Ophthalmology Quiz on Corneal Anatomy

Organs of Special Senses: Photoreceptor & Audio-receptor Systems

• The eye is a highly specialized organ for vision and photoreception.
• It analyzes the form, intensity, and color of reflected light.
• The human eye ball is approximately 24mm in diameter.
• It is located in the eye socket (orbit) of the skull and protected by eyelids.
• Extraocular muscles enable conjugate gaze, allowing both eyes to move in the same direction simultaneously.
• Each eyeball has a tough, fibrous globe to maintain shape, a system of transparent tissues to refract light and focus the image, and a layer of photosensitive cells and neurons to transmit visual information to the brain.

Structure of the Eye

• The fibrous tunic consists of the sclera and cornea.
• The vascular tunic contains the choroid, ciliary body, and iris.
• The retina is the innermost layer with pigmented and neural layers.
• The eye has three compartments (anterior chamber, posterior chamber and vitreous space) containing aqueous humor and vitreous humor.

Sclera

• Forms posterior 5/6 of the external eye layer.
• Opaque and white, relatively avascular.
• Made of dense connective tissue (collagen type I fibers).
• Contains fibroblasts.
• Episclera is the external layer of sclera, providing attachment sites for extraocular muscles.
• Tenon's capsule surrounds eyeball, creating Tenon's space for rotation.

Cornea

• Anterior 1/6 of the external layer.
• Convex anteriorly (dome-shaped).
• Colorless, transparent, and sensitive.
• Avascular, can be transplanted.
• Refracts light onto the lens.
• Provides protection.
• Consists of 5 layers including epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium.

Corneal Epithelium

• Stratified squamous non-keratinized epithelium of 5-6 layers.
• Surface cells are joined by tight junctions, other layers by desmosomes.
• Thick basement membrane for stability.
• Basal cells are responsible for regeneration (7 days).
• Surface cells with microvilli are provided by precorneal tear film of lipids, glycoprotein & water to keep cornea wet and prevent ulceration.
• Absorbs oxygen & nutrient from tear film.
• Rich sensory nerve supply stimulates eye lid blinking & tear flow.

Bowman's Membrane

• Basement membrane of the corneal epithelium.
• Consists of randomly arranged collagen I fibers.
• Non-cellular.
• Contributes to stability and strength.
• Cannot be regenerated.

Stroma (Substantia Propria)

• Thick layer (90%) of the cornea thickness.
• Composed of parallel lamellae of collagen fibrils running parallel within each layer and perpendicular to the next.
• Contains flattened fibroblasts (keratocytes) in between bundles.
• Ground substance rich in glycoproteins and chondroitin sulfate.

Descemet's Membrane

• Basement membrane of the corneal endothelium.
• Thick and homogenous.
• Contains fine collagen (VIII) filaments arranged in a three-dimensional network.

Endothelium

• Simple squamous epithelium.
• Connected by tight junctions.
• Possesses active transport properties for protein synthesis.
• Limited regenerative potential.
• Pumps excess fluid out of stroma to maintain cornea clarity.
• Forms Descemet's membrane.
• Important for maintaining corneal transparency through dehydration and Na+/K+ & passive transport of water & Cl-.

Corneo-scleral Junction (Limbus)

• Transition zone between cornea and sclera.
• Highly vascularized, supplying cornea by diffusion.
• Contains Schlemm's canal, draining aqueous humor to the venous system.
• Aqueous humor is similar to plasma composition but contains less than 0.1% protein.

Changes at the Limbus

• Corneal epithelium is continuous with the bulbar conjunctiva.
• Bowman's membrane terminates.
• Corneal stromal collagen fibers lose regularity.
• Descemet's membrane and its simple endothelium are replaced by a trabecular meshwork.
• Trabecular meshwork allows slow, continuous drainage of aqueous humor.

The Eye Contains 3 Compartments

• Aqueous humor is found in the anterior and posterior chambers.
• Vitreous humor is in the vitreous space.

Nutrition of the Cornea

• Cornea receives nutrients by diffusion from aqueous humor and scleral vessels.
• It obtains oxygen directly from air & tear film.

The Ciliary Body

• Anterior expansion (thickened ring) of the choroid at the lens level.
• Triangular in shape, with apex continuous with the choroid and base facing the iris.
• Plays a vital role in forming the aqueous humor.

Histological Structure of Ciliary Body

• Bulk is formed of smooth muscles.
• Surrounded by loose connective tissue.
• Rich in microvasculature and elastic fibers, including melanocytes.
• Composed of 4 layers from internal to external: double-layered epithelium, stroma, ciliary muscles and supraciliary layer.

