Eye Dr. Darren Hoffmann Lecture Objectives: After this lecture, you will be able to: -Differentiate the conjunctiva anatomically and histologically from other coats/layers of the eye. -Describe the muscles and nerves involved in opening and closing the eyes. -Explore a histological section of eyelid and identify all key structures. -Outline the pathway of tears from the lacrimal gland to the face or nasal cavity. -Discriminate the three principle layers of the eye, either at the anterior or posterior ends. -Describe the anatomical structures of the eye in histological terms. -Discuss how lens accommodation and pupillary constriction/dilation occur and which nerves mediate which processes. -Diagram the flow of aqueous humor and demonstrate how glaucoma can result from blockages. -Diagram the sequence of activation events that take place in the retina using a histological section as a visual aid Eye Anatomy and Histology - Hoffmann Page 1 Lecture Outline B1. The Eyelid and Associated Structures Conjunctiva A thin membrane covering the exposed surface of the eyeball (Bulbar) and the inner surface of the eyelids (Palpebral) Conjunctival sac- Space created between Bulbar conjunctiva and Palpebral conjunctiva if eyes are closed Conjunctiva Histology Stratified columnar epithelium with goblet cells (very unusual) Eyelid construction and muscles Each lid has a dense connective tissue band within called the Tarsus (Tarsal Plate) Closure of the eyelids Orbicularis Oculi Which cranial nerve supplies this? ______________ Soft Closure (resting eyes, blinking) Palpebral portion Hard Closure (squeezing eyes shut) Orbital portion Opening of eyelids 2 muscles with combined function to keep eyes open Levator Palpebrae Superioris m. Skeletal muscle (voluntary control) Innervated by CN III Tarsal Muscle (Mueller’s muscle) Smooth muscle (autonomic control) Innervated by Sympathetic nerves If either of the two nerves are injured, eyelid drooping (Ptosis) results Tarsal (Meibomian) glands Lipid-producing glands Prevent lids from sticking during blinking Hydrophobic barrier which keeps lacrimal fluid on eye Eye Anatomy and Histology - Hoffmann Page 2 Lacrimal Apparatus Lacrimal Gland Located in Superior-Lateral corner (posterior to conjunctiva) Lacrimal Ducts Convey tears from gland to conjunctival sac Tears build up in the medial angle of the eye – Lacrimal lake Lacrimal Punctum Tiny hole on medial end of each eyelid Leads into a thin duct called Lacrimal Canaliculus Lacrimal Canaliculi Convey tears into Lacrimal sac Nasolacrimal Duct Conveys tears from Lacrimal sac to Nasal cavity What leads to tear production - corneal irritation - emotion Lacrimal gland histology VERY similar to Parotid (contextual clues are the key to identification) Serous cells (basal-located nuclei in acini, heavy cytoplasmic staining) Acinar gland structure Eye Anatomy and Histology - Hoffmann Page 3 B2. General Organization of the Eyeball Coats of the Eyeball Outer Coat (Fibrous) Front: Cornea (Transparent layer) Back: Sclera (Opaque, white of eye) Middle coat (Vascular/Uvea) Front: Ciliary Body (ciliary muscle) Small ring of muscle which connects to lens via zonular fibers Iris Colored part of eye Controls amount of light to pass through lens Sphincter/Dilator pupillae muscles are contained within Back: Choroid Vascular layer between sclera and retina Inner coat (Retina) Front: Linings of Iris and Ciliary body Back: Retina (Photosensitive layer) Histology of the Eye Coats in the non-front parts of eye Fibrous coat: Sclera -Mostly collagen and elastin, very few cells Middle coat: Choroid -Lots of melanocytes, very dark staining, cells are not distinct Inner coat: Retina -Organized layers of cells Eye Anatomy and Histology - Hoffmann Page 4 B3. The Cornea, Lens & Humors 1. Cornea (high refractive power) Fibrous, but transparent 2. Aqueous humor (low refractive power) Watery substance produced and recycled constantly in the anterior part of the eye 3. Lens (high refractive power) Thick structure with clear cells 4. Vitreous humor (low refractive power) Gel substance posterior to the lens, present at birth and stays with you throughout life Cornea Histology Outer epithelium (stratified squamous, non-keratinized) Bowman’s membrane – Stability and structure to cornea Stroma – Thin collagen fibrils allow for light transmission Descemet’s membrane – Thinner layer Endothelium – Actively pumps water out of stroma into the anterior chamber (keeps cornea clear) Lens Histology Thin capsule Lens itself is cellular, composed of Lens Fibers running from front to back Cells mature from the perimeter inward Each cell runs the total width of lens (front to back) As cells mature, they destroy organelles (keeps lens clear) Cataracts: loss of lens transparency, caused by UV exposure Inability to see clearly Presbyopia: loss of lens mobility, flexibility due to condensation of lens fibers Inability to focus lens for near-objects Developing lens Mature lens Eye Anatomy