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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
**scleral spur**
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
**Emissaria**
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
**anterior emissaria**
located near insertion of muscle tendon anterior ciliary artery travel with EOMS enter sclera via anterior emissaria
28
**axenfeld nerve loop**
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
**posterior emissaria**
short posterior ciliary arter long posterior ciliary and nerves vortex veins
30
sclera nerve supply
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
scleritis
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
**Sclera nourishment and nutrients**
no direct blood, lymph or nerve supply nutrition derived by diffusion from overlying episclera and underlying choroidal vascular networks
33
**sclera composition**
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
sclera extracellular proteins
provide tensil support, stbaility strength and viscoelasticity - collagen and elastin fibrils, interfibrillar proteoglycans and glycoproteins - mainly collagen and some elastin: major insoluble proteins
35
**scleral fibroblasts**
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
**collagen of sclera**
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
**elastin of sclera**
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
**Marfan's syndrome**
fibrillin-1 gene mutated defective production of elastin defective fibrillin component of elastic fiber scleral pathology => high myopia dislocated lens
39
**proteoglycans of sclera**
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
matrix metalloproteinases of sclera
comprise a family of zinc-dependent endopeptidases degrade extracellular matrix proteins and collagen produced by scleral fibroblasts
41
scleral hydration
related to ECM water content sclera 68-70% proteoglycan regulate hydration with glycosaminoglycan side chains sclera contains lower concentration of proteoglycans than cornea
42
scleral biomechanical properties dependent on 3 things:
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
**high myopia and sclera**
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
**proposed model of scleral pathology in myopia development**
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
myopia development and effect on sclera
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
**high myopia and scleral changes**
scleral changes seen in high myopia may reflect abnormal growth and organization of collagen fibrils or increased breakdown of sclera overall scleral thinning
47
high myopia effects
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
posterior staphyloma
associated with axial elongation abnormal scleral has low mechanical resistance -stretches in response to IOP and EOM tensions
49
**scleral plaque**
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
**blue sclera**
thinned sclera pigmented uvea visible associated with osteogenesis imperfect -abnormalities in type 1 collagen
51
**icteric sclera - yellow sclera - jaundice**
bilirubin deposited predominantly in conj common causes: hepatitis, alcoholic liver disease, bile duct obstruction
52
age changes in sclera
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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