Ciliary Processes

• Extensions of the ciliary body.
• Contain connective tissue, fenestrated capillaries, and two epithelial layers.
• Zonular fibers or suspensory ligament connect ciliary processes with the lens capsule.

Ciliary Muscles

• Three arrangements: meridional, radial, and circular fibers arranged.
• Plays an important role in visual accommodation (changing the shape of the lens for focusing).

Visual Accommodation

• At rest, lens is under tension (flattened) for distant vision.
• Ciliary muscles contract, tension on zonula fibers is released, and the lens becomes more convex for close vision.

Function of Ciliary Body

• Non-pigmented outer layer of ciliary processes produces aqueous humor.

Formation of Aqueous Humor

• Cells of the non-pigmented ciliary process layer have tight junctions and basal infoldings for ion transport.
• Na+/K+-ATPase pumps fluid actively from vascular stroma into the posterior chamber.
• Aqueous humor travels through the posterior chamber to the anterior chamber through the pupil.

The Iris

• Extension of the choroid that partially covers the lens, with a central opening (pupil).
• Thin, pigmented, contractile, circular structure.
• Analogous to the diaphragm of a camera, dividing the anterior compartment into anterior and posterior chambers.

Structure of the Iris

• Consists of:
• Anterior discontinuous layer of pigment cells and fibroblasts.
• Stroma of loose connective tissue, highly pigmented (less vascular). Smooth muscle aggregations form the sphincter pupillae around the pupillary margin.
• Posterior layer of stroma with less pigment and more vascularization. Inner anterior epithelium is myoepithelial (light pigmented), forming dilator pupillae muscle. Cells of the outer posterior layer are highly pigmented epithelium to protect the eye's interior from excess light.

Function of Iris

• The heavily pigmented epithelium prevents light from entering the interior of the eye except through the pupil.
• Iris muscles regulate pupil diameter and thus the amount of light entering the eye.

Iris Muscles

• Two muscles: dilator pupillae (radially arranged fibers) and sphincter pupillae (concentric bundles at pupillary margin), influencing pupil diameter regulation.
• Dilator pupillae muscle increases pupil diameter in dark and dim light/fear.
• Sphincter pupillae muscle constricts pupil in bright light and for accommodation.

Color of the Eye

• Determined by the amount of melanin in the iris stroma.
• Ranges from blue to greenish blue, gray, and brown.
• Individuals with albinism have no melanin and their irises reflect the blood vessels in the stroma, appearing pink.

The Lens

• Transparent biconvex structure.
• Avascular and devoid of nerves.
• High in elasticity, but this decreases with age.
• Focuses light onto the retina.
• Held in place by the ciliary zonule.

Lens Capsule

• Thick basement membrane, homogeneous and refractile.
• Completely covers the lens, thickest near the equator.
• Composed mainly of collagen IV and rich in glycoprotein.

Subcapsular Epithelium

• Single layer of cuboidal epithelium.
• Present only on the anterior surface of the lens.
• Basal ends attach to the lens capsule with interdigitations that bind to the internal lens fibers.
• Cells at the equator responsible for lens growth during normal growth.

Lens Fibers

• Derived from subcapsular epithelium and flatten in shape, losing their nuclei and organelles.
• Filled with crystallins, increasing refractive index of the lens while maintaining transparency.

Lens Regions

• Regions based on fiber age: outer cortex (younger) and inner nucleus (older).
• Mature lens fibers have no organelles or nuclei.

Nutrition of the Lens

• Lens receives nourishment from the aqueous humor.
• Epithelial cells actively pump ions by Na+/K+-ATPase to maintain lens transparency.

Visual Accommodation Problems

• Presbyopia: difficulty with near-object vision due to reduced lens elasticity.
• Cataracts: lens becomes opaque and less transparent, often due to pigment accumulation, UV exposure, or diabetes mellitus.

Vitreous Body

• Clear, colorless, transparent, and gelatinous.
• Occupies space behind the lens, posteriorly surrounded by retina.
• Composed of 99% water, a small amount of collagen, hyaluronic acid, and few cells called hyalocytes.
• Maintains shape of the eye and transmits light.
• Allows transfer of substances to and from retina.
• Stagnant, nourished at periphery by vessels of retina & ciliary processes.

The Hyaloid Membrane

• Transparent membrane between vitreous humor and retina.
• Facilitates changes in the volume of the lens.
• Hyaloid canal is a transparent channel running from optic nerve to lens.

Refractive Media of the Eye

• Cornea, aqueous humor, lens, and vitreous humor.

