TBL29 - Eye

Question 1

Where are the optic vesicles found? What forms the optic vesicles?

Answer 1

1) The optic vesicles are found on the sides of the forebrain
2) Bilateral projections from the neuroectoderm of the diencephalon form the optic vesicles

Question 2

After contacting the surface ectoderm, what happens to the distal ends of the optic vesicles and what forms the lens placodes? What does invagination of the optic vesicles form and what does invagination of the lens placodes form?

Answer 2

1) After contacting the surface ectoderm, the distal ends of the optic vesicles invaginate and thickening of the surface ectoderm forms the lens placodes
2) Invagination of the optic vesicles forms the double-layered optic cups and invagination of the lens placodes forms the lens vesicles

Question 3

What do the optic cups remain attached to and by what? What forms the choroid fissure? What artery courses within the fissure to the lens vesicle?

Answer 3

1) The optic cups remain attached to the forebrain by the optic stalks
2) Invagination of the optic stalks and optic cups forms the choroid fissure
3) The hyaloid artery courses within the fissure to the lens vesicle

Question 4

What does mesenchyme invade the inside of the optic cups via and what does it convey to the developing lens? After the branches to the lens obliterate and disappear, what is the mesenchyme filled with?

Answer 4

1) Mesenchyme, which invades the inside of the optic cups via the choroid fissure, conveys branches of the hyaloid artery to the developing lens
2) After the branches to the lens obliterate and disappear, the mesenchyme is filled with a transparent gelatinous substance designated the vitreous body

Question 5

What does the outer layer (furthest from center) of the optic cups become and what does neuronal growth in the inner layer of the optic cups form? What do axons from the neural retina form and what does this nerve occupy and enclose?

Answer 5

1) The outer layer (furthest from center) of the optic cups becomes the thin pigmented layer of the retina and neuronal growth in the inner layer of the optic cups forms the thick neural layer of the retina
2) Axons from the neural retina form the optic nerve (CN II) that occupies the choroid fissure of the optic stalk and encloses the hyaloid artery

Question 6

What completes transformation of the optic stalk into the optic nerve and what does this nerve enclose?

Answer 6

Fusion of the choroid fissure completes transformation of the optic stalk into the optic nerve, which encloses the central artery of the retina, a remnant of branches of the hyaloid artery

Question 7

External to the optic cups, mesenchyme differentiates into what two layers and what is the outer layer continuous with posteriorly? What is the sclera continuous with anteriorly?

Answer 7

1) External to the optic cups, mesenchyme differentiates into an inner (closest to pigmented retinal layer) vascularized layer designated the choroid and an outer fibrous sclera, which is continuous posteriorly with cranial dura that covers the optic nerve
2) The sclera is continuous anteriorly with the cornea

Question 8

Anteriorly, what does the choroid terminate as and what is this structure continuous with anteriorly? At the ciliary body, what is the neural layer of the retina reduced to and what does this layer form?

Answer 8

1) Anteriorly the choroid terminates as the ciliary body, which is continuous anteriorly with the iris
2) At the ciliary body, the neural layer of the retina is reduced to an inner nonpigmented epithelial layer that, with the pigmented layer of the retina, forms the double-layered epithelium of the ciliary body and iris

Question 9

What forms the pupils?

Answer 9

Closure of the choroid fissure in the optic cups forms the pupils

Question 10

What defect causes Coloboma iridis?

Answer 10

Coloboma may occur if the choroid fissure fails to close

Question 11

What marks the exit of CN II from the posterior eyeball? What encloses the central retinal artery? What is the neural retina supplied by?

Answer 11

1) The optic disc marks the exit of CN II from the posterior eyeball
2) The optic nerve encloses the central retinal artery
3) The neural retina is supplied by branches of the artery

Question 12

What are the labeled arterioles branches of? What do these branches supply? What do the ciliary artery and the central retinal artery arise from and what is this artery the main derivative of?

Answer 12

1) The labeled arterioles (ignore names) are branches of the ciliary artery
2) The branches supply the choroid and sclera
3) The ciliary artery and the central retinal artery arise from the ophthalmic artery, the main derivative of the hyaloid artery

Question 13

What does the neural retina contain and what are all of these structures linked by? What do light impulses pass through before reaching the photoreceptors?

Answer 13

1) The neural retina contains outer photoreceptors (rods and cones), central bipolar cells, and inner ganglion cells, all linked by serial synapses
2) Light impulses pass through the ganglion and bipolar layers before reaching the photoreceptors

Question 14

What do light-sensitive segments of the photoreceptors adjoin, but lack? What do rods produce visual images in shades of and what do they outnumber the three types of cones with and by?

