambers - The posterior vitreous chamber, surrounded by the retina, lies behind the lens and its zonular fibers contain a large gelatinous mass of transparent CT called vitreous body Fibrous layer - This layer includes two major regions: 1. Posterior sclera 2. Anterior cornea - Both of which joined at the limbus Sclera - The fibrous, external layer of the eyeball protects the more delicate internal structures and provides sites for muscle insertion - The white posterior five-sixth of this layer is the sclera, which encloses a portion of the eyeball - It’s 0.5mm in thickness and consists mainly of dense CT with flat bundles of type 1 collagen. Microvasculature is present near the outer surface - Tendons of the extraocular muscles, which move the eyes insert into the anterior region of the sclera. - Posteriorly, the sclera thickens and joins the epineurium covering the optic nerve. - Where it surrounds the choroid, the sclera includes an inner suprachoroidal lamina, with less collagen and more fibroblasts, elastic fibers, and melanocytes. Cornea - In contrast to the sclera, the anterior 1-6th of the eye – the cornea – is transparent and completely avascular - A section of the cornea shows five distinct layers: 1. An external Stratified squamous epithelium: a. Nonkeratinized, five or six cell layers thick, and makes up about 10% of the corneal thickness 2. An anterior limiting membrane (bowman membrane) which is the basement membrane of the external stratified epithelium a. Very thick b. Helps protect against infection of the underlying stroma 3. The thick stroma (substantia propria) a. Makes up 90% of the cornea’s thickness  b. Made up of 60 layers of parallel collagen bundles aligned at approximately right angles to each other and extending the full diameter of the cornea c. Contributes to the transparency of this avascular tissue d. Between collagen lamellae are cytoplasmic extensions of flattened fibroblast-like cells called keratocytes. 4. A posterior limiting membrane (Descemet’s membrane), which is the basement membrane of the endothelium. 5. An inner simple squamous endothelium - Basal cells have a high proliferative capacity, which is important for renewal and repair of the corneal surface and emerge from stem cells in the corneoscleral limbus that encircles the cornea 1. The limbus encircles the cornea, which is a transitional area where the transparent cornea merges with the opaque sclera 2. 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). 3. As mentioned, the epithelial stem cells located at the limbus surface gives rise to rapidly dividing progenitor cells that move centripetally into the corneal epithelium. ▪ Note: an image of the limbus is in page 15 - Flattened surface cells have microvilli protruding into a protective tear film of: 1. Lipid 2. Glycoproteins 3. And Water Vascular layer (Uvea) - The eye’s more vascular middle layer. Consists of three parts (from posterior to anterior): 1. The Choroid 2. The Ciliary body 3. The Iris The Choroid - Located in the posterior two thirds of the eye - Consists of loose CT and contains numerous melanocytes 1. These form the characteristic black layer in the choroid and prevents light from entering the eye except through the pupil. - Two layers make up the choroid: 1. The Choroidocapillary lamina. 2. Bruch membrane Ciliary body - The anterior expansion of the Uvea that encircles the lens - Lies posterior to the limbus - Most of ciliary body rests on the sclera - Associated structures include: 1. Ciliary muscles: make up most of the ciliary body’s stroma and consists of three groups of smooth muscle fibers a. Contraction of these muscles affects the shape of the lens and is important in visual accommodation 2. Ciliary processes: radially arranged series of about 75 ridges extending from the inner highly vascular region of the ciliary body a. These provide a large surface area covered by a double layer of low columnar epithelial cells, the ciliary epithelium  b. The epithelial cells directly covering the stroma contain much melanin and correspond to the anterior projection of the pigmented retina epithelium. i. Surface area of cells lacks melanin and is continuous with the sensory layer of the retina c. The cells of this dual epithelium have extensive basolateral folds with Na+/K+ ATPase activity and are specialized for secretion of aqueous humor. i. So, 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 iris into the channels of the trabecular meshwork and the scleral venous sinus 3. Ciliary zonule: a system of many radially oriented fibers composed largely of fibrilin-1 and 2. a. Produced by the nonpigmented epithelial cells on the ciliary processes. b. These