4: iris pupil and lens copy)

uvea

middle layer of the eye

composed of 3 regions front to back- the iris, ciliary body and choroid

uvea is largest structure

choroid mainly composed of blood vessels

appearance of the iris

pupillary zone closest to the pupil

ciliary zone closest to the ciliary body

what is the iris

thin circular structure located anterior to the lens

the centre aperture- pupil, located slightly nasal and inferior to the iris centre

pupil size regulates retinal illumination

holes in the iris is crypt

ruff- ring in the iris

function- acts as a diaphragm to regulate the amount of light entering the eye. the two iris muscles are innervated separately.

the pupil

very small in brightly lit conditions and fairly large in dim illumination 

it is the thickest in the rgeion of the collarette

collarette encircles the pupil and ciliary zone which extends from the collarette to the iris root 

a. pupil and pupillary ruff

b. iris root

c. pupillary portion of the iris

d. ciliary portion

e. collarette

f. cellular anterior border layer

g. stromal tissue

h. sphincter muscle lies in stroma

i. posterior iris

j. anterior epithelium

k. uveal band

l. trabecular meshwork

m. canal of schlemm

n. iris

pupillary margin of the iris

rests on the anterior surface of the lens and in profile, the iris has a truncated cone shape such that the pupillary margin lies anterior to its peripheral termination , the iris root

iris root 

thinnest part of the iris and joins the iris to the anterior aspect of the ciliary body 

attaches to ciliary body 

iris divides the anterior segment of the globe into anterior and posterior chambers and the pupil allows aqueous humor to flow from the posterior into the anterior chamber with no resistance 

pupillary zone of iris

rests on the lens , lens fibres are fragmented

4 layers of the iris

anterior border layer

stroma and sphincter muscle

anterior epithelium and dilator muscle

posterior epithelium

anterior border layer of iris

thin condenstaion of the stroma

forms strands from connective tissue

composed of fibroblasts and pigmented meloncytes. the highly branching porcesses of the cells interweave to form a meshwork in which the fibroblasts are on the surface and meloncytes are located below

loose network of connective tissue- needs to be loose as iris is muscular

density and arrangement of meshwork differ amongst irises and are factors in iris colour

a. fibroblasts - long branching processes which interconnect 

b.superficial layer of fibrobtasts revoved 

c. capillaries 

where is anterior border of the iris absent at

absent at the oval shaped iris crypts

near the root, extensions of this layer form finger shaped iris porcesses that can attach to the trabecular meshwork.

iris stroma 

connective tissue is composed of pigmented and non pigmented cells, collagen fibrils, and extensive ground substance 

pigmented cells include meloncytes and clump cells 

non pigmented cells are fibroblasts, lymphocytes , macrophages and mast cells 

meloncytes and fibroblasts do not form a meshwork

vessels run root to pupil margin forming arterial circle at collarette

have tight junctios 

clump cells

large darkly pigmented cells and are likelu altered macrophages and are scavengers of free pigment within the iris. located in the pupillary portion of the stroma near phincter muscle 

collagen fibrils arranged in radial columns that are seen as white fibres in light coloured irises 

the iris arteries

branches of a circular vessel, the major circle of the iris.

the major circle of the iris is located in the ciliary body near the iris root

iris vessels usually follow a radial course from the iris root to the pupil margin

fibril network anchors the vessels in place and protects them from kinking and compression during estensive iris movement that occurs with miosis and mydriasis

the minor circle of the iris

an incomplete circle of the iris is located in the iris stroma inferior to the collarette

iris capillaries are not fenestrated and form part of the blood aqeous barrier

iris stroma is continours with stroma of ciliary body

sphincter muscle 

lies within the stroma and is composed of smooth muscle cells joined by tight junctions 

circular muscle located in the pupillary zone of the stroma - closest to pupil 

anchored firmly to adjacent stroma ad retains its function even if severed radially 

contraction of sphincter causes the pupil to constrict in miosis- pupil gets smaller  