Retina

• Evagination of the prosencephalon forms optic vesicle.
• Optic vesicle invaginates in surface ectoderm, forming a double-walled optic cup.
• Outer wall forms pigment epithelium.
• Inner wall forms neural retina.

Structure of Retina

• Two portions: inner optical (photosensitive/neural) and outer pigmented layers.
  • Retina composed of 9 layers

Ora Serrata

• Serrated junction between the retina and ciliary body.
• Pigment epithelium of the retina transitions into the pigment epithelium of the ciliary body.
• Inner portion of retina transitions into non-pigmented epithelium of ciliary body.

Pigment Epithelium

• Columnar cells connected by tight junctions.
• Apical melanin granules.
• Basal ion transport activity.
• Apical microvilli & sheaths that envelop photoreceptors.
• Contains lysosomes and SER for phagocytosis of shed components. Retina detachment occurs in this region.

Function of Pigment Epithelium

• Vital component of outer blood-retinal barrier.
• Absorbs light passing through the retina.
• Regulates ion homeostasis and eliminates metabolites.
• Phagocytoses shed components from adjacent rods and cones.
• Isomerizes & regenerates retinoids.
• Removes free radicals.

Neural Retina

• Three major layers of neurons.
• Outer layer of photoreceptor cells (rods and cones).
• Intermediate layer of bipolar neurons connects to rods & cones.
• Internal layer of ganglion cells, which synapse with bipolar cells and axons form the optic nerve.

Organization of Neural Retina

• Composed of 9 layers:
  • Outer layer of rods and cones
  • Outer limiting membrane
  • Outer nuclear layer (cell bodies and nuclei of rods and cones)
  • Outer plexiform layer (synapses between photoreceptors, horizontal & bipolar cells)
  • Inner nuclear layer (nuclei of bipolar, horizontal, amacrine, and Müller cells.)
  • Inner plexiform layer (synapse between bipolar, amacrine, and ganglion cells)
  • ganglion cell layer
  • Retinal (optic) nerve fibers layer (unmyelinated axons of ganglion cells, process of Müller cells & retinal blood vessels)
  • Inner limiting membrane (basal lamina of Müller cells separates retina from vitreous body)

Retinal Neurons and Glial Cells

• Neurons: rods, cones, bipolar, ganglion, horizontal, and amacrine.
• Glial cells: Müller cells, astrocytes & microglia.

Rod and Cone cells

• Rod cells: highly sensitive to light (dim light).
• Cone cells: specialized for color vision (bright light).
• Both are polarized neurons.
• Rods and cones are shaped for their function (outer segments)

Ultrastructure of Rods and cones

• Outer segment: flattened membranous disks, surround by plasma memberane.
• Constriction and modified cilium.
• Inner segment: proteins (polyribsomes), metabolic & energy production (mitochondria).
• External plexiform layer: synapses with bipolar cells.

Photopigments

• Rhodopsin (rod cells).
• Iodopsins (cone cells) – maximum sensitivity to red, green, and blue.
• Visual pigments contain opsin (transmembrane protein) and retinal (light sensitive chromophore).

Vision Reaction

• Light triggers photochemical reaction in rods & cones.
• Activates bipolar cells.
• Then activates ganglion cells.
• Information transmitted to the visual cortex.

Phototransduction

• Photons absorbed by retinal dissociate from opsin (opsin bleaching).
• Increases Ca diffusion to intracellular space of outer segment.
• Ca acts on cell membrane reduces permeability to Na, resulting in hyperpolarization.

Hyperpolarization

• Reduces synaptic release of neurotransmitter (unlike other receptors where action potential is generated through depolarization).
• Depolarizes bipolar neurons, sending action potentials to ganglion cells of the optic nerve.
• Reabsorbs Ca ions and pigment is reassembled.

Specialized Areas of the Retina

• Optic disc (blind spot).
• Fovea centralis contains concentrated cone cells for high visual acuity (no rods)

Optic Disk

• Site of optic nerve exit (axons of ganglion cells).
• Site of central retinal artery and vein entry.
• Lacks photoreceptors, insensitive to light.

Fovea Centralis

• Shallow depression with a very thin retina.
• Surrounded by macula lutea (yellow spot) rich in carotenoids.
• Protects cone cells (short-wavelength light filter and antioxidant).
• High cone cell concentration for precise high visual acuity. (no rod cells).
• Lacks blood vessels.

Nutrition of Retina

• Outer retina is supplied by choriocapillary layer.
• Inner retina is supplied by branches of the central retinal artery.

Medical Applications

• Color blindness and night blindness.

Accessory Structures of the Eye

• Conjunctiva (lines inside of eyelids and white of eye).
• Eyelashes protect eye from dirt.
• Lacrimal apparatus (produces tears).

