Lens & Sclera

vascularization of the lens

avascular

where is the lens located

in the posterior chamber between the vitreous and iris

main function of the lens

light transmission and focusing light onto the retina

refractive power of lens

20 D (1/3 of refractive power of the eye)

the (anterior/posterior) lens is steeper

posterior

the lens is (more/less) acidic than the surrounding aqueous humor and blood plasma

more acidic

the lens is (spherical/aspherical) and becomes (flatter/steeper) towards the periphery

aspherical

flatter

what attaches the posterior lens surface to the anterior vitreous

ring-shaped hyaloid capsular ligament

retrolental space of Berger

potential space between posterior pole of lens and anterior vitreous

what 2 characteristics of the lens help reduce spherical aberration

1. peripheral flattening

2. gradient index of refraction

3 main parts of lens

lens capsule, lens epithelium, lens cortex

BM of the lens

lens capsule

location of lens capsule and how it is made

surrounds the entire lens

secreted by the anterior lens epithelium

where is the lens capsule the thinnest and thickest

thinnest at the posterior pole

thickest in a circle around the anterior pole

what is the lens capsule composed of

type 4 collagen fibers and GAGs

function of lens capsule

barrier against large molecules entering the lens

anterior lens capsule is the attachment point for lens zonules that extend from the non-pigmented ciliary epithelium

lens epithelium histology

single layer of cuboidal epithelial cells adjacent to anterior lens capsule

where is there no lens epithelium and why

no posterior lens epithelium

posterior epithelium was used to form primary lens fibers during embryological development

what are the junctions of lens epithelium cells

maculae occludens and gap junctions

function of lens epithelial cells

secrete lens fibers and lens capsule

main site of lens metabolism

germinal zone

zone anterior to the lens equator that contains mitotic epithelial cells that become secondary lens fibers

where are secondary lens fibers produced and when

germinal zone

production of lens fibers is continuous throughout life

composition of lens cortex

65-70% water, 30-35% protein, 1% other

what makes up most (85-90%) of lens proteins

water soluble alpha, beta, and gamma crystallins tightly packed within the cytoplasm of lens fiber cells

__________ crystallins act as ___________ by helping other crystallins recover from injury

alpha crystallins act as molecular chaperones by helping other crystallins (beta & gamma) recover from injury

what causes structural damage to crystallins & lens fibers

UV exposure and oxidation

role of alpha crystallins in the lens cortex

prevent degradation of lens fibers and loss of transparency by acting as molecular chaperones and helping beta & gamma crystallins recover from injury

what creates a gradient of refraction throughout the lens

variable crystallin concentration

where is the index of refraction highest in the lens

higher in the nucleus than the anterior lens

index of refraction of aqueous and vitreous

1.336

how are lens fibers organized and why

joined together by multiple interlocking mechanisms

allows lens fibers to easily slide past one another during lens movement

other names for lens zonules

zonules of zinn, suspensatory ligaments

what produces lens zonules

basement membrane of the non-pigmented ciliary epithelium in the pars plana and pars plicata

what are zonules composed of

microfibrils that have fibrillin and ECM

zonules (HAVE/DO NOT HAVE) true elastic fibers

do not have

primary lens zonules

attach right to lens capsule in pre and post equatorial regions of the lens

secondary lens zonules

connect primary lens zonules to one another or to non-pigmented ciliary epithelium of the pars plana

tension zonules

connect primary lens zonules to the valleys between the ciliary processes of the pars plicata

function of the sclera

forms posterior 5/6 of the protective CT coat of the eye (other 1/6 is cornea)

helps maintain globe shape and protect intraocular structures

what is the point of attachment for the EOMs

sclera

thickest area of the sclera

1 mm at the posterior pole

thinnest area of the sclera

0.3 mm under the recti tendon insertions

weakest area of the sclera

lamina cribrosa (within the ON)

what is the vascularization of the sclera

sclera is avascular

where does sclera get blood supply

sclera gets minimal blood supply from the episcleral vessels, choroidal vessels, and branches of the LPCAs

