Eye

Question 1

name the 3 layers of the eye

Answer 1

• tunica externa/fibrosa
  
  • sclera + cornea
• tunica media/vasculosa
  
  • choroid + ciliary body + iris (uvea)
• tunica interna/nervosa
  
  • retina

Question 2

describe the 3 modifications of the corneoscleral coat (part of the tunica fibrosa)

Answer 2

• outermost layer of eye
• 3 main modifications:
  
  • cornea - anterior
    
    • anterior 1/6th
    • transparent
    • convexity
  • substantia propria - middle
    
    • collagen fibers
    • vascular
  • area cribrosa - posterior
    
    • perforated for passage of optic nerve and retinal vessels

Question 3

describe the cornea

Answer 3

• forms the ant. surface of the eye in an area largely corresponding to the pigmented iris
• avascular
• transparent
• richest sensory nerve supply

Question 4

describe the epithelium and stroma of the cornea

Answer 4

• anterior corneal epithelium
  
  • stratified squamous epi.
  • microvilli on most superficial cells (vermiform ridges)
  • basal cells mitotic activity
  • regeneration ~7 days
  • free nerve endings
• Bowman’s membrane
  
  • homogenous collagen fibers
  • provides stability and strength
  • does not regenerate
• corneal stroma
  
  • oriented at right angles to each other
  • fibroblasts - keratocytes - flattened end to end
  • keratin and chondroitin sulfates

Question 5

Answer 5

Question 6

describe the endothelium of the cornea

Answer 6

• Descement’s membrane
  
  • homogenous collagen filaments
  • meshwork of collagen
  • fenestrations
• posterior corneal endothelium
  
  • simple squamous cells
  • abundant Na pumps
  • mantains optimum hydration of corneal stroma

Question 7

describe the sclera

Answer 7

• dense CT: extension of dura
• collagen (type I) + elastic fibers
• 3 layers:
  
  • episcleral layer
  • substantia propria
  • suprachoroidal lamina
• Tenon’s space: between episcleral layer and substantia propria
• site of attachment of the ocular muscles
• posteriorly - area cribrosa
  
  • perforated for the exit of optic nerve fibers and entry of central retinal vessels

Question 8

describe the choroid

Answer 8

• function:
  
  • absorption of stray light
  • blood retinal barrier
• choroid proper:
  
  • presence of large blood vessels
  • melanocytes
• choroid capillary layer
  
  • fenestrated capillaries
• Bruch’s membrane:
  
  • between choroid and retinal pigment layer
  • component of blood retinal barrier

Question 9

Answer 9

Question 10

describe the ciliary body

Answer 10

• consists of:
  
  • ciliary muscles
  • ciliary processes
• ciliary muscle: smooth muscle
  
  • orientation of muscles - radial and circular
  • contraction and relaxation changes the shape of lens (bulges and flattens)
  • near and distant vision - accomodation
  • innervated by ciliary nerves

Question 11

descirbe ciliary processes

Answer 11

• functions:
  
  • provides attachment to the capsule of lens by means of zonule fibers (anchorage of lens) called ligaments of Zinn
    
    • zonule fibers - elastic (oxytalan) fibers
  • secretion of aqueous humor (non-pigmented epithelium)
  • characterized by the presence of fenestrated capillaries

Question 12

Answer 12

Question 13

Answer 13

Question 14

describe the anterior surface of the iris

Answer 14

• irregular contour
• lined by epithelial cells, stellate fibroblasts and stellate melanocytes
• stroma - collagen, fibroblasts and melanocytes

Question 15

describe the posterior surface of the iris

Answer 15

• lined by 2 layers of epithelium back to back
  
  • inner (posterior) epithelium: pigmented (continuation of retinal pigmented epithelium)
  • outer epithelium: pigmented myoepithelial cells (which make up dilator pupillae muscle)

