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Histology of The Eye

Copyright to Paul J. May, PhD, 2010

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External Features of the Eye

Pupil is dilated, are they interested?

Mascara

Place

where

sleep

stuff

endsup

Plucked?

Thicker,

Longer

Lashes

Get the red out of the conjunctiva

Brace

to keep

eyes

open

for Dr.

Mays

talks

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Optic Nerve

Sclera

Eye Lid

AnteriorCornealEpithelium

CornealStroma

Conjunctiva

Ciliary

Body

Lens

Choroid

VitreousChamber

AnteriorChamber

NeuralRetina

RetinalPigment

Epithelium

Zonule

of Zinn

PosteriorChamber

Iris

Primitive Eye Drawing

BY PAUL J. MAY

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Advanced

Eye

Drawing

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Histology of the Eye

Lacrimation

The tear film has two functions:

1. The aqueous smooth surface makes for better optics.

2. Oxygen is provided to the anterior cornea by first

being dissolved in the tear film.

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The lacrimal gland is an entirely serous gland like the parotid, but it lacks fat

cells that are present in the parotid gland.

Secretion is controled by preganglionic parasympathetic motor neurons in

the superior salivatory nucleus. They exit with the facial nerve (CN VII). The

postganglionics are located in the pterygopalatine ganglion.

Lacrimal Gland

Slide Set

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Lacrimal Gland

Junquiera & CarneiroAll Serous, Compound Tubuloalveolar Gland.

Produces a watery exudate containing lysozyme with antibacterial properties.

Myoepithelial

Cells

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Eyelid

The eyelids close in reflexive blinks to protect the

eye. They also blink rhythmically to spread the tear

film over the cornea. They help form the conjunctival

sac, which holds the tears. Uncontrolled repetitive

blinking is called blepharospasm.

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Levator Palpebrae Superioris Muscle

Holds upper lid up. Supplied by motor

neurons in the Oculomotor Nucleus.

Tarsal Muscles Helps hold lids open.Nerve supply:

Preganglionic sympathetics T1 and T2

intermediolateral gray

Postganglionic motor neurons in

superior cervical ganglion.

Orbicularis Oculi Muscle Closes eye.

Supplied by motor neurons in facial

nucleus via CN VII.

Tarsal Plate stiffens the lid.

Tarsal Gland of Meibohm: Compoundbranched alveolar sebacous gland that

secretes oil to limit evaporation of tears.

Lashes are sensors that trigger blinks.

Glands of Moll Apocrine gland

Product may be antimicrobial.

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Orbicularis Oculi

Muscle

Lash

GlandOf Moll

Tarsal GlandOf Meibohm

Lash

Slide Set

Here are the structures found distally in the eyelid.

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Superior Tarsal Muscle

Tendon of

Levator Palpebrae

Superioris Muscle

Tarsal Gland

Of Meibohm

Slide Set

Here are the structures found proximally in the eyelid.

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Histology of the

EyeCornea

Remember, the cornea is avascular, but highlyinnervated. The former characteristic allowedit to be transplanted long before other organs.

It is the main, albeit non-adjustable, elementof refraction. LASEKS/LASIKSsurgerytakesadvantage of this fact by resculpting the

corneal curve.

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Anterior Corneal Epithelium

Stratified, nonkeratinized, squamous

epithelium.

Bowmans Membrane (anterior

limiting membrane) The first layer of

underlying connective tissue.

Corneal Stroma or Substantia Propria

Very regularly arranged Type I

collagen fibers produced by

keratocytes.

Descemets Membrane (Posterior

limiting membrane) Basement

membrane of the posterior corneal

epithelium. Made up of type VIII

collagen.

Posterior Corneal Epithelium (formerly

the corneal endothelium) Simple

squamous epithelium that controls

passage of fluid and nutrients to the

corneal stroma.