and Histology - Hoffmann Page 5 Lens Accommodation Big Idea: Lens must change shape to refract light for different focal lengths (near vs. far vision) Ciliary body (muscle) is a ring of muscle around lens Under Parasympathetic Control (CN III) Connected to lens via Zonular fibers If Ciliary muscle is NOT contracted, the ring is large, which stretches the lens thin (Distant vision) If Ciliary muscle IS contracted, the ring is smaller, which allows the lens to relax and fatten (Near vision) B4. Uvea The uvea (vascular layer is composed of the ciliary body and processes and the iris in the front of the eye, continuous with the choroid in the back of the eye. Ciliary Body and Ciliary Processes Ciliary Body Smooth muscle Forms a circle around the lens (see above) Can be found at the corneoscleral junction Ciliary Processes Small projections Hold zonular fibers of the lens Lined by two layers of cell NOTE: these are from Retinal layer NOT uvea layer Inner layer: (‘Inner’ meaning toward inside of eye) Non-pigmented cells Continuous with the Retina These cells are source of Aqueous humor Outer layer: (‘Outer’ meaning toward outside of eye) Pigmented cells Continuous with Retinal pigmented epithelium Eye Anatomy and Histology - Hoffmann Page 6 Flow of Aqueous Humor Canal of Schlemm Circumferential canal in the Sclera at the corneoscleral junction Leads to Trabecular meshwork in sclera Here, Aqueous humor percolates into the tissue of the Sclera to be picked up by Scleral veins and returned to blood system Glaucoma: Damage to optic nerve/retina Often associated with increased pressure in the eye Can be caused by blockages in cycling of aqueous humor Open angle glaucoma: Flow to the canal of Schlemm is normal Microobstruction or increased venous pressure prevents fluid from leaving the eye 60% of glaucoma cases, slow painless onset Closed angle glaucoma: Flow to the canal of Schlemm is blocked by an iris that is positioned abnormally Sometimes sudden onset and painful Eye Anatomy and Histology - Hoffmann Page 7 Pupillary regulation Big Idea: pupil must change shape to allow different amounts of light in depending on the brightness of the environment Sphincter Pupillae- A circular muscle within the iris surrounding the pupil opening If constricted, it shrinks the pupil allowing less light in Under parasympathetic control (CN III) Dilator Pupillae- Straight muscles arranged in radial pattern around Sphincter pupillae If constricted, they pull the dilator pupillae open, enlarging pupil to let more light in Under sympathetic control (Sympathetic nerves) Iris Histology Iris is mostly loose connective tissue with lots of blood vessels (uvea layer) At the posterior, again two layers of cells: NOTE again: these are from the Retinal layer NOT the uvea This time, Inner layer (‘Inner’ relating to the inside of the eye) Heavily pigmented (cells can’t really be seen) Outer layer (‘Outer’ relating to the outside of the eye) Lighter pigmentation, actually forming myoepithelial cells to make Dilator pupillae (straight muscles mentioned above) Degree of pigmentation (eye color) is not related to these layers but is influenced more by pigment found in the stroma of the iris itself No pigment (albinism): iris color is pink (color of the blood vessels) Little pigment: iris color is blue Lots of pigment: iris color is brown Other colors are somewhere in between Eye Anatomy and Histology - Hoffmann Page 8 B5. Retina A true histological work of art. Layers of cells arranged in strict rows immediately recognizable. Sequence of activation: 1. Light passes through entire retina, activates photoreceptor cells (deep, next to pigment cells) 2. Photoreceptor cells activate bipolar cells 3. Bipolar cells activate Ganglion cells Ganglion cell axons leave and become the Optic nerve Accessory events: -Retinal pigmented epithelium (deepest layer) helps to absorb the light preventing scatter -Horizontal and Amacrine cells integrate/amplify signal -Muller cells (glial cells of retina) provide structure, create limiting membranes Eye Anatomy and Histology - Hoffmann Page 9 Retinal pigmented epithelium (RPE) Provides support for receptive processes of Rods and Cones -Delivers blood-borne nutrients to the photoreceptors -Phagocytoses the processes of rods/cones (continual, ongoing process to counteract UV radiation damage) -Pigment absorbs excess light -Tight junctions between cells creates a blood-eye barrier Macula (Fovea centralis) Point of central vision, greatest visual acuity Mostly cones (high concentration of color vision here) All other cell layers displaced for maximal stimulation Optic nerve Optic disc: blind spot on the retina where optic nerve exits (no photoreceptors at this location) Vessel in the nerve is the central artery of the retina, a branch of the ophthalmic artery that spreads across the surface of the inner surface of the retina. Eye Anatomy and Histology - Hoffmann Page 10 Eye Anatomy and Histology - Hoffmann Page 11 .