Question 16

describe the drainage of aqueous humor

Answer 16

sclerocorneal junction

• spaces of Fontana -> canal of Schlemm -> aqueous humor is drained into ciliary veins

Question 17

describe the retina (tunica nervosa)

Answer 17

• derived from ectoderm
• 2 basic layers:
  
  • outer retinal pigment epi. (non-sensory)
  • inner sensory retina or neural retina
• junction of these 2 = ora serrata (runs around the periphery)

Question 18

describe the pigmental epithelial layer of the retina

Answer 18

• single layer of cuboidal cells
• synthesize melanin
• adjacent cells - connected by a junctional complex
• abundant microvilli which surround and phagocytose shed discs of rods and cones
• establish blood retinal barrier
• esterification of vit. A

Question 19

describe the outer vs inner segment of photoreceptors

Answer 19

• outer segment:
  
  • light sensitive photoreceptor discs
  • the flattened membranous disk contains the photosensitive pigments
  • tips that contain disks are embedded within microvilli of pigment epithelial cells
• inner segments:
  
  • cell organelles
  • metabolic processing

Question 20

describe rods

Answer 20

~120 million

• cylindrical outer segment
• the flattened disks - visual pruple or rhodopsin (opsin -scotopsin)
• opsin + chromatophore (vit. A derived carotenoid -> retinal)
• activated when low levels of light are encountered
• black and white vision

Question 21

describe cones

Answer 21

~7 million

• thicker and shorter
• conical outer segment
• discs - contain pigment iodopsin (opsin - photopsin)
• sensitive only to light - better visual acuity

Question 22

describe the bipolar cells of the retina

Answer 22

• extend between inner and outer plexiform layers
• axons synapse with several ganglion cells
• this helps in communication with multiple cells
• in fovea, one bipolar to one ganglion cell -> increase visual acuity

Question 23

describe the layers of the retina

Answer 23

Question 24

describe the blood supply to the retina

Answer 24

• chorio capillaries supply layers 1-5 by diffusion
• central retinal artery supplies layers 6-10 as capillaries

Question 25

describe the optic disc

Answer 25

• site where optic nerve emerges from the retina
• it is also called as the blind spot since it is devoid of rods and cones

Question 26

Answer 26

Question 27

describe the macula lutea and fovea centralis

Answer 27

• macula lutea
  
  • center of the posterior retina at the visual axis of the eye
• fovea centralis
  
  • small depression in the center of the macula lutea
  • area of the highest visual acuity
  • contains only cones

Question 28

Answer 28

Question 29

describe the 3 components of the lens

Answer 29

• lens capsule
  
  • thick basal lamina
  • produced by anterior lens cells
• subcapsular epi.
  
  • single layer of cuboidal epi. cells
  • found only on the ant. surface of the lens
  • source of lens fibers
• lens fibers
  
  • thin flattened structures
  • lose the organelles and accumulate crystallins (proteins)

Question 30

describe the vitreous body

Answer 30

• located between lens and retina
• viscous translucent material
• spindle-shapred cells = hyalocytes
• fine collagen fibrils in the matrix
• extracellular matrix rich in GAGs, especially hyaluronic acid

Question 31

describe the glands of the eyelid

Answer 31

• Meibomian glands
  
  • sebaceous within tarsal plate
  • lubricates - delays drying of tears
• Glands of Zeiss
  
  • sebaceous glands within the eyelids
• Glands of Moll
  
  • sweat glands at the lid margins

Question 32

describe an advantage of corneal transplant

Answer 32

avascular feature of cornea means minimal immunological response

Question 33

describe formation of cataracts

Answer 33

• the lens is mostly made of water and protein
  
  • the protein, crystallin, is arranged in a precise way that keeps the lens clear
  • aging leads to clumping of the protein -> cloudy appearance

Question 34

name the most common location of retinal detachment

Answer 34

most common detachment occurs at the junction of pigment epithelial layer and the rods & cones layer