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Ocular Biology > Cornea and Sclera Development > Flashcards

Cornea and Sclera Development Flashcards

1
Q

Inductive mechanisms of cornea development

A
  • pax6 gene locus is transcription factor for the various
  • developing lens necessary to act as inucer for ectoderm to transform into cornea
  • morphogenesis requires coordinated cellular events: migration, proliferation, and differentiation
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2
Q

Overview of cornea development

A

Surface ectoderm => presumptive corneal epithelium

Mesenchyme => neural crest mesenchyme => corneal endothelium and stroma

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3
Q

Surface Ectoderm (5-6th week) => presumptive corneal epithelium

A

ECM-tissue interactions

initiated when lens vesicle separates from surface ectoderm

surface ectoderm: presumptive corneal epithelium (2 layers):

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4
Q

Primary Stroma

A

produced by primitive corneal epithelial cells

acellular matrix: collagen fibrils, hyaluronic acid and GAGs

hydration of hyaluronic acid causes swelling to create space for cellular migration

directional membrae to guide the migration of neural crest cells

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5
Q

1st wave Mesenchymal components produces what (7th wk)

A

the 1st wave of neural crest mesenchyme produces the corneal endothelium

1st wave mesenchyme: neural crest mesenchyme at anterior tip of optic cup

  • migrates to form single layer in front of lens
  • presumptive corneal endothelium
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6
Q

2nd wave mesenchymal components produces what (8th wk)

A

2nd wave of neural crest mesenchyme produces secondary cornea stroma.

2nd wave of neural crest mesenchyme invades primary stroma at margin of optic cup
-do not invade anterior 10 um primary stroma => becomes bowman’s layer

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7
Q

cornea stroma production

A

neural crest cells proliferate and differentiate into keratoblasts:

  • synthesize type 1 collagen fibrils and GAGs
  • layers fill in from posterior to anterior stroma
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8
Q

corneal stroma differentiation

A

process begins in posterior stroma and progresses anteriorly
-collagen lamellae: 1st organized in posterior stroma

keratoblasts differentiate into keratocytes

collagen and lamellae create most of stroma, but initally started by these keratoblasts

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9
Q

Descemet’s membrane development

A

endothelial cells produce basement membrane

DM seen 8wks and matures at 4 months

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10
Q

innervation development

A

tissue of origin of innervation: neural crest cells

3-5 month: first seen
6-9 month: nerve inc. in number and form network

supplied by CN5 V1

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11
Q

Fusion of eyelids and development

A

Eyelids fuse at 9th wk

in 24 weeks, eyelid open: cornea epithelium increase thickness and differentiates to basal, wing, squamous cells

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12
Q

factors affecting corneal size, curvature, transparency

A

Eyelid fusion: transparency gradually achieved due to maturation of stromal lamellae, metabolic activity of endothelium dehydration of cornea

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13
Q

cornea at birth and postnatally

A

newborn diameter: 9.5-10mm
diameter reaches adult size at 2 years of age

newborn radius of curvature: 47.00 (STEEEP)
-flattens with age

at birth: cornea may be thicker than adult (caused by increased water content caused by underdeveloped endothelial ion transport)

cornea translucent during fetal period

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14
Q

Sclera development tissue origin

A

neural crest: 95% of sclera

mesoderm: small portion from paraxial mesoderm

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15
Q

scleral development inductive mechanisms

A
  • RPE influences scleral development (neural crest differentiation)
  • IOP and vitreous formation: determines scleral shell size (expansion from IOP, increased vitreous volume)
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16
Q

Scleral development timeline

A

7th week - scleral condensation– limbus

8th week - equator

3rd month - optic nerve

5th month - lamina cribosa

develops anterior to posterior

17
Q

scleral condensation

A

anterior to posterior development

neural crest cells differentiate to fibroblasts

fibroblasts: collagen, elastic fibers, and GAGs

neuro crest cells meet myoblasts of EOMs

neural crest cells form sheaths for muscle fibers near equator of globe

18
Q

Fibroblasts synthesize what

A

collagen fibrils - initially small diameter that steadily increases

elastic fibrils

proteoglycans and GAGs (13 wk)

19
Q

postnatal scleral development

A

birth: sclera thin, highly distensible, translucent
postnatal: collagen synthesis most active in posterior eye

diameter of fibers and rigidity change: outer scleral layers (larger diameter) collagen fibrils compared to inner layers (smaller)

  • response to circumferential streses on scleral by IOP
  • regional differences in fibril diameter likely contribute to scleral resistance to IOP

sclera increases diameter and thickness in relatively rapid manner, loses distensibility during first 3 years

growth continues at a slow pace after 3 years:
anterior sclera adult size by age 2
posterior sclera develops until age 13-16

20
Q

differences between cornea and sclera development

A

cornea development:

  • derived from surface ectoderm and neural crest mesenchyme
  • induced by lens
  • formed by 1st and 2nd waves of neural crest mesenchyme that migrates anteriorly with a guiding matrix of primary stroma with a resulting regular lamellar arrange ment

sclera development:

  • derived from neural crest mesenchyme and some mesodermal mesenchyme
  • induced by RPE
  • formed from scleral condensation that migrates posteriorly with a resulting irregular interweaving arrangement
21
Q

differences between developed cornea and sclera

A

cornea - transparent

  • regular lamellar arrangement
  • uniform fibrils with uniform diameter
  • regularly spaced with regular size
  • less interweaving and regular directed bundles
  • steeper curve – clear cap on front of eye
  • lacks elastin components

sclera - opaque

  • irregular lamellar arrangement
  • more nonuniform in diameter
  • more irregularly spaced, arranged in variously sized
  • high interweaving, irregularly directed bundles
  • flatter curve-opaque covering of eye
  • elastin components
22
Q