muscle innervated by parasympathetic system

dilator muscle

dilator for pupil -mydriasis 

radial muscle is the dilator muscle when dilator muscle contracts the pupil gets bigger 

runs around the pupil outwards - runs underneath sphincter

top image- constricted dilator, relaxed sphincter 

bottom- contricted sphincter, relaxed dilated 

anterior epithelium of the iris

posterior to the stroma are two layers of epithelium; anterior iris epithelium and dilator muscle

anterior iris epithelium is composed of myoepithelial cell. the apical portion is pigmented cuboidal epithelium joined by tight junctions and desmosomes, whereas basal portion is composed of elongated , contractile smooth muscle processes

it is the same as dilator muscle -

anterior epithelial cells are flatter with lower density of pigment, long extesniosn which form strands of dilator muscle

anterior epithelium forms dilator muscle

dilator muscle present from iris root to a point in the stroma below the midpoint of sphincter

stroma separating sphincter and dilator is dense band of connective tissue

anterior iris eithelium continues to pupillary margin as cuboidal epithelial cells and anterior iris epith continues posteriorly as the pigmented epithelium of the ciliary body

posterior epithelium of iris 

the second epithelial layer posterior to the stroma is posterior iris surface 

single layer of heavily pigmented columnar cells joined by tight junctions and desmososomes 

they rest on the anterior surface of the lens

in periphery the posterior iris ep begins to lose pigment as it continues into siliary body as the non pigmented epithelium 

thin basement membrane covers basal aspect of this cellular layer which lines the posterior chamber

positioin of anterior and posterior iris epithelial layers

positioned apex to apex , result of embryonic development

apical microvilli extend from both surfaces and desmosomes join the 2 apical surfaces

the epithelial cells curl from posterior iris to anterior surface at pupillary margin forming the pigmented pupillary rudd , which encicles the pupil

anterior iris surface

thin radial collagenous columns or trabeculae are evident in lightly pigmented irises

thicker, radially oreinted branching encircle depressions or openigns in the surface called crypts

they allow the aqueous quick exit into spaces in the iris stroma as volume of iris changes with contraction and dilation

cirucular contraction folds 

evident on anterior surface of ciliary zone 

result from tissue moving forward toward the iris root during pupill dilation

posterior iris surface

smooth but small circular furros evident near the pupil

radial contraction furrows located in pupillary zone , and the deeper structural furros run throughout the ciliary zone and continue into the ciliary body

colour of the iris

made up of differenr areas of pigment

overall pigment of colour based on wavelengths of light hitting iris, some absorbed, some reflected

iris colour determined by meloncytes in stroma and anterior border layer

when iris is brown, meloncytes are well pigmented , longer wavelenghts are reflected bacl

in blue iris, wavelengths of light that are reflected back are shorter

microscopic section of iris 

blood vessels flowing through the muscle layers 

iris regulates pupil size and keeps regulates light entering the eye 

most anterior layer - anterior borfer layer 

anterior epithelum- quite posterior 

surface and layers of iris 

right to left shows how each layers look brown iris ha slots of melalocytes 

sections of the crystalline lens

posterior surface is more curved

curvature:

anterior radius = approx 10.50 mm

posterior radius= approx 6.00mm

centre thickness- 4mm approx

diameter approx 9mm

refractive idnes approx 1.43

purpose of the crystalline lens

avascular, transparent structure that aids in focusing light rays onto the retina

lens is located within posterior chamber, anterior yo the vitreous chamber and posterior to the iris

suspended from surrounding the ciliary body by zonular fibres

what can cause lens shape to change 

its malleable, and the ciliary muscle contraction causes lens shape to change, increasing its dioptric power of the eye 

the mechanism  that causes increase in lens power is accomodation, which allows near objects to be focused on the retina 

cornea and lens power

lens structure -external to internal

external capsule

anterior epithelium

equator : zonule (external) and formation of lens fibres ( internal)

cortex - young lens fibres

nucleus - old lens fibres

lens capsule 

a transparent envelope that surrounds enitre lens 

capsule is a basement membrane and with time, becomes the thickest in the body 

at posterior pole its the thinnest 

thickens at anterior pole increasing with age 

capsule primarily collagen, has no elastic fibres high highly elastic due to lamellar arrangement of fibres 

