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Histology / Embryology Unit 2 > The Eye > Flashcards

Flashcards in The Eye Deck (82):

What are the three chambers of the eye?

Anterior, posterior, and vitreous chambers


What are the three tunics of the eye and what are their major parts?

Fibrous: Sclera, cornea
Vascular: Choroid, ciliary body, and iris
Sensory: Pigment epithelium, neural retina


What does the anterior chamber contain?

Aqueous humor, similar in composition to CSF or blood plasma (high Na+ and low K+), but very few proteins (i.e. platelets or immunoglobulins)


What is the posterior chamber? What are its boundaries?

It is the annular (donut-shaped) space bounded by the epithelium of the iris, ciliary process, zonular fibers, and anterior lens


What is the function of the ciliary body?

Produces aqueous humor (~3 mL/day), helps nourish the lens and other structures bordering anterior/posterior chambers.
Fluid from here flows from posterior to anterior chamber between iris and lens


What is the blood-aqueous barrier?

A blood-ocular barrier isolating the aqueous humor from the blood and cornea. Formed by occluding zonules of the blood vessels of the iris, ciliary body, and corneal endothelial cells.


What are the boundaries of the vitreous cavity?

Space bounded by the retina, posterior ciliary body, zonular fibers, and posterior lens


What produces the vitreous humor and what is it made of?

Produced by hyalocytes, it is gelatinous, containing hyaluronic acid and cross-linked collagen fibers of collagens which do not produce gritty fibers (i.e. mostly Type II collagen).


What are the functions of the vitreous humor?

Supports the shape of the eye, maintains the position of the retina to prevent detachment


What is the cornea and what cannot be found in it?

It is the transparent chief refractive component of the eye. It is avascular and normally free of white blood cells (to prevent occluding the visual axis


What are the five layers of the cornea from top to bottom?

1. Anterior corneal epithelium
2. Bowman's layer
3. Corneal stroma
4. Descemet's membrane
5. Corneal endothelium


What are the properties of the anterior corneal epithelium and what does it contain?

Consists of stratified squamous nonkeratinized epithelium. It is renewed about once a week and is capable of repair. It contains many sensory free nerve endings (you can feel when your cornea is cut)


What is Bowman's layer? Does it regenerate?

2nd layer of cornea, it is an acellular condensation of the underlying collagenous (T1 collagen) layer that resists penetration and slows bacterial invasion. Though it is below the corneal epithelium, it is NOT a basement membrane, and is NOT regenerated.


What is the corneal stroma and its organization? Does it regenerate?

It is dense irregular connective tissue, the thickest layer of the cornea. Made of Type 1 collagen fibrils sheets which are oriented perpendicularly with eachother. The fibrils have uniform diameter and interfibrillar spacing. Allows penetration of light, and damage results in scarring as it does not regenerate


What are the cells + ground substance of the stroma?

Cells: specialized fibroblasts called a corneal keratocyte
Ground substance: a corneal proteoglycan rich in keratan and chondroitin sulfates


What is Descemet's membrane? Does it regenerate?

It is the thickened basal lamina of the adjacent corneal endothelium (unlike Bowman's layer)

It does regenerate


What is the corneal endothelium? What does it make? What does it do?

A simple cuboidal epithelium which becomes more squamous overtime, because when damaged, current cuboidal cells spread out to fill the gaps (does not regenerate). Cells have occluding zonules to make part of the blood-aqueous barrier.

Helps regulate ion balance of corneal stroma to keep it hydrated. Damage to this layer can lead to permanent damage of corneal stroma.


What causes corneal opacity?

Caused by damage to endothelium, disrupting ion balance in the stroma. Also could be due to invasion of WBCs due to infection


Why do corneal transplants have good success?

The cornea is avascular, thus decreasing the likelihood of graft rejection


Why might you surgically remodel the cornea?

Permanent refractive changes can be produced by reshaping it, as in radial keratotomy (LASIK)


What is the bulbar conjunctiva?

