St. George's University Histology of the Eye PDF

All year-1 course materials, whether in print or online, are protected by copyright. The work, or parts of it, may not be copied, distributed, or published in any form, printed, electronic, or otherwise. As an exception, students enrolled in year 1 of St. George’s University School of Medicine and t...

All year-1 course materials, whether in print or online, are protected by copyright. The work, or parts of it, may not be copied, distributed, or published in any form, printed, electronic, or otherwise. As an exception, students enrolled in year 1 of St. George’s University School of Medicine and their faculty are permitted to make electronic or print copies of all downloadable files for personal and classroom use only, provided that no alterations to the documents are made and that the copyright statement is maintained in all copies. ‘View only’ files, such as lecture recordings, are explicitly excluded from download, and creating copies of these recordings by students and other users is strictly illegal. The author of this document has made the best effort to observe current copyright law and the copyright policy of St. George's University. Users of this document identifying potential violations of these regulations are asked to bring their concerns to the attention of the author. Histology – A Text and Atlas Pawlina 9th Edition Chapter 24: Eye Eye | Histology: A Text and Atlas: With Correlated Cell and Molecular Biology, 9e | Premium Basic Sciences | Health Library (lwwhealthlibrary.com) Your Objectives will show here! SOM.MK.I.BPM2.1.NB.1.HCB.1177 Describe the microscopic structure and appearance, and functions of the corneoscleral coat (sclera, cornea) SOM.MK.I.BPM2.1.NB.1.HCB.1178 Describe the microscopic structure and appearance, and function of the components of the uveal tract (choroid, ciliary body, iris) SOM.MK.I.BPM2.1.NB.1.HCB.1179 Describe the functional significance of choriocapillary layer of choroid SOM.MK.I.BPM2.1.NB.1.HCB.1180 Describe the mode of synthesis and drainage of aqueous humor SOM.MK.I.BPM2.1.NB.1.HCB.1181 Identify & describe the microscopic structural organization and blood supply of the retina SOM.MK.I.BPM2.1.NB.1.HCB.1182 Identify & describe the microscopic structure of optic disc and fovea centralis SOM.MK.I.BPM2.1.NB.1.HCB.1183 Identify & describe the microscopic structure of the lens and its function SOM.MK.I.BPM2.1.NB.1.HCB.1184 Identify & describe the microscopic structure of vitreous body and its function SOM.MK.I.BPM2.1.NB.1.HCB.1185 Describe the microscopic structure, location and clinical significance of the glands of the eyelids SOM.MK.I.BPM2.1.NB.1.HCB.1186 Describe the microscopic structure of the conjunctiva SOM.MK.I.BPM2.1.NB.1.HCB.1187 Describe the structural changes in the retina in retinal detachment SOM.MK.I.BPM2.1.NB.1.HCB.1188 Explain the effect of multiple sclerosis on the optic nerve SOM.MK.I.BPM2.1.NB.1.HCB.1189 Explain the clinical significance of corneal transplant SOM.MK.I.BPM2.1.NB.1.HCB.1190 Describe the changes in the lens in cataract SOM.MK.I.BPM2.1.NB.1.HCB.1191 Describe the changes in the eye that result in glaucoma SOM.MK.I.BPM2.1.NB.1.HCB.1177 SOM.MK.I.BPM2.1.NB.1.HCB.1178 SOM.MK.I.BPM2.1.NB.1.HCB.1181 General structure Chambers of the eye Layers of the eye Microscopic structure of the eye Lens Vitreous body Special regions of the retina Accessory structures of the eye AC, anterior chamber CB, ciliary body FC, fovea centralis L, lens OS, ora serrata R, retina V, vitreous cavity C, cornea Ch, choroid I, iris ON, optic nerve PC, posterior chamber S, sclera arrows, muscle insertions SOM.MK.I.BPM2.1.NB.1.HCB.1177 SOM.MK.I.BPM2.1.NB.1.HCB.1178 Ora serrata 3 layers or coats 1. Corneoscleral coat Outer fibrous layer Cornea (transparent portion) Sclera (white portion) 2. Vascular coat Middle layer or uvea Iris Ciliary body Choroid 3. Retina Inner layer Inner neural retina (‘Retina Proper’) Outer pigment epithelium SOM.MK.I.BPM2.1.NB.1.HCB.1181 SOM.MK.I.BPM2.1.NB.1.HCB.1177 SOM.MK.I.BPM2.1.NB.1.HCB.1178 