Answer 14

1) Light-sensitive segments of the photoreceptors adjoin the retinal pigmented epithelium but lack secure junctional attachments
2) Rods produce visual images in shades of gray from dim light perception, and outnumber the three types of cones with selective sensitivities to blue, green, and red wavelengths by 15:1

Question 15

What are common causes of retinal detachment and what has improved its prognosis?

Answer 15

1) The neural retina and REP (retinal pigmented epithleium) may separate if fluid (such as vitreous, blood, or exudate) accumulates in the potential space
2) Retinal detachment may occur in diabetic retinopathy and intraocular infection
3) Laser treatment has greatly improved the prognosis for this condition

Question 16

What is retinitis pigmentosa and how can its progression be slowed?

Answer 16

1) A group of genetic diseases, called retinitis pigmentosa (RP), leads to progressive visual loss
2) In one RP subtype, an amino acid alteration in rhodopsin leads to photoreceptor cell death and hypertrophy or atrophy of RPE
3) Patients have problems with night vision, which progresses to tunnel vision and loss of visual acuity and color discrimination
4) High daily doses of antioxidants such as vitamin A may help slow disease progression

Question 17

Why is the optic disc the blind spot of the retina?

Answer 17

The optic disc is called the blind spot of the retina because it lacks photoreceptors and is insensitive to light

Question 18

Where is the fovea centralis located in respect to the retina?

Answer 18

The fovea centralis is located in the posterior aspect of the retina

Question 19

Why is the fovea the site of highest visual acuity?

Answer 19

The fovea is the site of highest visual acuity because ganglion and bipolar cells are compressed to the periphery and the remaining photoreceptors are all cones

Question 20

What holds the lens in a fairly fixed position? What regulates tension on the zonular fibers, where do these fibers extend from and to, and what do they regulate?

Answer 20

1) The zonular fibers and vitreous body hold the lens in a fairly fixed position
2) Smooth muscle in the ciliary body regulates tension on the zonular fibers that extend from the ciliary processes to the lens and thereby regulate lens thickness

Question 21

What are the ciliary body and ciliary processes covered by? Where does the epithelial bilayer continue onto?

Answer 21

1) The ciliary body and ciliary processes are covered by the double layer of pigmented and nonpigmented epithelium
2) The epithelial bilayer continues onto the iris

Question 22

What does the nonpigmented layer secrete into the posterior chamber? Where does this substance circulate around into the anterior chamber? From the anterior chamber, where does aqueous humor drain into and where does this structure empty into?

Answer 22

1) The nonpigmented layer secretes aqueous humor (similar to CSF) into the posterior chamber (i.e. posterior to the iris)
2) Aqueous humor circulates around the pupillary aperture into the anterior chamber
3) From the anterior chamber, aqueous humor drains into the canal of Schlemm that empties into venules of the sclera

Question 23

How can glaucoma lead to blindness?

Answer 23

1) The dense connective tissue of the sclera and cornea becomes more fibrous with age, and obstruction of the canal of Schlemm may lead to glaucoma, a common condition involving abnormally increased intraocular pressure
2) Untreated, it leads to visual impairment and blindness

Question 24

What two muscles reside in the stroma of the iris? What determines eye color?

Answer 24

1) The pupillary dilator and constrictor muscles reside in the stroma of the iris
2) Relative numbers of melanocytes in the stroma determine eye color

Question 25

What types of epithelium cover the anterior and posterior surfaces of the cornea? What are the corneal epithelia and avascular stroma (dense connective tissue) nourished by?

Answer 25

1) Nonkeratinized stratified squamous epithelium and simple squamous epithelium cover the anterior and posterior surfaces of the cornea, respectively
2) The corneal epithelia and avascular stroma (dense connective tissue) are nourished by the aqueous humor

Question 26

Why is the cornea a good candidate for transplantation?

Answer 26

Being avascular, the cornea is immunologically privileged and a good candidate for transplants

Question 27

What is the lens surrounded by and what is this structure an attachment site for?

Answer 27

The lens is surrounded by a capsule that is the attachment site for the zonular fibers

Question 28

Where does the single layer of lens epithelium reside inside of? What does this epithelium generate and what makes up these structures?

Answer 28

1) The single layer of lens epithelium resides inside the capsule
2) The epithelium generates lens fibers, which are elongated, anucleate epithelial cells with distinct cytoplasmic proteins called crystallins

Question 29

What part of the lens does epithelium only reside along? What does the embryonal zone represent? Where do lens fibers develop from?

Answer 29

1) Epithelium only resides along the anterior surface of the lens
2) The embryonal zone represents the earliest formed lens fibers
3) Lens fibers develop in successive waves from the embryonal zone through adulthood