NB8-1 - Special Senses - Eye

Question 1

What are the 3 walls of the eye and their alternate names? What do these layers primarily consist of?

Answer 1

• Tunica Externa, aka Tunica Fibrosa, aka Corneoscleral Coat which consists of the cornea, sclera, and area cribrosa
• Tunica Media, aka Tunica Vasculosa, aka Uvea which consists of the choroid, ciliary body, and iris
• Tunica Interna, aka Tunica Nervosa which consists of the retina

Question 2

Describe the two major layers of the retina and their distribution.

Answer 2

The retina consists of two primary layers. The more superficial layer consists of nervous tissue and extends only up to the choroid. The deeper layer is a pigmented epithelial layer that extends to the tip of the iris

Question 3

Answer 3

Question 4

List the chambers of the eye and their boundaries

Answer 4

• Vitreous Chamber - bounded anteriorly by the lens and ciliary bodies; bounded everywhere else by the retina
• Posterior Chamber - bounded anteriorly by the iris and posteriorly by the lens
• Anterior Chamber - bounded anteriorly by the cornea and posteriorly by the iris

Question 5

Describe the blood supply to the eye including clinically relevant information.

Answer 5

There are two circulations of the eye: the retinal and uveal, supplied in humans by posterior ciliary arteries, originating from the ophthalmic artery. The arteries of the uveal circulation, supplying the uvea and layers 1-5 of the retina and DO NOT enter the eyeball via the optic disk. The retinal circulation, on the other hand, derives its circulation from the central retinal artery, also a branch of the ophthalmic artery, which enters the eye via the optic disc alongside the optic nerve. The retinal circulation does not anastomose so damage to it often leads to retinal damage

Question 6

Describe the cellular and matrix composition of the cornea. Include any clinically releveant information.

Answer 6

The cornea consits of 5 layers:

1. Anterior corneal epithelium consisting of stratified squamous epithelium whose most superficial cells contain microvilli and most basal cells appear cuboidal/columnar and are mitotically active (7 day regeneration). This layer contains free nerve endings
2. Bowman’s membrane consisting of homogenous collagen fibers that provide stability and strength and does not regenerat__e.
3. Corneal stroma consisting of many lamellae of collagen fibers oriented at right angles to each orther interspersed with keratan and chondroitin sulphates. The keratocytes (fibroblasts) are flattened and span from one end of the cornea to the other between the lamellae, every keratocyte is oriented at a right angle to the next/previous lamellae’s keratocyte.
4. Descemet’s membrane consists of a meshwork of homogenous collagen filaments with fenestrations that allow fluid from the stroma to pass through to the endothelium
5. Posterior corneal endothelium consists of simple squamous cells which have abundant Na/K ATPases used to pump water out of the stroma and into the anterior chamber to maintain ideal stromal hydration. Refer to image

Question 7

What is and what causes a corneal opacity?

Answer 7

A corneal opacity is scar tissue creating an opaque region in the cornea. It is caused by damage to Bowman’s membrane (because it cannot regenerate)

Question 8

Describe the cellular and matrix composition of the sclera. Include any clinically relevant information.

Answer 8

The sclera is a dense connective tissue extension of the dura and it consists of 3 layers and a space:

1. Episcleral layer consists of type 1 collagen and elastic fibers
2. Tenon’s space is the space between the episcleral layer and the substantia propria. It contains large blood vessels and is where extraocular muscles attach to the eyeball.
3. Substantia propria, aka sclera proper or tenon’s capsule, consists of predominantly type 1 collagen
4. Suprachoroidal lamina lays just superficial to the choroid and consists of

At the posterior end there is an opening known as the area cribrosa which is where the opic nerve fibers and retinal vessels enter/exit the eye.

Question 9

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Question 10

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Question 11

What common features do all the components of the tunica vasculosa have?

Answer 11

1. Pigmented cells
2. Loose connective tissue (more cells than fibers)
3. Vascularization

Question 12

What layer is the choroid a part of? Where is it within the eye? Describe its cellular and matrix composition. Include any clinically relevant information.

Answer 12

The choroid is part of the tunica vasculosa (uvea) and it is situated right between the sclera and retina. The choroid consists of three prinicipal layers:

1. Choroid proper (closest to the sclera) which contains large branching vessels, loose connective tissue, and melanocytes that serve to absorb light the retinal cells do not absorb.
2. Chorio-capillary layer which contains fenestrated capillaries (from the vessels of the coroid proper)
3. Bruch’s membrane which is a connective tissue layer between the choroid capillary layer and the retinal pigment layer, serving as a blood-retinal barrier

Question 13

Describe the cellular and matrix composition of the ciliary body. Include any clinically relevant information.