SOM.MK.I.BPM2.1.NB.1.HCB.1181 Posterior Layers of the Eye Sclera Anterior Layers of the Eye Choroid Retina cornea lens SOM.MK.I.BPM2.1.NB.1.HCB.1177 Cornea Anterior surface of the eye Transparent – pigmented iris is visible behind it Diameter of the cornea is ~11 mm Thickness ranges from ~0.5 mm centrally to ~1mm along the margins of the cornea. Richest sensory nerve supply Function Refraction of light Avascular Blood vessels are not normally found in the cornea Limbus Cornea Sclera SOM.MK.I.BPM2.1.NB.1.HCB.1177 H&E x260 1 2 5 layers 1.Corneal epithelium 2.Bowman’s membrane 3.Corneal stroma 4.Descemet’s membrane 5.Corneal endothelium 3 4 5 H&E x140 H&E x280 SOM.MK.I.BPM2.1.NB.1.HCB.1177 1. Corneal epithelium Stratified squamous epithelium (~5 layers) Continuous with conjunctival epithelium Basal cells have mitotic activity Stem cells reside at corneoscleral limbus Regeneration ~7 days Numerous free nerve endings 2. Bowman’s membrane Homogenous layer on which epithelium sits Random orientation of collagen fibers Provides strength and acts as barrier to infection Does not regenerate H&E x280 Corneal stroma 90% corneal thickness 60 lamellae, each made of collagen fibrils Oriented at right angles to each other Corneal fibroblasts (keratocytes) – flattened between the lamellae Corneal proteoglycans Keratan sulfate (lumican) Chondroitin sulphate Uniform spacing of fibrils, lamellae and orthogonal array of lamellae responsible for transparency TEM x16,700 3. SOM.MK.I.BPM2.1.NB.1.HCB.1177 4. Descemet’s membrane Thick basal lamina of the corneal endothelial cells Interwoven meshwork of collagen fibers and pores Can regenerate Thickens with age 5. Corneal endothelium Metabolic exchange between cornea & aqueous humor Regulates water content (& transparency) of the stroma Numerous Na+ / K+ ATPase pumps Simple squamous epithelium Limited proliferation Clinical correlation: Corneal dystrophy & corneal transplant H&E x280 SOM.MK.I.BPM2.1.NB.1.HCB.1189 Indications Keratitis Trauma Keratoconus Corneal degeneration Types of transplant Penetrating keratoplasty Lamellar keratoplasty Endothelial keratoplasty Advantages Cornea is avascular Minimal immunological response or graft rejection H&E x140 SOM.MK.I.BPM2.1.NB.1.HCB.1177 Sclera Thick fibrous layer Dense connective tissue Flat collagen bundles interspersed with fine networks of elastic fibers Lamina cribosa (LC) Posterior openings of the sclera (arrows) Allows for entry/exit of nerves and vessels S S P S- Sclera D- Duramater P- Piamater D P SOM.MK.I.BPM2.1.NB.1.HCB.1177 3 ill-defined layers 1. External Episcleral layer Extraocular muscle 2. Middle layer/Substantia propria (Tenon’s capsule) 3. Inner Suprachoroidal lamina Retina Choroid Sclera SOM.MK.I.BPM2.1.NB.1.HCB.1177 Transition zone between cornea & sclera (black arrow on bottom image) Bowman’s membrane ends Epithelium thickens to from ~5 layers of the cornea (C) to ~10-12 layers of the conjunctiva (Cj) The basal layer contains corneolimbal stem cells H&E x45 Generate & maintain corneal epithelium Apparatus for the outflow of aqueous humor 1. Trabecular meshwork (spaces of Fontana) 2. Canal of Schlemm (CS) CEp- Corneal epithelium Cj Conjunctiva CjEp- Conjunctival epithelium C- Cornea S- Sclera AC- Anterior chamber PC- Posterior chamber I- Iris CB- Ciliary body CS- Canal of Schlemm H&E x130 SOM.MK.I.BPM2.1.NB.1.HCB.1178 Features: CT, blood vessels and pigmented cells Components: A. Iris Anterior part of the vascular coat Forms a contractile diaphragm in front of lens Pupil (central opening) B. Ciliary body Thickened anterior portion between the iris & choroid Ciliary processes Ciliary muscle Iris Vascular Ciliary Body coat C. Choroid Posterior portion Lies between sclera and retina Choroid SOM.MK.I.BPM2.1.NB.1.HCB.1178 Arises from anterior border of ciliary body Contractile diaphragm Dilator pupillae Sympathetic Sphincter pupillae Parasympathetic Pupil is the central aperture Separates anterior & posterior chambers of the eye Anterior surface Discontinuous layer of fibroblasts and melanocytes Highly