Eyelashes, Glands of Zeis and Glands of Moll

• Meibomian glands are sebaceous glands in tarsal plate that prevent tear evaporation by forming oil layer.
• Glands of Zeis are small sebaceous glands associated with follicles of eye lashes.
• Glands of Moll are modified sweat glands connected to follicles of eye lashes.

Lacrimal Apparatus

• Consists of lacrimal gland, canaliculi, lacrimal sac, and nasolacrimal duct.
• Lacrimal gland: tubuloalveolar with secretory ducts and serous columnar cells (similar to parotid acinar).
• Myoepithelial cells also present.
• Canaliculi, lined by squamous epithelium.
• Sac and nasolacrimal duct lined by respiratory epithelium.

Tears

• Alkaline fluid rich in lysozyme.
• Tear film formed by 3 layers:
  • A mucous layer formed by goblet cells of conjunctiva
  • A watery layer formed mainly by lacrimal glands
  • A lipid layer formed by tarsal glands

Vestibulo-cochlear Apparatus (Vestibuloauditory System)

• Hearing and maintaining equilibrium.
• External ear receives sound waves.
• Middle ear transmits sound to inner ear fluids via bones.
• Inner ear contains auditory organs for transduction to nerve impulses, including the Vestibular organ for equilibrium.

External Ear

• Composed of:
  • Auricle (ear pinna) - elastic cartilage covered with skin, determines and amplifies sound source
  • External auditory canal (acoustic meatus)- lined by st. squamous epithelium (skin) with hair follicles, sebaceous, and ceruminous glands (modified apocrine). supported by cartilage (outer part) & temporal bone (inner part).
  • Tympanic membrane (ear drum) - CT layer of collagen, elastic fibers and fibroblasts. Outer layer : thin skin. Inner layer: simple low cuboidal epithelium. Cerumen (earwax) secreted by ceruminous glands for protection (oily/waxy material).

Middle Ear (Tympanic Cavity)

• Air-filled cavity lined with simple cuboidal epithelium, connected anteriorly to pharynx via eustachian tube.
• Tube lined with ciliated pseudostratified columnar epithelium (opening during swallowing for air pressure equalization).
• Posteriorly continuous with the air-filled cavities of mastoid process of temporal bone.
• Other Boundaries include lateral wall (tympanic membrane), medial wall (inner-ear containing two membrane-covered regions: oval & round windows).
• Contents of tympanic cavity: 3 auditory ossicles (malleus, incus, stapes) that transfer mechanical vibrations to inner ear from tympanic membrane.
• 2 skeletal muscles (tensor tympani and stapedius)
• restricting ossicle movement and protect inner ear from loud noises.
• Eustachian tube equalizes air pressure.

Protection of Middle Ear

• Eustachian tube equalizes pressure on both sides of tympanic membrane.
• Shape of ossicle articulation restricts movement and protects the inner ear from high intensity sounds (own speech and chewing).

Internal (Inner) Ear

• Located within temporal bone.
• Consists of bony and membranous labyrinths (osseous and membranous).
• Bony labyrinth houses the membranous labyrinths.

Bony Labyrinths

• Cavities within petrous bone, formed of 3 semicircular canals and vestibule & cochlear canal.
• Filled with perilymph (similar to CSF and ECF composition).
• Perilymph formed by microvasculature of periosteum draining via perilymphatic duct into subarachnoid space.

Membranous Labyrinths

• Fluid-filled, epithelium-lined tubes within the bony labyrinth.
• Formed of semicircular ducts, utricle, saccule, and cochlear duct.
• Filled with endolymph (different ionic composition to perilymph).

Sensory Structures in the Inner Ear

• Specialized neuroepithelium (neuro-epithelium) lining membranous labyrinths.
• Includes organ of Corti, maculae, and cristae ampullaris.
• Maculae and cristae are structures responsible for equilibrium (head position/linear acceleration/deceleration ; rotational acceleration)

Maculae

• Sensory receptors containing mechanoreceptors (hair cells).
• Located in floor of utricle and anterior wall of saccule.
• Maculae function in registration of head position & sense of linear acceleration/deceleration.
• Composed of two types of receptor cells and supporting cells and afferent nerve endings (endings of vestibular branch of the 8th cranial nerve). - Components include otoliths, otolithic membrane, kinocilium & stereocilia.
• Supporting cells are columnar shaped cells with apical microvilli. The neuroepithelium is covered with gelatinous glycoprotein and surface depositions (otoliths or otoconia)

