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Ocular Biology > Scleral Anatomy > Flashcards

Scleral Anatomy Flashcards

1
Q

Sclera background

A

white, rigid dense connective tissue covering the globe posterior to the cornea

avascular

3 layers: episclera, scleral stroma proper, lamina fusca

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2
Q

sclera vs. cornea

A

sclera more opaque

sclera more rigid

sclera no epithelium or endothelium

scerla has zone of vascularity - episclera

sclera collagen fibrils more interwoven and larger interfibrillar spaces

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3
Q

scleral 4 functions

A

1) provide strong tough external framework and coating to protect the eye
2) maintain shape of globe so inner eye is undisturbed
3) expansile-resistant structure maintaining the forces generated by the intraocular pressure
4) attacment sites for EOMs

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4
Q

Size of sclera

A

incomplete sphere surrounding posterior 5/6ths of the globe

17 cm2 outer surface area and outer diameter of 24 mm

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5
Q

Limbus

A

2mm transition zone as sclera approaches cornea

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6
Q

2 major openings of sclera

A

1) anterior scleral foramen or canal - cornea and limbus

2) posterior scleral foramen or canal - opening for the optic nerve

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7
Q

radius of curvature and thickness of sclera

A

radius of curvature - 12mm

thickness (thickest to thinnest):

  • posterior pole 1.0
  • anterior sclera close to limbus ~0.8
  • equator 0.4-0.6
  • behind insertion of recti muscles 0.3
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8
Q

Tenon’s capsule

A

dense collagenous layer

facial sheath

located between conjunctival stroma and episclera

inner surface smooth = allows free gliding of adjacent structures internal to it

external to sclera

attached to episclera with strongest attachment at limbus and posterior pole

~3 mm posterior to limbus, tenon’s capsule becomes freely mobile over episclera

anteriorly nourished by conjunctival vascular plexus and episcleral plexus

sandwiched between conjunctival stroma and episclera

separates orbital fat from contact with sclera

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9
Q

sub-tenon’s injection

A

route of drug delivery by injection to the area of sub-tenon’s space (episcleral)

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10
Q

internal surface sclera

A

sclera adjacent to choroid posteriorly

sclera adjacent to ciliary body anteriorly

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11
Q

suprachoroidal space

A

superficial to choroid posteriorly

serves as a conduit for nerves and blood vessels

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12
Q

supraciliary space

A

superficial to ciliary body anteriorly

serves as a conduit for nerves and blood vessels

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13
Q

posterior surface sclera

A

posterior scleral foramen

outer portion of scleral fibers fuse with dural and arachnoid sheaths of optic nerve

inner portion crosses posterior scleral foramen as lamina cribosa

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14
Q

posterior scleral foramen

A

inner 2/3 of sclera forms fenestrated scaffold across the canal or foramen to form the lamina cribosa -> supports optic nerve axons

lamina cribosa is weakest point of globe to expansile forces -> optic nerve vulnerable to damage

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15
Q

Lamina Cribosa

A

interwoven collagen fibrils forming canals through which optic nerve bundles pass

weakest area of sclera

  • glaucoma damage
  • at posterior scleral foramen
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16
Q

Episclera layer of sclera

A
  • thin, well vascularized: fibers blend with scleral stroma
  • elastic and loos connective tissue
  • collagen fibrils vary in diameter
  • fibroblasts- primary cell type
  • some melanocytes and macrophages
  • thickest anterior to muscle insertions and thins posteriorly to optic nerve insertion
  • lymphatics absent
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17
Q

episclera blood supply

A

1) anterior ciliary arteries (anterior to insertion of rectus muscles):
a- superficial episcleral capillary plexus - anastomose at limbus with conjunctival vessels and with deep plexus
b- deep episcleral capillary plexus - close to scleral stromal layer

2) long posterior cil arteries (posterior to muscle insertions)

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18
Q

episcleritis

A

conjuctival and superficial episcleral vascular plexi involved

vessels blanch with 2.5% phenylephrine

underlying deep episcleral plexus not involved, lies flat against normal sclera

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19
Q

Sclera stroma layer of sclera

A

thickest layer

collagen fibers

variable diameter

collagen fibrils:

  • variability in fibril diameter
  • irregular spacing
  • interweaving of bundles
  • differences in water binding substances
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20
Q