capsule shape

helps mold the shape of the lens

it prefers to take on a spherical shape but this tendency is counteracted by the pull from the zonular fibres, these fibres insert into capsule merging with it

lens capsule barrier function

prevents large molecules such as albumin and hemoglobin from entering the lens

anterior lens capsule is produced by anterior epithelium and thickens with age

posterior lens capsule may recieve contribution from basal membrane of lens fibres but thickeness of posterior capsule changes minimall y

more capsule functions

mould lens into a more convex shape in accom

insertion of zonular fibres which attac the lens to cilary body

ciliary body controls accomodation

as capsule gets thicker, it gets less easier to stretch causnig decrease in accom 

thickest at equator - very top, very bottom, very nasal/temporal 

where zonule attaches to the capsule 

at the equator 

lens epithelium

adjacent to anterior lens capsule is a layer of cuboidal epithelium - anterior lens epithelium

these cells secrete the anterior capsule throughout life and are the site of metabolic mechanisms

they form the lens fibres - constantly elongating to become fibres

to see lens use the parallelepiped

why is there no posterior epithelium present

it was used during embryonic development to form the primary lens fibres

the lateral membranes of epithelial cells are joined by desmosomes and gap junctios

what is germinal zone of lens epithelium

the band of cells in the preequatorial region that lies anterior to the equator

is the location of cell mitosis

fibre formation

lens epithelium elongate and form fibres

epithelial cells have other functions like secretion of capsule, active transport of ions and water for lens transparency

transport of metabolites into the lens from the aq humour

lens fibres

picture shows lens fibres afte elongation

growth results into concentric layers of secondary lens fibres

section through equator of les shows that fibres cut in cross section are hexagonal in shap and arranged in concentric rings

reason why transparent, is because theyre regularly arranged , and cannot change shape 

what happens if lens fibres change shape

can form cataracts

fibres are usually locked together with a ball and socket and tongue and groove formations

why do the fibres of lens lose their cellular organelles as they age 

its because they have no vascular supply 

theres an extensive network of gap junctions throughout the lens along the fibre to account for facility with which nutrients and ions move within the lens 

gap junctions of lens fibres

have different packing arrangement and different protein connexins , forming the channel

gap junctions are not evenly distributed throughout the lens near the poles, more toward equator

epithelium- fibre interface

border between apical membrane of anterior epithelium and apical mem of elongating fibre known as epithelium fibre interface

nutrients and ions exchange across the EFI.

which iris strucrure is most posterior 

anterior epithelium 

what is this called

crypt of fuchs

cilliary zone

insertion of lens zonule

they are attached at equator to the capsule.

inbetween cillary body and lens

formed of extracellular matrix that includes fibrillin and elastin

fibres arise from basement embrane of non pigmented ciliary epithelium

zonules are interwoven into components of the capsule 

those that attach to the lens are called primary zonles, secondary join the primary zonules with eachother 

zonules on slit lamp 

shows zonule with pigment dispersion

lens divisions

cortex is constantly growing, epithelium turn into cortical cells

nucleus stays same size

nucleus is subdivided - does grow from embryonic stage

growth of lens nucleus

embryonic lens  

the primary lens fibres from elongating posterior epithelium form centre of the lens, embryonic nucleus

cell mitosis begins in preequatorial region of epithelium, new cell migrates toward equator and then elongates forming lens fibre 

all fibres formed are secondary lens fibres 

fetal nucleus includes embryonic nucleus and fibres surrounding it that are formed before birth

regions of adult lens 

includes the embryonic and fetal nuclei and the fibres formed from birth to sexual maturation

lens cortex contains fibres formed after sexual maturation

A. fetal nucleus : a is anterior suture b is posterior

B. adult lens cortex

C. adult lens

lens fibre zones in vivo

seen using the optic section

light coming from right hand side

lens sutures

as the lens fibres reach the poles they meet with the other fibres in their layer, forming a junction known as a suture