Part of the fibrous layer, it is a mucous membrane on the anterior surface of the eyeball around the cornea. It reflects onto the inner surface of the eyelids as the palpebral conjunctiva


What makes up the bulbar conjunctiva?

A layer of stratified epithelium that is continuous with the anterior corneal epithelium. It contains goblet cells which will produce the mucus portion of the tear film. It is highly vascularized and can turn red (as in conjunctivitis)


What is the sclera? What are the principle cells, fibrous component, and ground substance?

A dense irregular connective tissue which is opaque and vascularized.
Cells: Fibroblasts
Ground substance: Keratan sulfate and chondroitin sulfate, but different from corneal proteoglycan
Fibrous component: Type 1 collagen fibers that vary in both diameter and interfibril spacing


What is a cause of myopia via sclera?

Scleral thinning can lead to axial elongation of the vitreous chamber, focusing the image in front of the retina


What is the limbus? What are its functions?

The corneoscleral junction. It is an annular ring which is highly vascularized.
1. Supplies nutrients for maintenance of cornea
2. Provides corneolimbal stem cells for replacement of corneal epithelium
3. Drains the aqueous humor


What is the trabecular meshwork?

A structure of sieve-like squamous endothelium-lined channels in the wall of the anterior chamber at the limbus level. It is adjacent to the peripheral rim of corneal endothelium. It drains the vitreous to the channel canal of Schlemm


What is the canal of Schlemm?

A canal that drains aqueous humor from the trabecular meshwork into the venous network


What is the lamina cribrosa?

A sieve-like opening in the posterior sclera which allows the exiting retinal ganglion cell axons to form the optic nerve. It is the opening for the optic nerve, and contains type I collagen.


What is the function of the vascular layer?

Carries out nutritive and contractile functions, it is supplied by two ciliary arteries and four vortex veins. Gives oxygen to outer retina, sclera, iris, and ciliary muscles.


What is the choroid? What are its layers?

A pigmented vascular layer between the sclera and the retina.
1. Choroidal melanocytes: pigmented cells containing melanin to absorb light passing through the retina
2. Choriocapillaris: Layer of fenestrated capillaries bordering the retinal pigment epithelium (Bruch's membrane is between technically)


What is Burch's membrane?

A basal lamina of the retinal pigment epithelium reinforced by collagen and elastic fibers. It lies between the choriocapillaris and the retinal pigment epithelium


What is the ciliary body?

Continuation of vascular layer and partially the retina from the ora serrata, it is an annular structure which borders the iris and choroid. It is relatively flat near the retina, but thickens anteriorly towrads the iris


What is the ciliary muscle?

A circumferentially orientated smooth muscle. It contracts to release tension from the zonular fibers, which allows the lens to become more round (how it wants to be) and increases its power. This allows accommodation, or regulation of the focal point of the lens


What are the four layers of the ciliary body from outer wall to inner wall?

1. Smooth muscle (ciliary muscle)
2. Vascularized connective tissue beneath the epithelium (pointing internally)
3. Outer pigmented epithelial layer
4. Inner nonpigmented epithelial layer


What are the ciliary processes?

Projections of the two-layered epithelium of the ciliary body, closer to the iris.
Outer layer is continuous with RPE from ora serrata


What are the functions of the inner nonpigmented epithelial layer of the ciliary body?

1. Secretes aqueous humor
2. Joined together by occluding zonules, forming part of the blood-aqueous barrier
3. Anchor fibers of the suspensory ligament (zonular fibers).


What are zonular fibers?

Aggregations of noncollagenous microfibrils, predominately fibrillin


What is the iris?

A thin, annular fibromuscular flap of tissue containing pigmented cells, vascularized connective tissue, and smooth muscle (part of the vascular layer, anterior to the ciliary body)


What does the free margin of the iris form?

The pupil


What covers the anterior surface of the iris?

A discontinous layer of fibroblasts and melanocytes, NOT pigmented epithelium


What is the stroma of the iris?

