Histology Of The Eye Flashcards
What are the layers of the eye?
3 layers or coats
- Corneoscleral coat
- Outer fibrous layer
- Cornea (transparent layer)
- Sclera (white portion)
- Outer fibrous layer
- Vascular coat (uvea)
- Middle layer or uvea
- Iris
- Ciliary body
- Choroid
- Middle layer or uvea
- Retina
- Inner layer
- Inner neural retina
- Outer pigment epithelium
- Inner layer
Describe the cornea
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
What are the layers of the cornea?
5 layers
- Corneal epithelium
- Bowman’s membrane
- Corneal stroma
- Descemet’s membrane
- Corneal endothelium
Describe the corneal epithelium
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
Describe the Bowman’s membrane
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
Describe the corneal stroma
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)
• Regulates fibril assembly & spacing
• Chondroitin sulphate
• Uniform spacing of fibrils, lamellae and orthogonal array of
lamellae responsible for transparency
Describe the Descemet’s membrane
Descemet’s membrane
• Thick basal lamina of the corneal endothelial cells
• Interwoven meshwork of collagen fibers and pores
• Can regenerate
• Thickens with age
Describe the corneal endothelium
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
Whhat are the clinical correlations of the cornea
Clinical correlation: Corneal dystrophy & corneal transplant
What are the indications of corneal transplant?
Indications • Keratitis • Trauma • Keratoconus • Corneal degeneration
What are the types of corneal transplant?
Types of transplant
• Penetrating keratoplasty
• Lamellar keratoplasty
• Endothelial keratoplasty
What are the advantages of corneal transplant?
Advantages
• Cornea is avascular
• Minimal immunological response or graft rejection
Describe the sclera
Thick fibrous lauer
-Dense connective tissue
Flat collagen bundles interspersed with fine networks of elastic fibers
Lamina cribosa (LC)
- Posterior openings of the sclera (black arrows)
- Allows for entry/exit of nerves and vessels
- fibers of optic nerve
- central artery and vein
What are the 3 ill-defined layers of Sclera?
ill-defined layers
1. External Episcleral layer
• Loose CT next to periorbital fat
- Middle layer Substantia propria (Tenon’s capsule)
• Investing fascia of the eye
• Dense network of collagen fibers
• Tendons of extraocular muscles attach here - Inner Suprachoroidal lamina
• Inner layer adjacent to the choroid
• Thinner collagen fibers and elastic fiber
What are the components of the vascular coat (uvea)?
Features: CT, blood vessels and pigmented cells
Components:
A. Iris
• Anterior part of the vascular coat
• Formsacontractilediaphragmin front of lens
• Pupil
B. Ciliary body
• Thickened anterior portion between the iris & choroid
• Ciliary processes
• Ciliary muscle
C. Choroid
• Posteriorportion
• Lies between sclera and retin
What’s the function of choroid?
- Pigmented, dark brown vascular layer
- Functions:
- Bloodsupply
- Absorption of scattered light
What are the 2 layers of the choroid?
- Choriocapillary layer
• Inner vascular layer
• Fenestrated capillaries - Bruch’s membrane (lamina vitrea)
• Amorphoushyalinemembrane between the choriocapillary layer and retinal pigment epithelium (RPE)
• AttachesRPEtochoriocapillarylayer
• Sheet containing 5 layers:
What are the functions of Iris?
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
Describe the anterior surface of the Iris
- 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
- Double layer of epithelium
How is pupil diameter regulated?
• 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
What is the function of ciliary muscle?
- Ciliary muscle
- Smooth muscle
- Parasympathetic
- Fibers spread out in several directions
- Longitudinal, radial & circular
• Reduces tension of zonular fibers causing lens to get more
round for near vision* Relaxation of muscle * Increases tension of zonular fibers causing lens to flatt - Smooth muscle
What are the major functions of the ciliary process?
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
How are distant objects accomodated for?