Answer 13

Ciliary body is composed of two parts:

1. Ciliary muscles which are smooth muscles oriened radially and circularly so that when they contract the lens bulges and when they relax the lens flattens Innervated by ciliary nerves
2. Ciliary processes are ridge like extensions from the ciliary muscles that provide attachments for zonule fibers (ligaments of Zinn) that attach to the capsule of the lens. These processes also secrete aqueous humor which comes form the fenestrated capillaries within the processes

Question 14

Describe the path of aqueous humor flow.

Answer 14

Aqueous humor is secreted by the inner epithelium of the ciliary body (ciliary processes) into the posterior chamber which will then pass through the pupil into the anterior chamber and drain into the canal of schlemm via the iridocorneal junction (space of fontana). From there is will drain into the ciliary veins.

Question 15

What layer is the iris a part of? Where is it within the eye? Describe its cellular and matrix composition. Include any clinically relevant information.

Answer 15

The iris is part of the uvea and is the anterior extension of the choroid. It is located just anterior to the lens and it forms a diaphragm with an aperture in the center (pupil).The anterior surface of the iris has an irregular contour lined by epithelial cells, stellate (star shaped) fibroblasts, and stellate melanocytes. The stroma consists of loose connective tissue, fibroblasts, melanocytes, blood vessels, and the pupilary muscles.The pupilary dilator muscles run almost the entire length of the iris. The pupillary sphincter muscles are also within the iris but form a circle right around the pupil. When they contract the pupils constrict. The posterior surface is lined by two layers of epithelium back to back - an inner (posterior) epithelium that is a continuation of the retinal pigmental epithelium, and an outer pigmented myoepithelial layer which forms the pupilary dilator muscles.

Question 16

Answer 16

Question 17

What types of innervation cause pupil dilation and constriction?

Answer 17

PSNS innervation causes pupil constriction and SNS innervation causes pupil dilation

Question 18

What is the name of the boundary where the retinal pigment epithelium transitions into the inner epithelium of the iris?

Answer 18

The ora serrata

Question 19

What layer of the eye is the retina a part of? What germ layer is the retina derived from? List the layers of the retina?

Answer 19

The retina is the tunica nervosa and it is derived from Ectoderm. It consists of two basic layers with the inner layer containing 9 more layers:

• Outer retinal pigment epithelium which is non sensory
• Inner sensory retina, aka neural retina which consists of:
  
  • Outer segment of photoreceptors layer
  • Outer limiting membrane (outer limit of Muller cells)
  • Outer nuclear layer (inner segment of photoreceptors)
  • Outer synaptic layer, aka outer plexiform layer
  • Bipolar cell layer, aka inner nuclear layer
  • Inner synaptic layer, aka inner plexiform layer
  • Ganglion cell layer
  • Optic nerve fiber layer
  • Inner limiting membrane (inner limit of muller cells)

Question 20

Describe the cells of the retinal pigment epithelium and their functions.

Answer 20

The retinal pigment epithelial (RPE) layer consists of cuboidal cells with abundant microvilli and joined by tight junctions. These cells perfrom many functions:

• Establish blood retinal barrier
• Synthesize melanin
• Phagocytose photobleached discs from the rods and cones
• Esterify vitamin A

Question 21

Describe the cells of the retinal photoreceptor layer and their function

Answer 21

The photoreceptor layer consists of two types of photoreceptors - rods and cones. Each of these cells consists of two general segments:

• An outer segment which contains flattened membranous disks containing a photosensitive pigment - rhodopsin in rods (black and white vison and sensitive to low levels of light) and iodopsin in cones (color vision and sensitive to higher levels of light). This outer segment is partially embedded in the microvilli of the RPE cells
• An inner segment, aka outer nuclear layer, that contains the cell organelles and is where metabolic processing occurs

Question 22

Describe the cells of the bipolar cell layer and their function.

Answer 22

This layer consists of the cell bodies of bipolar neurons, association neurons, and supporting cells.