Posterior

Corneal

Epithelium

Descemets

Membrane

Corneal

Substantia

Propria

Keratocytes

Anterior

Corneal

Epithelium

Bowmans

Membrane

Corneal Layers

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Slide Set

The layers of the cornea are easily seen. Note the greater

thickness of the external surface.

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Corneal

irregularities

can cause

astigmatismuneven

focusing of

the image.

Anterior

Corneal

Epithelium

Surface processes

on epithelial cells

help hold the tearfilm in place.

Stratified squamous

epithelium, usually

5-6 cells thick, is

supplied by freenerve endings.

TEM Cross & Mercer

Junquiera & Carneiro

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Oxygen is supplied to the anterior corneal epithelium by way of

the tear film. Without sufficient tears, the surface ulcerates.

Slide Set

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Note the extremely regular arrangement of Type I collagen fibers in the

stroma. Individual layers lie orthoganal (perpendicular) to one another.

The keratocytes are specialized fibroblasts that maintain the collagen

fibers. It is the stroma that is resculpted in laser surgeries.

TEM Cross & Mercer

Keratocyte

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Type VIII

collagen in

Descemets

membrane

forms a

hexagonal

array.

Posterior Limiting Membrane Descemets Membrane

Descemets

membrane is

actually the

thickened

basement

membrane ofthe posterior

corneal

epithelium.

Posterior Limiting Membrane

Geneser

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Posterior Corneal Epithelium

This simple

squamous

epithelium hastight junctions

between the

cells and it

controls the

flow of nutrients

to the corneal

stroma.

It keeps the

stroma slightly

dehydrated, for

better optics.

Slide Set

Anterior

Chamber

P.C.E.

Cross & Mercer

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Slide Set

The limbus (corneal-sclera junction) contains Tenons capsule, where the

collagen fibers run parallel to the edge of the cornea to maintain its bulging

shape. Unfortunately, this difference in orientation is not visible in sections.

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Iris & Pupil

The iris controls the amount of light entering the eyethrough the pupil in order to keep the light levels falling onthe retina in the best range of the photoreceptors. Thus,the pupil constricts in response to higher luminancelevels. It also constricts to increase depth of field when

viewing nearby targets. The pupil dilates when luminancelevels fall, but also in response to arousal caused by fearor desire. Sympathomimetics are used to dilate the eyefor easier visualization of the retina with a fundoscope.

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The iris is a

delicate flap of

tissue lying in front

of the lens.

Junquiera & Carneiro

View on Next Slide

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Components of the Iris

Anterior iridial border Just an

edge made by fibroblasts and

melanocytes.

Iris stroma Loose connective

tissue with vessels and

melanocytes.

Anterior iridial epithelium A layer

of inner tunic cells with melanin

granules. These cells have

processes that contain myosin and

form the dilator pupillae muscle.

Posterior iridial epithelium

Densely pigmented back layer of

the iris, also part of inner tunic.

Constrictor (sphincter) pupillae

muscle band of smooth muscle

that encircles pupillary margin.

Slide Set

Ant & Post

IridialEpithelium

Anterior

IridialBorder

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Dilator pupillae muscle Sympathetic: Preganglionics in T1

intermediolateral gray, Postganglionics in Superior cervical ganglion.

Sphincter pupillae muscle: Parasympathetic: Preganglionics in Edinger-

Westphal N., exit with CN III, Postganglionics in ciliary ganglion.

The dilator pupillae muscle is actually made of a series of processes from

anterior iridial epithelium cells. These processes contain myifibrils and

constrict in response to adrenergic sympathetic input.

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Slide Set

Anterior iridial epithelium A layer of inner tunic cells with melanin

granules. Their processes contain myosin and form the dilator

pupillae muscle.

Posterior iridial epithelium Densely pigmented back layer of the

iris, also part of inner tunic.

PosteriorIridial

Epithelium

AnteriorIridial

Epithelium

Dilator Pupillae M.

Use your oil objective to see the myoepithelial anterior iridial epithelium.