vascular CT stroma Posterior surface Double layer of epithelium Posterior pigment epithelium Anterior pigment myoepithelium Processes form the dilator pupillae x570 SOM.MK.I.BPM2.1.NB.1.HCB.1178 Autonomic reflexes regulate pupil diameter Bright light – Parasympathetic neurons stimulate circular muscles (sphincter pupillae) – Decrease in size (constriction) Dim light – Sympathetic neurons stimulate radial muscles (dilator pupillae) – Increase in size (dilation) SOM.MK.I.BPM2.1.NB.1.HCB.1178 x120 Ciliary muscle Smooth muscle Fibers spread out in several directions Contraction of muscle Longitudinal, radial & circular Reduces tension of zonular fibers causing lens to get more round for near vision Relaxation of muscle Parasympathetic x480 Increases tension of zonular fibers causing lens to flatten Ciliary Processes ~75 radial ridge-like extensions Vascular Ciliary body & processes are covered by a double layer of columnar epithelium 2. 3. Secretion Tight junctions Zonular fibers primarily composed of fibrillin 3 major functions Produce and anchor zonular fibers that form the suspensory ligament of the lens Secretion of aqueous humor Part of blood-aqueous barrier 1. Outer pigmented layer Inner nonpigmented layer Clinical correlation: Marfan syndrome & dislocation of lens SOM.1ai.BPM2.1.NB.1.HCB.SS.0307 SOM.MK.I.BPM2.1.NB.1.HCB.1178 Distant Object (> 20 feet) Ciliary Muscle Zonule Fibers Lens Relaxed Flattened Tightened Close Object (< 20 feet) Ciliary Muscle Zonule Fibers Lens Contracted Rounded Relaxed SOM.MK.I.BPM2.1.NB.1.HCB.1178 Pigmented, dark brown vascular layer Functions: Blood supply Absorption of scattered light 2 layers 1. Choriocapillary layer Inner vascular layer Fenestrated capillaries 2. Bruch’s membrane (lamina vitrea) Amorphous hyaline membrane between the choriocapillary layer and retinal pigment epithelium (RPE) Attaches RPE to choriocapillary layer Sheet containing 5 layers: H&E x325 SOM.MK.I.BPM2.1.NB.1.HCB.1184 1. Anterior chamber Space between cornea & iris 2. Posterior chamber Space between the posterior surface of the iris & anterior surface of the lens 3. Vitreous chamber Space between the posterior surface of the lens & the neural retina Segments Anterior Posterior SOM.MK.I.BPM2.1.NB.1.HCB.1180 Lens A C AC PC Posterior Chamber Anterior Chamber PC Cornea CS CP Sclera CP- Ciliary processes CS- Canal of Schlemm SOM.MK.I.BPM2.1.NB.1.HCB.1191 Increased intraocular pressure Symptoms: Headache Eye pain Loss of vision Vomiting “Halo” around light Signs: Peripheral field of vision is decreased Redness Raised intra ocular pressure (tonometry) SOM.MK.I.BPM2.1.NB.1.HCB.1191 Sudden blockage can lead to increase in the fluid in the eye. This causes eye pain and optic nerve compression, which might lead to permanent visual loss. Tonometer –used to measure IOP SOM.MK.I.BPM2.1.NB.1.HCB.1184 Posterior segment of the eye Transparent jelly-like substance Homogenous gel Collagen fibrils GAGs - especially hyaluronan Spindle shaped cells -> hyalocytes Attached to surrounding structures Maintains shape of eye & helps keep retina in place Clinical correlation: Composition can change with age resulting in vitreous detachment & retinal detachment Vitreous body SOM.MK.I.BPM2.1.NB.1.HCB.1181 Consists of 2 Basic layers 1. Neural retinal (retina proper) from inner layer of optic cup Non-photosensitive region anterior to ora serrata Photosensitive region posterior to ora serrata 2. Retinal pigment epithelium (RPE) Outer layer of optic cup Pigmented epithelium Retina Neural retina Photosensitive part x440 x440 SOM.MK.I.BPM2.1.NB.1.HCB.1181 Layers of the Retina 10. Inner limiting membrane (inner limit of Muller cells) 9.Optic nerve fiber layer (axons of ganglion cells) 8. Ganglion cell layer 7. Inner plexiform layer (synapses of bipolar cells & ganglion cells) Limit of Muller’s cells 6. Inner nuclear layer (cell bodies of bipolar, horizontal, amacrine & Muller’s cells) 5. Outer plexiform layer (synapses of photoreceptors & bipolar cells) 4. Outer nuclear layer (inner segments of rods & cones) 3. Outer limiting