fibers extend from grooves between the ciliary processes and attach to the surface of the lens, holding that structure in place Iris - The iris is the most anterior extension of the middle uveal layer, which covers part of the lens, leaving a round central pupil. - The anterior surface of the Iris, exposed to aqueous humor in the anterior chamber, consists of a dense layer of fibroblasts and melanocytes with interdigitating processes, and lacks epithelial covering - Deeper in the iris, the stroma consists of loos CT with melanocytes and sparse microvasculature. - Posterior surface of the iris has a two-layered epithelium continuous with that covering the ciliary processes, but heavily filled with melanin - The highly pigmented posterior epithelium of the iris blocks all light from entering the eye except that passing through the pupil. - Myoepithelial cells form a partially pigmented epithelial layer and extend contractile processes radially as the very thin dilator pupillae muscle. - Smooth muscle fibers form a circular bundle near the pupil as the sphincter pupillae muscle. - The dilator (sympathetic) and sphincter muscles (parasympathetic) of the iris have sympathetic and parasympathetic innervation 1. Dilator (sympathetic): enlarging the pupil 2. Sphincter muscles (parasympathetic): constricting the pupil - Melanocytes of the iris stroma provide the color of one’s eyes Other structures: Lens - The lens is a transparent biconvex structure suspended immediately behind the iris, which focuses light on the retina - Derived from an invagination of the embryonic surface ectoderm, it is a unique avascular tissue and is highly elastic, a property that normally decreases with age. - Lens has three principal components: 1. A thick homogenous lens capsule composed pf proteoglycans and type IV collage surrounding the lens and provides attachment for the fibers of the ciliary zonule 2. A subcapsular lens epithelium consisting of a single layer of cuboidal cells present only on the anterior surface of the lens ▪ Epithelial cells attach basally to the surrounding lens capsule and their apical surfaces bind to the internal lens fibers 3. Lens fibers are highly elongated, terminally differentiated cells that appear as thin, flattened structures ▪ The cytoplasm typically becomes filled with a group of proteins called crystallin, and the organelles and nuclei undergo autophagy. ▪ Lens fibers are packed tightly together and form a perfectly transparent tissue highly specialized for light refraction - Lens is held in place by fibers of the ciliary zonule, which extend from the lens capsule to the ciliary body. 1. Together with the ciliary muscles, this structure allows the process of visual  accommodation, which permits focusing on near and far objects by changing the curvature of the lens 2. When eye is at rest or gazing at distant objects, ciliary muscles relax and resulting shape of ciliary body puts tension on the zonule fibers, which pulls the lens into a flatter shape 3. To focus on a close object, the ciliary muscles contract, causing forward displacement of the ciliary body, which relieves some of the tension on the zonule and allows the lens to return to a more rounded shape and keep the object in focus (figure 23-12) Vitreous body - The vitreous body occupies the large vitreous chamber behind the lens - It consists of a transparent gel-like CT that is 99% water (vitreous humor), with collagen fibrils and hyaluronate, contained within an external lamina called the vitreous membrane. - The only cells there are a small mesenchymal population near the membrane called hyalocytes, which synthesize they hyaluronate and collagen, and a few macrophages Retina - The innermost tunic of the eye develops with two fundamental sublayers from the inner and outer layers of the embryonic optic cup: 1. The outer pigmented layer is a simple cuboidal epithelium attached to Bruch’s membrane and the choroido-capillary lamina of the choroid ▪ This heavily pigmented layer forms the outer part of the dual epithelium covering the ciliary body and posterior iris. 2. The inner retinal region, the neural layer, is thick and stratified with various neurons and photoreceptors.  ▪ Although Its neural structure and visual function extend anterior only as far as the Ora serrata, this layer continues as part of the dual cuboidal epithelium that covers the surface of the ciliary body and posterior iris. Retina pigmented epithelium (outer pigmented layer) - It consists of simple cuboidal or low columnar cells with basal nuclei and surrounds the neural layer of the retina - The cells have a well-developed junctional complexes, gap junctions, and numerous invaginations of he basal membrane. - 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. - Functions of retinal pigmented epithelium include: 1. Absorb scattered light that passes through neural layer 2. Retinal regeneration 3. Phagocytosis and removal of free radicals for protection 4. And more Neural retina - Functions as an outpost of the CNS with Glia and several interconnected neuronal subtypes in well-organized strata. - 9 distinct layers comprise the neural retina, with three major layers containing the nuclei of the interconnected neurons: 1. Outer nuclear layer (ONL): near the pigmented epithelium. Contains cell bodies of photoreceptors 2. The inner nuclear layer (INL): contains the nuclei of various neurons, notably the bipolar cells, amacrine cells, and horizontal cells (integrate signals from rods and cones over a wide area of the retina) 3. Near the vitreous, the ganglionic layer (GL) has ganglion cells with much longer axons (axons make up the nerve fiber layer “NFL”).  - Between the three layers with cell nuclei are two fibrous or “plexiform” regions containing only axons and dendrites connected by synapses: 3. Outer plexiform layer (OPL) 4. Inner plexiform layer (IPL) - Rods and cone cells are located in the: 5. Rod and Cone layer (RCL) - Further, supporting cells called Muller cells are organized into two boundaries that appear as very thin retinal layers: 6. The outer limiting layer (OLL) 7. The inner limiting membrane (ILM) - The last two layers include: 8. Nerve fiber layer (NFL) 9. Non-neural pigmented layer Rods and Cone cells: - Rods: o Human retina has on average 92 million rod cells o Extremely sensitive to light, and allow some vision even at low light levels o Very thin, elongated cells, composed of two functionally distinct segments o Proteins on the cytoplasmic surface of each disc include abundant rhodopsin (or visual purple) which is bleached by light and initiates visual stimulus - Cone cells: o Less numerous and less light-sensitive o 4.6 million of them in the typical human retina produce color vision in adequately bright light o Three morphologically similar classes of cones, each containing one type of visual pigment Iodopsin (or photopsins) o Each of the iodopsin has maximal sensitivity to light of different wavelengths, in the red, blue, or green regions of the visible spectrum, respectively Specialized areas of the retina - The optic disc (blind spot) occurs at the posterior of the retina, where the axons in the retina’s nerve fiber layer (NFL)converge at the optic nerve head and leave the eye as the optic nerve o Lacks photoreceptors and other retinal layers except the NFL from ganglion cells - Near the optic disc and directly opposite the pupil lies a specialized retinal area called the fovea centralis o Here, visual acuity or sharpness is maximal o It is a shallow depression within the retina where cell bodies of the ganglionic and inner nuclear layers are dispersed peripherally leaving primarily cone cells. o Devoid of most conducting neurons as well as capillaries, the fovea allows the light to fall directly on its cones with very little light scatter. Accessory structures of the eye Conjunctiva - A thin, transparent mucosa that covers the exposed anterior portion of the sclera and continues as the lining on the inner surface of the eyelids. - Consists of stratified columnar epithelium with numerous small goblet cells supported by a thin lamina propria of loose vascular conjunctiva tissue - Mucous secretions from conjunctiva cells are added to the tear film that coats the epithelium and the cornea. - Conjunctivitis: o Eyelids - Pliable structures containing skin, muscle, and conjunctiva that protects the eyes. - The skin is loose and elastic, lacks fat, and only has 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 - Adjacent to the conjunctiva is a dense fibroelastic pate called the tarsus that supports the other tissues o The tarsus surrounds a series of 20-25 large sebaceous glands, each with many acini secreting into a long central duct that opens among the eyelashes o Oils in the sebum are produced by these tarsal glands, also called Meibomian glands, forma surface layer on the tear film, reducing its rate of evaporation, and helping lubricate the ocular surface. Lacrimal glands - Produce fluid continuously for the tear film that moistens and lubricates the cornea and conjunctiva and supplies O2 to the corneal epithelial cells - The lacrimal glands have acini composed of large serous cells filled with lightly stained secretory granules and surrounded by well-developed myoepithelial cells and a sparse, vascular stroma A good summary for the layers of the eye