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Histology of the

Eye

Ciliary Body

The ciliary body has three functions:

1. It controls the curvature of the lens, allowing theimage to be focused on the retina.

2. It produces aqueous humor, which flows through theposterior chamber, nourishing the lens, and into theanterior chamber to nourish the cornea.

3. It maintains the vitreous humor.

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Ciliary Body

The Cilary Body has 2 main parts: Pars plicata (folded part) which produces the aqueous humor

for the posterior and anterior chambers

Pars Planum (flat part) which maintains the vitreous humor.

The Ciliary Body has 4 layers (from outside in): Ciliary Muscle smooth muscle which controls tension in the

zonnule of Zinn, to maintain the focus of the lens

Stratum Vasculosum - which contains fenestrated capillaries thatare the source of the aqueous humor

Pigmented Ciliary Epithelium - which absorbs any stray lightstriking the ciliary body.

Unpigmented Ciliary Epithelium - which regulates thecomponents of the aqueous and vitreous humor.

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Pars plicata makes aqueous

Pars planum maintains the vitreous

Slide Set

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Fenestratedvessels in the

stratum

vasculosum are

the source of the

fluids flowing in

the aqueous

humor.

However, the

unpigmented

ciliary epithelium,with its tight

junctions, controls

the components of

the aqueous. Slide Set

Pars Plicata of the Ciliary Body

PigmentedCiliary

Epithelium

Ciliary Muscle

Unpigmented

CiliaryEpithelium

Stratum

Vasculosum

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TEM of Ciliary EpitheliumUnpigmented

Cross & Mercer

Pigmented

Note the large number of vacuoles in these cells that support

movement from the fenestrated capillaries into the aqueous humor.

Posterior

ChamberFenestratedCapillary

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Histology of the

Eye

Aqueous Humour

Aqueous humor has high levels of NaCl, bicarbonate, amino acids

and ascorbate, but relatively low levels of glucose and almost noprotein.

The rate of aqueous humor production is controlled by the bloodpressure and flow rate of the blood in stratum vasculosum.

The content of the aqueous humor is regulated by the unpigmented

ciliary epithelium, which forms the blood-aqueous border.

Aqueous humor is produced in pars plicata of the ciliary body, flowsthrough the posterior chamber, pupil and anterior chamber. Itsustains the lens and cornea, but not the iris, which is well suppliedwith vessels. It empties into the trabecular meshwork at theirideocorneal angle, and from there drains into the canal of Schlemm.

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Posterior Chamber

Anterior

Chamber

Flow ofAqueous Humour

Slide Set

Aqueous humor is produced in pars plicata of the ciliary body,

flows through the posterior chamber, pupil and anterior chamber. It

empties into the trabecular meshwork at the irideocorneal angle.

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Slide Set

The cornea meets the sclera at the limbus. The largest holes found in the

limbus near the irideocorneal angle belong to the Canal of Schlemm. This

canal is not continuous, so it may not be present in every slide.

Slide Set

Limbus

Cornea

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Glaucoma is caused by a lack

of balance between the

production and outflow of

aqueous humor. The build up inpressure is painless, but it will

eventually cause compression

of the vessels that supply the

retina, leading to blindness. You

can recognize this problem with

a simple ophthalmoscopic(fundoscopic) exam. Look for

cupping of the optic disk where

the retinal arteries enter.

In narrow angle glaucoma the

irideocorneal angle impedesoutflow.

In open angle glaucoma the

problem may be ciliary

overproduction or clogging of

the trabecular meshwork.

Cornea

Trabecular

Meshwork

Slide Set

Canal of

Schlemm

Aqueous empties

into the trabecular

meshwork, and from

there drains into the

canal of Schlemm.

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Junquiera

& Carneiro

Ora Serata

Retina

Ciliary BodySlide

Set

The ora

serata is the

border

between the

ciliary body,with its 2-

layered

epithelium

and the retina,

with a 10-layered

epithelium.