Sclera color

A

opaque white

scatters all frequencies of visible light

due to refractive index of tissue and composition of stroma

reduces internal light scattering

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21
Q

lack of transparency in sclera

A

variability in collagen fibril diameter

irregular spacing between fibrils

interweaving of bundles

differnces in water binding substances

22
Q

scleral stroma fxn and properties

A
  • organization of collage and elastic fibers support fxn of sclera:
    1) resist penetrating injury
    2) provide stable support - withstand expansive force of IOP, EOM, cil muscle contraction
  • support mechanical properties:
    1) Viscoelasticity: rapid lengthening of fibrils and rebound, slow stretching on prolonged pressure -> protects eye from injury
    2) strong tensional properties
23
Q

sclera stroma vasculature

A

anterior ciliary vessels enter sclera anterior to rectus muscles

short and long posterior ciliary vessels, vortex veins, and nerves enter posterior to the muscles

scleral stroma relatively avascular -> lacks extensive capillary networks

stroma derives nutrients from episcleral and choroidal vascular networks

24
Q

Lamina fusca layer of sclera

A

innermost layer

pigmented - proiximity to choroid

inerface- a potential space: nerves and blood vessels, blood and fluid can collect during inflammation, weak attachment

25
Q

scleral spur

A

innermost scleral structure

deep fibrils of sclera condense in a ring at limbus (more elastin fibrils)

contribute to stbaility of corneal contour with circumcorneal annulus

  • trabecular meshwork tissue inserts into anterior aspect
  • ciliary body muscle takes origin in posterior scleral spur
26
Q

Emissaria

A

channels in sclera

allows passage of arteries, veins and nerve into or out of the eye

nerves and arteries travel in suprachoroidal/supraciliary space

sclera collagen fibers form weak attachments to emissaria

27
Q

anterior emissaria

A

located near insertion of muscle tendon

anterior ciliary artery travel with EOMS

enter sclera via anterior emissaria

28
Q

axenfeld nerve loop

A

2nd anterior emisarrium

branch of long posterior ciliary nerve

leaves supraciliary space -> travels through sclera

appears pigmented due to association with choroid - carries pigment with it

29
Q

posterior emissaria

A

short posterior ciliary arter

long posterior ciliary and nerves

vortex veins

30
Q

sclera nerve supply

A

primary sensory innervation, richly innervated

posterior sclera: short ciliary nerves pierce sclera around optic nerve

anterior sclera: branches of long ciliary nerves, divides at equator

  • return to posterior scler or reenter choroid in area of lamina fusca
  • most enter ciliary body or form nerve loops of axenfeld
  • rest penetrate sclera 3mm from limbus (branch to supply trabecular meshwork, schlemm’s canal and episclera)
31
Q

scleritis

A

innervation prominent in anterior sclera

scleral inflammation is painful

  • direct stimulation: inflammatory cytokines
  • distension and stretching of nerve bundles from swelling, cellular infiltration and EOM movement
32
Q

Sclera nourishment and nutrients

A

no direct blood, lymph or nerve supply

nutrition derived by diffusion from overlying episclera and underlying choroidal vascular networks

33
Q

sclera composition

A

composed of 68 - 70% water
diffuse population of cells found within stroma of sclera

stabilized by extracellular proteins: 
-soluble proteoglycans 1%
-insoluble collagen 27%, elastin 1%
-fibrocytes 1%
other 1%
34
Q

sclera extracellular proteins

A

provide tensil support, stbaility strength and viscoelasticity

  • collagen and elastin fibrils, interfibrillar proteoglycans and glycoproteins
  • mainly collagen and some elastin: major insoluble proteins
35
Q

scleral fibroblasts

A

found between lamellae in stroma sclera

synthesize collagen fibrils, elastic fibrils, and proteoglycans

cell-cell communication is mediated by growth factors within scleral extracellular matrix

36
Q

collagen of sclera

A

up to 90% dry weight

synthesized by fibroblasts

primarily type 1 - tensile strenght

type 5 - impt in regulating fibril diameter during fibrillogenesis

less type 5 collagen than cornea

collagen fibrils have wide range of diameters (25 - 300 nm)