Relaxed ciliary muscle
Tightened zonule fibers
Flattened lens
How are close objects accomodated for?
Contracted ciliary muscle
Relaxed zonule fibers
Lens are rounded
Describe the corneocleral limbus (junction)
Transition zone between cornea & sclera (black arrow in bottom image)
• Bowman’s membrane ends
- Epithelium thickens to from ~5 layers of the cornea (C) to ~10-12 layers of the conjunctiva (CjEp)
- The basal layer contains corneo-limbal stem cells
- Generate & maintain corneal epithelium
• Apparatus for the outflow of aqueous humor
1. Trabecular meshwork (spaces of Fontana)
2. Canal of Schlemm (CS)
• Scleral venous sinus
Describe the corneocleral limbus (junction)
Transition zone between cornea & sclera (black arrow in bottom image)
• Bowman’s membrane ends
- Epithelium thickens to from ~5 layers of the cornea (C) to ~10-12 layers of the conjunctiva (CjEp)
- The basal layer contains corneo-limbal stem cells
- Generate & maintain corneal epithelium
• Apparatus for the outflow of aqueous humor
1. Trabecular meshwork (spaces of Fontana)
2. Canal of Schlemm (CS)
• Scleral venous sinus
What are the symptoms of Glaucoma?
Increased intraocular pressure
Symptoms: • Headache • Eye pain • Loss of vision • Vomiting • “Halo” around light
What are the signs of the Glaucoma?
- Peripheral field of vision is decreased
• Redness
• Raised intra ocular pressure (tonometry)
How is intraocular pressure memorized?
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
Describe the vitreous body
- Posterior segment of the eye
- Transparent jelly-like substance
- Homogenous gel
- Collagenfibrils
- 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
What are the 2 basic layers of the retina?
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
- Retinal pigment epithelium (RPE)
• Outer layer of optic cup
Describe 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 • Clinical correlation: Night blindness
Describe cones
~7 million • Thicker and shorter • Conicaloutersegment • Visual pigment = iodopsin • Sensitive only to light – better visual acuity • Colorvision • Largest density of cones is in the fovea • Clinical correlation: Color blindness
Describe retinal pigment epithelium
- Singlelayerofcuboidalcells
- Junctionalcomplexes
- Tight junctions, adherens junctions and gap junctions
- Blood-retina barrier
- Basal lamina attached to Bruch’s membrane
What are the functions of retinal pigment epithelium?
- 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
- Clinical correlation: Link to retinal detachment
What are the causes of retinal detachment?
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
Whats the significance of the. Optic disc?
• Optic disc
• Site where optic nerve emerges from the retina
• Aka “blind spot”→no rods and cones.
• Clinical correlate: Optic neuritis in Multiple
sclerosis
Whats the importance of the macula lutea?
• Macula lutea
– Center of the posterior retina at the visual axis of the eye
– Clinical correlate: Age-related macular degeneration
Whats the purpose of the fovea centralis?
Fovea centralis
– Small depression in the center of the macula lutea
– Area of highest visual acuity
– Contains only cones
Describe the lens
- The lens is a biconvex structure
• Avascular
• Held in place by a radially oriented group of zonular fibers
What are the 3 principal components of the lens?
3 principle components
1. Lens capsule:
• Thick basal lamina of 10 – 20 μm
• Produced by anterior lens cells
- 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 - Lens fibers
• Thin flattened structures
• Lose organelles and accumulate crystallins (proteins)
• Formed from the epithelial cells at the germinal zone
Clinical correlations: cataract & presbyopia
Explain what is lens cataract
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.
Describe the conjctiva
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
What are the glands of the eyelids?
Glands of the eyelid • Meibomian glands • Sebaceous within tarsal plate • Lubricates – delays drying of tears • Clinical correlation: Chalazion
• Glands of Zeiss
• Sebaceous glands associated with
eyelashes
• Clinical correlation: Stye (hordeolum)
- Glands of Moll
- Sweat glands at the lid margins
