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Ocular Biology > Cornea Anatomy > Flashcards

Cornea Anatomy Flashcards

1
Q

Corneal Functions*

A
  1. Transparent: smooth optical surface, avascular
  2. Major refractive surface: 2/3 refractive power of eye
  3. mechanical strength - collagen: protection, maintain ocular contour
  4. barrier - ocular biodefense system
  5. Protection: highly innervated, barriers, and mechanical strength
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2
Q

corneal-scleral junction is called ______

A

aka limbus

richly vascularized transition zone

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3
Q

cornea radius of curvature

A

anterior surface: 7.8 mm (steep center, flatter peripheral)

post sufrace: 6.5 mm (steeper than anterior)

optical zone: 4.mm

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4
Q

cornea refractive power

A

~43 D

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5
Q

Cornea diameter?*

A

anterior surface - oval: 10.6 mm vertical and 11.7 mm horizontal

posterior surface - round: 11.7 mm around

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6
Q

megalocornea and microcornea

A

megalocornea: cornea diameter > 13 mm
microcornea: cornea diameter < 10 mm

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7
Q

Thickness of cornea*

A

center average: 544 +/- 34 um

periphery: 670 um

hydration determines thickness of cornea

thickness fluctuates on diurnal basis -> AM thicker

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8
Q

Layers of the cornea*

A
  1. Epithelium
  2. Bowman’s membrane
  3. Stroma
  4. Decemet’s membrane
  5. Endothelium

(mnemonic: erect penises blasting organisms stopped the doctors exam needlessly)

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9
Q

Epithelium*

A

non-keratinized stratified squamous (4-6 layers thick)

superficial cells exfoliate (7-10 day lifespan)

barrier function and light transmission

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10
Q

Epithelium - squamous layer*

A

terminally differentiated cells: 2-3 layers thick

no mitosis

less active metabolically: fewer organelles

most superficial (slough off 7-10 days)

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11
Q

epithelium squamous layer surface modifications*

A

microplicae and microvilli

glycocalyx: interacts with mucin layer of tear film
- maintain hydrophilic properties of epithelium, enhance tear film stability, barrier against pathogens

barrier: tight jxn, desmosomes, gap jxn
- anterior corneal barrier, lost when cell exfoliate

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12
Q

fluorescein dye

A

evaluate barrier jxn,

focal epithelial defects allow penetration of dye: spk (superficial punctate keratitis)

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13
Q

epithelium - wing cells

A

extensive interdigitations: desmosomes, gap jxns,

highly innervated

2-3 cell layers

cytokeratins: abundant intermediate filaments
- influences shape of wing cells

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14
Q

epithelium - basal cells*

A

single layer

desmosomes

mitotically active: originate from stem cells, transient amplifying layer

metabolically active: glycogen granules, metabolic pumps, secretes basal lamina

dendritic cells and lymphocytes

adherence of epithelial cells (hemidesmosome send anchoring fibers to lamina densa of BM) – essential to wound healing

hemidesmosomes, anchoring fibrils (7): penetrate to stroma, anchoring plaques

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15
Q

epithelium - dendritic langerhans cells

A

antigen-presenting cells:
immune response cells in cornea

numerous in periphery of cornea - “professional”

central cornea - “immature”

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16
Q

epithelial regeneration*

A

constantly state of turn-over with exfoliating apical cells being replaced by underlying cells in a weekly basis

basal cells are only epithelial cells capable of mitosis

during normal apical cell exfoliation basal cell proliferates and replace lost cells in 7-14 days

loss of basal cells and defective regeneration will lead to corneal scar formation

17
Q

how is BM connected to bowman’s layer*

A

2 part process: cell attached to BM and BM to bowman’s to secure epithelium to stroma

anchoring fibrils and anchoring plaques running from lamina densa to bowman’s layer

18
Q

Bowman’s layer*

A

anterior limiting lamina

no cells, no orgnization to fibrils, and thin

8-14 um thick

randomly arranged, small diameter collagen: 1,3,5,6
-not ordered into bundles - dense fibrous sheet

acellular - modified condensation of the anterior stroma

secured to stroma -> hard to penetrate and prevents shearing
-provides shears strength (stromal resistance to sublayer sliding)

barrier

striated collagen fibrils from anterior and sometimes deeper stroma become contiguous with bowman’s layer
-anchor bowman’s layer to stroma: strong attachment

bowman’s layer can not be stripped away from cornea

19
Q

stroma

A

made of collagen (lamallae), fibroblasts (keratocytes), corneal connective tissue (substantia propia)

20
Q

stroma lamallae*

A

sheets of collagen fibers

~200-300 lamellae

unique arrangement of collagen fibrils in adjacent lamellae (more disorganized anteriorly and peripherally)

extend limbus to limbus

tensile strenght

stable shape

transparency importance

collagen: type 1 and type 5 collagen heterodimers

collagen fibers are uniform and regular in diameter

distance between fibers is consistent (fibrils surrounded by EC matrix)

21
Q

stroma keratocytes*

A

fibroblasts that sandwich between lamallae

  • connected to each other by gap jxn and tight jxn
  • form a fxnal synctium: (communicating network) tight junction, gap junction

majority in intralaminar space

sig. degree of mobility when activated: can migrate to wound margins and synthesize collagen GAGs, and MMPs

synthesize intracellular crystallins

some leucocytes also found in stroma

22
Q

circumcorneal annulus

A

approaching limbus (1.5 - 2.0 mm wide)

believed to maintain curvature of cornea

collagen fibrils within a lamellae turn and run circumferentially around the cornea