The central portion, consists of loose, pigmented, highly vascularized connective tissue. Predominately fibroblasts, melanocytes are responsible for eye color. The endothelial cells lining the iris vessels will have tight junctions to prevent aqueous / blood exchange.


What are the two layers of pigmented epithelium on the posterior surface of the iris?

1. Posterior / inner pigment epithelium - heavily pigmented epithelial cells, continuous with inner layer of ciliary body epithelium (which was nonpigmented)
2. Anterior / outer pigment epithelium - pigmented myoepithelial cells, continous with the outer pigmented layer of the ciliary body. Their processes form the dilator pupillae musclle


What are the two muscles of the iris? What forms them?

1. Sphincter pupillae - Pupillary constrictor muscle adjacent to the pupillary margin. Formed by circumferentially arranged fibers (helically contract)
2. Dilator pupillae - Pupillary dilator muscle, found along pigmented myoepithelial cells (anterior layer). Fibers are radially arranged so contraction shrinks the iris.


What is the cause for glaucoma involving the trabecular meshwork?

Collagen can be deposited at trabecular meshwork during aging, and may block aqueous outflow channels, producing increased IOP. Laser surgery or carbonic anhydrase inhibitors to slow aqueous humor production are current treatments.


What is the retina and where does it extend?

It is the innermost wall on the posterior 2/3 of the eye from the ora serrata to the optic disc. It is the sensory layer and is derived from brain, thus having CNS features.


What is the retinal pigment epithelium?

Simple cuboidal epithelium lying on Bruch's membrane. The pigmented cells contain melanin to absorb any light not captured by photoreceptors. Its cells also function to phagocytose the used-up tips of photoreceptors.


What are drusen?

Opaque nodules the accumulate between Bruch's membrane and the RPE. They hinder the transfer of oxygen and nutrients from the choriocapillaris to the retina, and often appear in AMD, which results in central vision loss.


What are the two parts of the photoreceptor layer?

Bordering the RPE, the outer segment captures photons to produce visual signal via G-protein coupled receptors. The inner segment stains darker because it has organelles like mitochondria


What are rods?

Photoreceptors containing rhodopsin. They provide high sensitivity but poor acuity and no color discrimination


What are cones?

Photoreceptors containing iodopsin, photopigments that provide for color vision and fine acuity.


What is the outer nuclear layer of the retina (ONL)?

The nuclei of the photoreceptors


What are synaptic / nuclear layers of the retina and how many are there?

Synaptic - plexiform layers - 2 of them
Nuclear - nuclear layers - 3 of them


What is the outer plexiform layer of the retina (OPL)?

Synapse between the photoreceptors and next layer of neurons (Generally bipolar)


What is the inner nuclear layer of the retina (INL)?

Nuclei of several cell types, mostly bipolar cells which relay signals from photoreceptors to innermost ganglion cell alyer


What is the inner plexiform layer of the retina (IPL)

Synapse between bipolar cells (and other cell types of INL) and retinal ganglion cells


What is the ganglion cell layer (GCL)?

Nuclei of ganglion cells that send their axons out into the nerve fiber layer


What is the optic nerve fiber layer (NFL)?

Contains ganglion cell axons which will form the optic nerve


What is the macula lutea / fovea centralis?

There is a depression on the visual axis from which the inner layers of the retina have been displaced to a peripheral hump (macula). The center of the depression is the fovea. Light falls directly on photoreceptors here, which is 100% cones, with straight-through neuronal connections for highest visual acuity.


What is the optic papilla?

The optic disc, where nerve fibers leave the eye through the lamina cribrosa. It is a blind spot because there are no receptors.


What is the optic nerve?

Where the ganglion cell's axons acquire myelin sheathes and run centrally with accompanying glial cells and a meningeal sheath as a CNS tract.


What runs through the intraorbital section of the optic nerve?