37
Q

elastin of sclera

A

most dense in peripapillary area (around optic nerve)

tissue exposed to multidirectional streching

biomechanical modeling: IOP creates tensile stresses distributed within plane of sclera

elastin arranged to reinforce weaker scleral foramen

elastin ring allows reversible expansion and contraction with variable IOP

mainly found inner layer of sclera

superficial layer contains min. elastin

elastic fibers = elastin core surrounded by microfibrils containing glycoproteins (fibrillin)

38
Q

Marfan’s syndrome

A

fibrillin-1 gene mutated

defective production of elastin

defective fibrillin component of elastic fiber

scleral pathology => high myopia

dislocated lens

39
Q

proteoglycans of sclera

A

produced by fibroblasts

consist of protein core with glycosaminoglycans

dermatan sulfate and chondroitin sulfate are main proteoglycans

  • present throughout sclera
  • influence collagen fibril assembly and arrangement
40
Q

matrix metalloproteinases of sclera

A

comprise a family of zinc-dependent endopeptidases

degrade extracellular matrix proteins and collagen

produced by scleral fibroblasts

41
Q

scleral hydration

A

related to ECM

water content sclera 68-70%

proteoglycan regulate hydration with glycosaminoglycan side chains

sclera contains lower concentration of proteoglycans than cornea

42
Q

scleral biomechanical properties dependent on 3 things:

A

1) scleral structure: thickness, collagen fibril characteristics, organization of fibril bundles and rate of turnover of scleral ECM
2) hydration: glycsaminoglycan content
3) scleral fibroblats
- biomechanical properties influence shape and size: changes in proteoglycan synthesis in posterior sclera are closely correlated with changes in ocular size and refractive state

43
Q

high myopia and sclera

A

high myopia: associated with lengthening of posteiror globe

scleral changes seen in high myopia may reflect abnormal growth and organization of collagen fibrils

proposed that chnages in proteoglycan synthesis in posterior sclera influence axial length and refractive state

genetic or enviromental factors may lead to alterations in scleral ECM

44
Q

proposed model of scleral pathology in myopia development

A

normal eye growth proposed to be influenced by visual feedback based on quality of retinal image

sclera undergoes constant remodeling during childhood eye growth to achieve emmetropia and adult eye size

feedback believed to depend on paracrine cytokines or growth factors from retina and or RPE

45
Q

myopia development and effect on sclera

A

initiatting factors: environmental stimulus, genetic stimulus or both

effect on scleral fibroblasts: dec. production of type 1 collagen, inc. activity of MMP, dec. synthesis of GAG

sclera of myopic eye more extensible and less resistant to expansive forces of IOP

46
Q

high myopia and scleral changes

A

scleral changes seen in high myopia may reflect abnormal growth and organization of collagen fibrils or increased breakdown of sclera

overall scleral thinning

47
Q

high myopia effects

A

stretching of ocular coat

straightening of temporal retinal vessels

muopic conus

thinning of retina and choroid

choroiretinal damage resulting in permanent vision loss in high myopes

lacquer cracks - tears in bruch’ membrane

supretinal neovascularization

rpe hyperplasia in macula (fuch’s spot)

48
Q

posterior staphyloma

A

associated with axial elongation

abnormal scleral has low mechanical resistance
-stretches in response to IOP and EOM tensions

49
Q

scleral plaque

A

anterior to horizontal rectus muscle insertions

dark patches due to inc. translucency of sclera

inc. scleral density posterior to limbus due to deposition of calcium between colalgen fibrils

50
Q

blue sclera

A

thinned sclera

pigmented uvea visible

associated with osteogenesis imperfect
-abnormalities in type 1 collagen

51
Q

icteric sclera - yellow sclera - jaundice

A

bilirubin deposited predominantly in conj

common causes: hepatitis, alcoholic liver disease, bile duct obstruction

52
Q

age changes in sclera

A

human sclera adult size by 12-13 years

as aging:

  • progressive degeneration of collagen and elastic fibers
  • loss of GAGs
  • scleral dehydration
  • becomes more yellow: deposition of fat globules b/n fibrils
  • collagen become thicker, less uniform
  • disrupted fibers can cause calcium deposits ->hyaline plaques
  • collagen inc. in rigiity

Ocular Biology (9 decks)

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