Hair Cells of Maculae

• Two types: Type I (flask-shaped, large cup-shaped afferent nerve ending at the base) and Type II (cylindrical, with multiple afferent nerve endings).
• Both have one kinocilium and many stereocilia. Tips of the tallest stereocilia of outer hair cells are embedded in tectorial membrane

Cristae Ampullaris

• Sensory receptors.
• Located in ampullae of semicircular ducts.
• Function in registering rotational head movement (angular acceleration).
• Structure similar to maculae, but cupula (conical glycoprotein layer) is thicker and no otoliths

Maintaining Balance (Vestibular Function)

• Fluid flow in semicircular ducts moves cupula over crista, bending stereocilia toward kinocilium increasing impulses carried by vestibular nerve. Uniform movement returns cupula to normal position. Maculae detect gravity/tilt (bending of stereocilia).

Endolymphatic Duct & Sac

• Lined by simple squamous epithelium that shifts to tall columnar near the sac.
• Epithelial cells with apical microvilli for absorption of endolymph.
• Pinocytotic vesicles for endocytosis of foreign material and cellular remnants.
• Drained from the vestibule to venous sinuses of dura mater by endolymphatic duct.

Cochlea

• Snail-shaped bony canal (35mm long).
• Contains cochlear duct in the middle.
• Makes 2 ½ turns around the modiolus (bony core).
• Modiolus contains spiral ganglion and blood vessels.
• Divided into three spaces: Scala vestibuli, Scala media (cochlear duct) & Scala tympani.

Cochlear Duct

• Site of hearing receptor (organ of Corti).
• Ends at apex of cochlea via helicotrema.
• Scala tympani and vestibuli form one long tube.
• Begins at the oval window, ends at the round window.
• Contains endolymph. Other scalea contain perilymph.

Borders of Cochlear Duct

• Above (roof): vestibular (Reissner's) membrane.
• Lateral: Stria vascularis.
• Below (floor): basilar membrane.
• Vestibular membrane consists of two layers of squamous epithelium form scala media & sclera vestibuli, cells joined by tight junctions. High ionic gradient and endolymph and perilymph.

Stria Vascularis

• Lateral wall of cochlear duct.
• Unique vascularized epithelium.
• Involved in ion and water transport to maintain endolymph ionic composition.

Basilar Membrane

• Floor of cochlear duct.
• Extends from spiral lamina (projection from modiolus) to spiral ligament.
• Spiral limbus is a periosteal CT extension over osseous spiral lamina forming the floor.
• Covered by columnar epithelium.
  • Composed of 20,000-30,000 fibers that vary in length and thickness. (longer and thinner toward apex) Resonate at different frequencies, like a tuning fork.

Organ of Corti

• Location: rests on upper surface of basilar membrane longitudinally from spiral limbus to spiral ligament.
• Covered by tectorial membrane (acellular layer).
• Formed of mechanoreceptors (hair cells) and supporting cells.
• Three basic structures: single row of inner hair cells (flask shaped) , three to five rows of outer hair cells (cylindrical), and supporting cells.

Hair Cells

• Both inner and outer hair cells have stereocilia.
• IHC have one linear row of short stereocilia.
  • OHC have a curved row of longer stereocilia, W-shaped.
• They both lack a kinocilium.
• OHC stereocilia tips embedded in tectorial membrane.
• They are within supporting cells (phalangeal cells). Bipolar afferent neurons form spiral ganglion of the modiolus.

Function of Hair Cells

• Both hair cells have afferent and efferent nerve endings.
• IHC greatly innervated, are true receptors for hearing.
• OHC contract when stimulated , pulling on tectorial membrane and stimulating IHC.

Supporting Cells

• Pillar cells shape space between inner and outer hair cells (inner tunnel). Important for sound transduction. Have microtubules and keratin for stiffness. -Hensen's cells are lateral to organ of Corti.
• Phalangeal cells surround the base of the outer and inner hair cells (almost completely enclosing IHC). They only extend basally to the OHCs

Medical Applications

• Conductive hearing loss (otosclerosis, otitis media)
• Sensorineural deafness.

Ear Parts & Functions

• Ear pinna: determines sound source, amplifies frequency.
• Ear drum: vibrates in response to air pressure fluctuations, transforms sound waves into mechanical energy.
• Eustachian tube: equalizes air pressure on both sides of the eardrum.
• Middle ear: amplifies sound pressure through ossicles.
• Inner ear: hearing and balance organs.

This quiz tests your knowledge on the anatomy and functions of the cornea and lens in the human eye. Questions cover components such as Descemet's membrane, Bowman's membrane, and the ciliary muscles. Assess your understanding of how these structures contribute to vision and corneal health.