Cryptophtalmos

A

absent cornea

  • facial skin replaces eyelids, covers orbit, obliterating cornea
  • bilateral
  • associated with other congenital birth defects
23
Q

megalocornea

A

megaocornea newborn: 12 mm (normal 10 mm)

in adults: >13 mm (normal 12 mm)

-hereditary or secondary to congenital glaucoma

24
Q

megalocornea - hereditary

A

resulting from anomalies of differentiation of neural crest

greater than normal steepness (myopia)

defect in growth of optic cup

typically isolated but many ocular and systemic associations

25
Q

megalocornea - 2nd to congenital glaucoma

A

buphthalmos - ox eye

  • elevated IOP stretches cornea and sclera
  • ocular enlargement
  • descemet’s may rupture or tear
  • -haab’s stria: old, healed horizontal ruptures in descemet’s
  • -curved lines or scars at endothelial level (tend to be horizontal)

other findings with congenital glaucoma: tearing, photophobia, blepharospasm

26
Q

microcornea

A

horizontal diameter < 10mm

nonprogressive, can be unilateral or bilateral

flat cornea

2 proposed cause:

1) arrest in growth of cornea (after 5th gestational month)
2) overgrowth of anterior lips of optic cup

27
Q

sclerocornea

A

non-progressive

cornea opaque and limbus indistinct

corneal flat (< 43 D)
cornea plana
cornea curvature < scleral curvature

occur alone or with other ocular defects

usually bilateral, commonly asymmetric

caused by abnormailty of 2nd wave of mesenchyme (severity variable)

central cornea usually clearer than periphery (differnt than peter’s anomaly)

28
Q

posterior corneal defect (peters’ anomaly)

A
  • rare, sporadic and frequently bilateral disorder
  • range of severity, from barely detectable focal indentation to total corneal scarring and ectasia
  • results from delay or failure of separation of lens vesicle from surface ectoderm
  • results in adhesion between cornea and lens

–migration of mesenchyme is impeded by this adhesion, causing characteristic features of this anomaly

29
Q

peter’s anomaly characteristic features

A

opacity of central cornea

central concave defect in posterior corneal stroma

corneal endothelium and descemet’s membrane absent in posterior ulcer

lens adheres to cornea

iris adheres to cornea (iridocorneal adhesions)

severe form: corneal staphyloma, scarring, ectasia

30
Q

tears in endothelium and desecemet’s membrane

A

birth trauma: vertical or oblique lines in inner corneal layers

congenital glaucoma: haab striae: horizontal breaks or circumferential breaks

31
Q

corneal dystrophy

A

occur early in life

familial inheritances - AR or AD

bilateral, central cornea involved

  • almost all autosomal dominant
  • not associated with any other systemic diseases
  • onset of dystrophy is by age 20
  • bilateral
  • slowly progressive
  • primary involved in a single cornea layer (either epithelium, stroma, endothelium)
32
Q

anterior corneal dystrophies

A

corneal epithelium

epithelial basement membrane

bowman’s layer

33
Q

epithelial-basement membrane dystrophy (EBMD)

A

most common anterior corneal dystrophy

abnormal basement membrane synthesized

onset 30-40

central

3 patterns:
maps - gray epithelial patches: islands of anomalous basement membrane
dots - multiple intraepithelial cysts + cellular debris; intraepithelial opacities
fingerprints - gray or refractile fine lines; basement membrane separating sheets of duplicated epithelium

34
Q

stromal dystrophies

A

abnormal substances appear within keratocytes or among collagen fibrils

  • Granular (hyaline)
  • Lattice (amyloid)
  • Macular (GAGs/ MMP)

mnemonic: Mickey Mouse Goes Home to LA

35
Q

granular dystrophy (hyaline deposits) - stroma

A

hyaline deposits in anterior stroma

gray-white opacities do not extend to limbus

visual loss relative late
in life

36
Q

macular dystrophy (GAGs deposits/MMP deposits) - stroma

A

least common, most severe

begins as central gray-white superficial stromal opacities

clear corneas that cloud between age 3-9

autosomal recessive

accumulation of GAGs

37
Q

lattice dystrophy (amyloid deposit) - stroma

A

most common stromal dystrophy

localized amyloidosis

glasslike branching lines in stroma, seen best with retroillumination

autosomal dominant

38
Q

Endothelial dystrophy: Fuch’s endothelial dystrophy

A

thickening of descemet’s membrane (corneal guttata: droplets)

compromise of endothelial fxn

corneal edema

rare symptomatic before 50

corneal decompensation

marked stromal edema

microcystic and bullous epithelial edema

subepithelial fibrosis

39
Q

anterior keratoconus - ectatic dystrophy

A
  • progressive thinning of central stroma
  • leads to conical configuration of cornea
  • present in teens
  • bilateral, central, and slowly progressive
  • munson’s sign - pushes on lower lid when looking down
  • fleisher’s ring (iron) in deep epithelium and BM at base of cone
  • vogt’s lines (vertical striae/stress lines)
  • stromal thinning
  • acute hydrops (stromal edema
  • scissors reflex in ret

Ocular Biology (9 decks)

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