Retinal artery and vein, which enters and leaves the retina through the optic disc


What is the ora serrata?

Junction of retina and ciliary body, where the retina goes from multi-layered to a bi-layered ciliary epithelium. The choroid becomes the ciliary body stroma and muscle. The RPE becomes the pigmented (outer) ciliary epithelial cells. The neural retina becomes the single-layered non-pigmented inner ciliary epithelial cells.


What is the neural retina?

The many layers of retina which are inner to the RPE. Do not confuse retinal photopigments (photoreceptors) with pigment epithelium (RPE) with contains melanin


What supplies blood to the inner 2/3 of the retina?

Central retinal artery, which enters via optic disc and spreads across retinal surface. (Can be seen via ophthalmoscope). This includes the ganglion and bipolar cells


What supplies blood to the outer 1/3 of the retina?

The choriocapillaris of the choroid, which is supplied by the ciliary arteries. The includes blood supply to the photoreceptors + RPE


What is the blood-retinal barrier (inner + outer)?

Inner: Blood vessels within the retina have occluding zonules
Outer: RPE cells have occluding zonules (to prevent leakage from fenestrated capillaries of choriocapillaris). They must selectively pass materials to the retina.


What are prominent features of diabetic retinopathy?

Blood-retina barrier leakage and capillary loss


What is vasculation to the lens in postpartum humans?

Lens is avascular and aneural in post-partum humans


What is the anterior lens epithelium? What does it form?

A simple cuboidal epithelium on the lenses anterior surface. These cells on the periphery will undergo mitosis and elongate in the equatorial region and become lens fiber cells which occupy the posterior lens


What forms the lens nucleus?

Lens fiber cells elongate and approach the center, where they lose their nuclei and most organelles, but remain functional for many years.


What are three important characteristics of lens fiber cells?

1. Have extensive gap junction contacts in their plasma membranes
Maintain clarity by:
2. High concentration of proteins called lens crystallins
3. Filensin and phakinin, intermediate filament proteins which combined to form beaded filaments


What makes the lens capsule and what is its function?

Thick basal lamina of all lens cells. Anchors the zonular fibers, which will also anchor into the ciliary epithelium on the ciliary processes


What covers the external surface of the palpebrae?

Thin skin containing sebaceous glands, eccrine glands, and fine hairs. (Palpebrae = eyelids)


What is at the free margin of the eyelids?

Eyelashes, which are thick hairs


What are the three layers under the skin of the eyelids, from outside to inside?

1. Orbicularis oculi - skeletal muscle closing eyelid
2. Tarsal plate
3. Palpebral conjunctiva


What is the tarsal plate? What is the function of its product?

A connective tissue plate which contains Meibomian glands, which have their own openings on the lid margin. The oily secretion from these glands forms a hydrophobic ring around the eye to prevent loss of tear fluid (stops evaporation)


What is the palpebral conjunctiva?

A thin epithelium that is continuous with the thin skin of the eyelid. It is the opposite side of the bulbar conjunctiva after it goes through the arch (fornix) making a pocket.


What is the epithelium of the fornix and bulbar conjunctiva?

Highly unusual - stratified epithelium with goblet cells (for mucus secretion for mucus portion of tear film)


What is the lacrimal gland?

Compound tubulo-alveolar serous gland (lacking striated ducts) and heavily infiltrated with adipocytes and plasma cells.


What is the lacrimal tear fluid made of (lacrimal gland's contribution)?

Watery, contains secretory IgA and lysozyme to combat infection. This is why the lacrimal gland has plasma cells in it (to produce IgA)


What are the layers of the tear film?

From bulbar to palpebral conjunctiva
1. Mucus - from conjunctival goblet cells
2. Watery - from lacrimal gland
3. Oily film - from Meibomian gland, prevents evaporation by floating on surface


What is the outermost layer of the fovea?

The outer plexiform layer, since the fovea only contains the photoreceptors. The outermost layer is the synaptic contact between the bipolar cells and the photoreceptors.