what innervates the sclera

sclera is minimally innervated by LPCNs and SPCNs

layers of the sclera

episclera, sclera proper, lamina fusca

episclera

loose CT with a capillary network that surrounds the cornea

ciliary flush definition and cause

circumlimbal injection of the episclera

inflammation of the CB or iris causes inflammation of the anterior ciliary arteries that innervate the episclera

where are the 2 networks of the ACAs

anterior conjunctiva

episclera

sclera proper

thick, dense, avascular CT continuous with corneal stroma

what is the sclera proper composed of

irregular collagen bundles, ground substance

collagen composition of sclera

irregular collagen bundles that provide strength but no transparency

composition of the sclera compared to the stroma

both have collagen bundles but stromal collagen is organized and evenly spaced (transparent) while scleral collagen is irregular and strong but opaque

both sclera and stroma have ground substance, but sclera has less fibroblasts and GAGs that stroma

episclera vs sclera proper

episclera: loose CT, highly vascular

sclera proper: dense CT, avascular

lamina fusca

innermost layer of sclera adjacent to choroid

lamina fusca composition

elastin fibers and melanocytes

conditions associated with blue sclera

ehlers-danlos syndrome

osteogenesis imperfecta

why does sclera have blue tint in infants

underlying uveal pigment is visible through thinner sclera

why does sclera become yellow in elderly

lipids become trapped in dense irregular CT over time

can signify liver disease

other name for tenon’s capsule

fascia bulbi

tenon’s capsule

thin transparent CT layer that covers the episclera

begins 2 mm behind limbus and encircles the rest of the globe

separates globe from surrounding adipose tissue

what layers surround and fuse with tenon’s capsule

episclera behind it

conjunctival submucosa in front of it

what perforates tenon’s capsule

tenon’s is perforated posteriorly by ON, ciliary vessels, ciliary nerves, and tendons of 4 recti muscles

anterior scleral foramen

area occupied by cornea (no sclera) 11.7 mm in diameter

posterior scleral foramen

area where ON enters eye

what is the lamina cribrosa composed of

scleral collagen and elastin fibers that associate with axon bundles and astrocytes in the ON

why is the ON in the lamina cribrosa most likely to be damaged by high IOP

the lamina is the weakest part of the sclera

3 emissaria (networks for veins, arteries, nerves) in the sclera

anterior emissaria, middle emissaria, posterior emissaria

anterior emissaria vessels and nerves

deep and intrascleral venous plexi, anterior ciliary arteries, episcleral artery branches, aqueous veins of ascher, LPCNs

deep and intrascleral venous plexi

travel through sclera to connect with ciliary vein and ciliary body

anterior ciliary arteries

travel through sclera to provide blood to anterior structures of the eye

episcleral arteries

travel through sclera to reach the AC angle

aqueous veins of ascher

travel through sclera to drain aqueous humor from Schlemm’s canal

LPCNs

form axenfeld loops in sclera

travel through sclera to innervate anterior structures of the eye

middle emissaria

vessels near equator traveling through sclera

what vessels travel through the middle emissaria of the sclera

vortex veins to drain the choroid

posterior emissaria

vessels that travel through the sclera near the ON

what vessel channels run through the posterior emissaria

LPCAs, SPCAs, LPCNs, SPCNs

how thick is the sclera at the posterior pole

1 mm (thickest point)

how thick is the sclera at the recti insertions

0.3 mm (thinnest points)

do the recti and obliques insert anterior or posterior to the equator

recti: anterior

obliques: posterior

index of refraction at the center of the lens

1.41

where are most lens zonules produced and where do they attach

produced at pars plana

attach to anterior mid-periphery of lens

in lens development, does anterior or posterior lens form first

posterior

why are posterior cells primary in lens development

they are first ones to act and first ones to form a lens nucleus

why does lens growth happen throughout life

anterior lens epithelium divides throughout life

where does nuclear sclerosis start

embryonic nucleus

why does embryonic nucleus have highest index of refraction

has the most crystallins

what generates the juvenile and adult nucleus

anterior lens epithelium

what generates the embryonic nucleus

posterior lens epithelium

what drugs can cause blue sclera

tetracyclines, minocyclines, topical steroids

what inflammatory eye disease can cause blue sclera

scleritis (2nd to inflammation)

which lens nucleus is demarcated by lens sutures

fetal