23
Q

Stroma - non collagenous proteins *

A

hydrophilic mucopolysaccharide in ground substance

  • proteoglycans: protein with GAG side chain
  • highly negatively charged
  • surround collagen fibrils to create uniform spaces between fibrils

corneal crystallin in keratocytes

24
Q

stroma - transparency of cornea*

A

regularity of collagen fibers thickness

regularity of distance between collagen fibers

lamellar arrangement in stroma (any disruption (edema, scarring) loss of transparency)

25
Q

Stroma anterior 1/3 *

A

collagen fibrils are thicker than in bowman’s layer, but thinner than posterior stroma

lamellae cross at oblique angles (more disorganized)

lamellae are thin

highest density of keratocytes

contribute to shear resistance

less of a tendency to accumulate water than posterior stroma

26
Q

stroma posterior 2/3*

A

collagen fibers thicker

precise arrangement of lamellae (orthogonal)

provides tensile strength

greater tendency to accumulate water more anterior stroma

27
Q

descemet’s membrane*

A

basal lamina of endothelium
10-15 um thick

anterior layer (banded): lattice of precisely arranged sheets - does not change in thickness

posterior layer (non-banded): less ordered arrangement, type 4 collagen, laminin, fibronectin, secreted thorughout life, provides strong barrier to infection and perforating injury

connected to endothelium by hemi-desmosomes, but not strongly attached to stroma

28
Q

endothelium*

A

monolayer of hexagonal cells (400k - 500k)

metabolically active: ion transport systems, synthesize Descemet’s

no mitosis; does not divide in vivo

thickness: 10um birth; 4uq adult

consumes a lot of energy (2nd to photoreceptors in the eye)

pinocytosis (vesicles)

microtubules

mitochondria

lateral interdigitation

desmosomes, gap jxn

focal tight jxn “leaky” -> allow duffusion of glucose and nutrients from aqueous

overlapping marginal folds

club-like microvilli

29
Q

cell junctions

A

zonula occludens: claudins, occludens, actin, protein ZO-1
-transmembrane protein complex determines tightness of jxns; the more the tighter

zonula adherens: cadherin E

macula adherens: attachment plaques, proteins, keratin fibers

gap jxn: cell to cell communication, connexion proteins

30
Q

endothelium - cell density*

A

cell density changes with age

birth 5k - 6k cells/mm2
child 3k - 4k cells/mm2
adult 1.4k - 2.5k

critical cell density for normal endothelial fxn: 400 - 700 cell/mm2

density is greatest in periphery then paracentral, and finally central

31
Q

endothelium - cell density dysfxn*

A

polymegthism - different sizes

pleomorphism - different shapes

guttata - endothelium thinning over thickened descemet membrane in the center

32
Q

cornea innervation*

A

sensory cholinergic innervation

long posterior ciliary nerves responsible for sensory innervation

limbal (pericorneal) plexus - radial arrangement

70-80 large nerve trunks; loss of myelin 1-2 mm within cornea, rich afferent innervation, 300-400x epidermis

33
Q

cornea innervation - plexi of nerve fibers *

A
  • limbal plexus
  • stromal plexus
  • sub-epithelial (beneath bowman’s layer) - loss of schwann cell covering -> naked nerve ending -> highly sensitive to stimuli
  • sub basal plexus: between basal cell and basement membrane
34
Q

cornea innervation details*

A

overlap in innervation
-sensitive to small stimuli, approximate sense of spatial location of stimulus, greatest sensitivity –central cornea (has more free nerve endings)

little innervation in posterior stroma

no innervation in descemet’s membrane and endothelium

respond to mechanical, thermal, and chemical stimuli

  • some fibers are stimuli specific but majority are polymodal
  • primarily perceived as pain
  • neurotrophic fxn
  • inflammatory response: sustained pain

sympathetic stimulation may inhibit epithelial wound healing

35
Q

cornea innervation - neurotrophic fxn*

A

sensory afferent innervation maintains normal epithelial structure and fxn particularly the epithelium

cornea sensory nerves release:
neuropeptides (substance P), growth factors (insulin like growth factor and epidermal growth factor), induce survival development and fxn of neurons

loss of corneal sensation: destruction of normal corneal integrity – epithelial breakdown

neutrophic keratitis: dec. corneal sensation, persistent epithelial defects, delayed epithelial wound healing, ulcers infections melting perforation

dec. corneal sensation: acoustic neuroma, viral infections - herpes, diabetes mellitus

36
Q

cornea innervation - reflex blinking

A

stim. of corneal nerves -> reflex arc

afferent pathway CN 5
efferent pathway CN 7

37
Q

cornea innervation - reflex tearing

A

stimultion of corneal nerves -> reflex arc

parasympathetic of lacrimal gland

38
Q

cornea blood supply*

A

arteries:
ophthalmic -> muscular -> anterior ciliary -> episcleral -> conjunctival -> superficial marginal plexus breaks to:

peripheral corneal arcades (1mm to cornea)
and
recurrent conjunctival vessels

point: does not do much/not much nutrients/ essentially avascular

Ocular Biology (9 decks)

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