membrane (outer limit of Muller cells) 2. Outer segments of rods & cones Retinal detachment occurs here! 1. Pigment epithelial layer SOM.MK.I.BPM2.1.NB.1.HCB.1181 Rods ~120 million Cylindrical outer segment Visual pigment = rhodopsin Activated when low levels of light are encountered such as night-time or dusk. Black & white vision Highest density outside the fovea centralis Clinical correlation: Night blindness Rod Cone Cones ~7 million Thicker and shorter Conical outer segment Visual pigment = iodopsin Sensitive only to light – better visual acuity Color vision Largest density of cones is in the fovea centralis Clinical correlation: Color blindness X32,000 29 SOM.MK.I.BPM2.1.NB.1.HCB.1181 Single layer of cuboidal cells Junctional complexes Tight junctions, adherens junctions and gap junctions Blood-retina barrier Basal lamina attached to Bruch’s membrane Functions: Absorption of light Major component of blood-retina barrier via tight junctions Active in visual pigment pathways Vitamin A metabolism Phagocytosis of membranous discs from rod & cone photoreceptors X20,000 Clinical correlation: Link to retinal detachment SOM.1ai.BPM2.1.NB.1.HCB.SS.0310 SOM.MK.I.BPM2.1.NB.1.HCB.1187 Site of detachment: Most common detachment occurs at the junction of pigment epithelial layer and the rods & cones layer. Some causes: Following cataract surgery Trauma Extreme dehydration Retinal tears Common site of retinal detachment SOM.MK.I.BPM2.1.NB.1.HCB.1181 SOM.1ai.BPM2.1.NB.1.HCB.SS.0310 Branches of central retinal arteries 10 Supplied by central retinal arteries 9 8 7 6 Choroidal vessels Posterior ciliary arteries Central retinal arteries (end arteries) 5 Diffusion from Choroidal capillaries 3 4 2 1 Choroidal capillaries Retina H&E courtesy, K Wright, Loma Linda University SOM.MK.I.BPM2.1.NB.1.HCB.1182 Optic disc/papilla Site where optic nerve emerges from the retina Aka “blind spot” →no rods and cones. Clinical correlate: Optic neuritis in Multiple sclerosis Macula lutea – Center of the posterior retina at the visual axis of the eye – Clinical correlate: Age-related macular degeneration Fovea centralis – – – – Small depression in the center of the macula lutea Area of highest visual acuity (specialized for discrimination of details and color vision) Contains only cones Foveola (central region of the fovea) Fovea centralis Foveola SOM.MK.I.BPM2.1.NB.1.HCB.1183 The lens is a biconvex structure Avascular Held in place by a radially oriented group of zonular fibers 3 principal components 1. Lens capsule: Thick basal lamina of 10 – 20 µm Produced by anterior lens cells 2. Subcapsular epithelium: Single layer of cuboidal epithelial cells Found only on the anterior surface of the lens Epithelial cells proliferate at the germinal zone at the lens equator & migrate 3. Lens fibers Thin flattened structures Lose organelles and accumulate crystallins (proteins) Formed from the epithelial cells at the germinal zone Clinical correlations: Cataract & presbyopia x570 SOM.MK.I.BPM2.1.NB.1.HCB.1190 Loss of transparency of the lens. The lens fibers are filled with proteins called crystallins. Development linked to cross-linking of proteins resulting in a loss of transparency & cloudy appearance. Other causes include metabolic conditions, trauma, genetic conditions Cataracts can severely impair vision. Lens can be replaced with an artificial lens. Artificial lens implantation SOM.MK.I.BPM2.1.NB.1.HCB.1185 Conjunctiva Stratified epithelium with goblet cells Varies depending on location Lines the inside of the eyelid (palpebral) and the visible part of the sclera (bulbar) Secretes fluids to lubricate the eye Clinical correlation: Conjunctivitis x60 Glands of the eyelid Meibomian glands Glands of Zeiss Sebaceous within tarsal plate Lubricates – delays drying of tears Clinical correlation: Chalazion Sebaceous glands associated with eyelashes Clinical correlation: Stye (hordeolum) Glands of Moll Sweat glands at the lid margins x20 36 SOM.MK.I.BPM2.1.NB.1.HCB.1185 Bulbar conjunctiva 1.Chalazion 3. Conjunctivitis Palpebral conjunctiva 2. Stye