the secondary fibres formed during embryonic meet in 3 branches , forming Y sutures

how is anterior and posterior suture formed 

by joining of the apical aspects of the fibres- anterior is an upright Y shape 

posterior formed by joining of basal aspects - inverted Y shape 

as growth continues and lens becomes larger, the sutures become asymmetric so get less transparent 

lens fires forming new shells

the fibres elogate around the equator

form a belt around equatory of the lens

this prodces a complete shell of fibres

the anterior and posterior ends of each fibre are attached to sutures near the lens surface

lens nucleus and cortex sutures

when the shell is larger it has more fibres and irregular sutures. no fibres run from pole to pole

all fibres meet in the suture

sutural cataract 

lens fibres dont meet together neatly so get opacification

crotex sutures under microscope

lens- accomodation

unaccomodated state- zonules are tight , pulling on the lens. this flattens the lens

accomodated- ciliary body becomes smaller and contracts, zonules relax. causes elasticity of capsule to produce outward force increasing curvature and centre thickness . anterior lens becomes more convex

what happes during accomodation

lens shape changes by contracting ciliary muscle - increases power when looking at something that is not in the distance 

les thickens increasing anterior to posterior 

lens thins along the equator 

anterior lens surface moves forward and anterior chamber becomes shallower 

posterior pole remains in same position

lens shape in accomodation

changes as you get older

younger can accomodating more as front surface of lens is becoming a lot more convex

viteeous role in accomodation - when cilary muscle contracts

when ciliary muscle contracts, the choroid is pulled forward slightly,

contraction of ciliary body by decreasing cicumference of sclera, may lead to an elongation of axial length of eye

accom can cause a widening of intertrabecular spaces, causing aq outflow and result in a decrease of IOP

Vitreous role in accom- when ciliary muscles are relaxed

muscle is moved outward, and ciliary body is stretched posteriorly by elastic tissue of Bruchs membrane 

ciliary ring expands and tension of zonules stretches capsule , restoring lens to its unaccomodated state 

growth of lens

lens is constantly growths.

cortex and capsule become thicker

in adult the cortex is thicker - reason why it restricts accomodation

types of cataract

most common is nuclear cataract

cortical cataracts- affect cortex

age related with nuclear cataract 

nucleus has opacified

an opacity located in the embryonic, fetal or adult nucleus is called nuclear cataract 

cortical cataract

located in cortex, thicker in periphery and tapering towards the lens centre , it follows the shape of the fibre

fluid accumulatesand membrane rupture in equator can occur

only affect vision when it spreads to centre of the lens

sub- capsular cataract- shown on retro illumination of lens from retina

in the middle at the back

located beneath posterior capsule

impacts vision early and significantly along visual axis

risk factor is high dose steriod use

sutural cataract

affects space where lens fibres are supposed to meet

grading cataracts

vitreous chamber 

filled with the gel like vitrous body and occupies the largest prtion of the globe 

all surfaces that interface with vitrous are basemement membrane 

helps maintain shape of eye and keeps retina on the inside surface of eye 

canal running from anterior to posterior there for embryology

some liquid bits some solid

attachments of vitreous

vitrous is attached to the inside surface of eyeball both anteriorly and posteriorly

attachment of vit at the macula- macula is centre of retina posteriorly

also attached to optic disc posteriorly

relations of the vitreous

vit base is attached to pars plana

ora serrata - where retina finishes

these are anterior attachmets of vitreous

divisions of vitreous space 

1. pre retinal tract : separates tge vit cortex from intermediate substance 

2. median and coronary tracts : running to median and coronary ligaments of the pars plana 

3. hyaloid tract: seperates intermediate substance from central channel   

growth and ageing of the vitreous

gradually becomes more liquid as collagen fibres break down

when it gets too liquidly it can detach from retina which can result to a retinal detachment

whats this

anterior cortex

what does the lens epithelial do 

secretes capsule

active transport of ions 

transport metabolites 

elongates to become cortical cells