• Bipolar neurons synapse with photoreceptors in the outer synaptic layer and with ganglion cells in the inner synaptic layer.
• Associatoin neurons, horizontal and amacrine cells, synapse with many photoreceptors and the bipolar cells and serve to integrate information.
• Supporting cells, Muller’s cells, span from the outer edge of the inner segment of the photoreceptors (outer limiting membrane) to the optic nerve fiber layer (inner limiting membrane) and serve to support the cells within.

Question 23

What are the macula lutea and fovea centralis

Answer 23

The macula lutea is the center of the posterior retina at the visual axis of the eye it contains a small depression known as the fovea centralis which contains the highest concentration of cones in the retina and does NOT contain rods. The fovea is the area of highest visual acuity.

Question 24

Describe the structure, composition, and function of the lens and its components.

Answer 24

The lens has three components:

1. The lens capsule which is a thick basal lamina produced by anterior lens cells and serves to encapsulate the rest of the lens and act as an anchoring point for the zonule fibers
2. The subcapsular epithelium which is a single layer of cuboidal epithelial cells found ONLY on the anterior surface of the lens and serves to secrete lens fibers
3. The lens fibers which are thin flattened cells completely devoid of organelles and filled with crystallins (proteins)

Question 25

Describe the composition and function of the vitreous body.

Answer 25

The vitreous body is the viscous translucent material that fills the vitreous chamber. Is consits of a fine collagen fibril matrix rich in GAGs, especially hyaluronic acid (which holds water very well). Spindle shaped cells called hyalocytes float in the vitreous body and secrete the matrix. The vitreous body serves to give the eyeball its shape.

Question 26

What is conjunctiva? Where is it found and what does it do?

Answer 26

The conjuctiva is an epithelial layer rich in goblet cells. It lines the inside of the eyelids and the visible parts of the sclera. It’s primary purpose is to secrete mucin for lubrication.

Question 27

List and describe the function of the many glands of the eyelid.

Answer 27

Meibomian glands - long sebaceous glands located just superior to the conjunctiva which secretes sebum to lubricate and delay the drying of tears

Glands of Zeiss - small sebaceous glands at the root of the eyelashes which secrete sebum to lubricate the follicle

Glands of Moll - sweat glands at the lid margins

Refer to image

Question 28

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Question 29

Label image and say from which tissue this sample was taken.

Answer 29

Cornea

Question 30

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Question 31

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Question 32

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Question 33

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Question 34

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Question 35

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Question 36

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Question 37

Label all of the relevant layers.

Answer 37

Question 38

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Question 39

Answer 39

3

Question 40

What feature of the cornea is useful for transplants and why?

Answer 40

Corneal tissue is avascular which means there is no rejection and very little bleeding involved in a transplant

Question 41

Where is the incision made during a corneal transplant?

Answer 41

As wide out as possible without contacting the corneoscleral junction

Question 42

What is a cataract? What causes it? How is it treated?

Answer 42

If the crystallin protein within the lens begins to crosslink with itself, the lens will become cloudy and interfere with vision (cataract). This is usually caused by ageing but can also be caused by metabolic disorders, congenital disorders, or overuse of corticosteroids. It is treated by removing all of the protein from within the lens capsule and replacing it with artificial lens material.

Question 43

What is glaucoma? What causes it and what are its signs and symptoms?

Answer 43

Glaucoma is increased intra ocular pressure typically causes by a blockage of the drain pathway for aqueous humor. The aqueous humor will then collect around the eyeball and press it into the optic nerve, compressing it. Signs and symptoms include headache, eye pain, loss of vision, vomiting, light halos, tunnel vision, and redness.

It is most easily diagnosed by using a tonometer to measure the intraocular pressure.

Question 44

Explain the most common cause of night blindness.

Answer 44

Vitamin A deficiency.

Rods (black/white and night vision) utilize the pigment protein rhodopsin which is composed of the reuseable scotopsin protein and retinal which is derived from Vitamin A and has to be frequently replaced due to photobleaching. Therefore, a Vitamin A deficiency leads to Rod insensitivity and night blindness.

Question 45

When retinal detachment occurs, which layers usually detach and what usually causes it?

Answer 45

The most common retinal detachment site is between the RPE and the outer segment of the photoreceptor layer. This is most commonly caused by cataract surgery, trauma, extreme dehydration (vitreous body shrinks), and retinal tears

Question 46

What is the defect?

Answer 46

Central Retinal Artery Occlusion

Question 47

What is the defect?

Answer 47

Retinal detachment

Question 48

What is the defect?

Answer 48

Increase intraocular pressure (possibly glaucoma)