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Histology of the

Eye

Actions of the Ciliary Body,Suspensory Ligament

(Zonule of Zinn) upon the Lens

The Near Triad refers to the three actions that occur when looking at closeby targets. They are all produced by axons of motor neurons traveling in

the oculomotor nerve (C.N. III) to the extraocular muscles (1) and the

ciliary ganglion (2&3).

1. Convergence: The skeletal muscle fibers of the medial rectus m. contract

to converge the eyes, due to input from alpha motor neurons.2. Accommodation: The smooth muscle in the ciliary m., activated by

postganglionic parasympathetics, constricts so that the anterior surface

of the lens will become more convex, focussing the image on the retina.

3. Pupillary Constriction: The smooth muscle of the sphincter pupillae m. is

activated by postganglionic parasympathetics, to increase depth of field.

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The suspensory

ligaments (zonnule

of Zinn) form a

series of spokes all

along the equator

of the lens. Theyoriginate from pars

planum and in

between the folds

of pars plicata.

They insert into the

lens capsule.

Junquiera

& Carneiro

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Slide Set

The fine

filaments of

thesuspensory

ligament

can be

seen lying

betweenthe lens

and ciliary

body, and

along thesurface of

the ciliary

body.

Suspensory

Ligaments

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Dista t Foc s:iliary scle Relaxed

Zo le te seterior le s s rface flatte ed

Near Foc s:iliary scle o stricted

Zo le slack terior le s s rface c rved

The ciliary muscle

contracts when

we attempt tofocus on nearby

targets.

This decreases

the tension in the

zonule, whichallows the anterior

surface of the lens

to assume a more

curved shape dueto its own internal

elasticity.

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The cornea provides the strongest refraction, but

only the lens provides adjustable refraction.

Myopia loss of distance vision.Hyperopia and, with age, Presbyopia- loss of near

vision.

These can be due to the length of the eyeball or

problems in the refracting elements, e.g. cornea.

Diseases ofAccommodation

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The Lens

The anterior lens is coated by a simple squamous tocuboidal epithelium the anterior lens epithelium.

The posterior lens consists of an innernucleus and an

outercortex. They are made up of elongated cells calledlens fibers.

The entire structure is encased by a very thick basementmembrane called the lens capsule.

Cataracts are any opaque areas in the lens. These canbe due to age or other sources such as trauma orinfection. They are generally treated by replacing thenatural lens with an artificial one.

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Cortex

Nucleus

Due to divisions in the

anterior lens

epithelium, over time

cells are added to the

cortex at the equator ofthe lens. These are in

turn converted into

nuclear fibers.

The nucleus is not as

flexible, so as it growswith age, the lens

becomes more difficult

to focus. Eventually, in

presbyopia, the lens

loses so much of itsnatural flexibility that

the actions of the

ciliary muscle can no

longer bring close

objects into focus.

Embryonic Lens

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Cortex

Lens Nucleus

Zonule of Zinn

The nucleus of

the lens has

cells that lack

nuclei.This region

has the best

optics.

The cells in

the cortex still

have nuclei,

so they are

alive, and are

more flexible,allowing the

lens to

accommodate

for focus.

Slide Set

The Equator Region of the Lens

Anterior

Lens Epithelium

Lens

Capsule

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Lens fibers in the

nucleus are

practically dead

cells. They are

simply bags filled

with a protein groupcalled crystalins.

Numerous gap

junctions

(arrowheads)

keeping the lensfibers ionically

connected.

Cross & Mercer

TEM

SEM

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Histology of the

Eye

Choroid & Sclera

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Pig e t it eli f t e eti

ri c ill ris

st tiPr ri f t ec r i

r c se r e

Layers of the Choroid

Bruchs membranebasement membrane of

the pigment epithelium.

Choriocapillaris thin layer

with the capillary bed that

feeds the pigmentepithelium and

photoreceptors.

Substantia propria dark

layer containing large

vessels and numerous

melanocytes.

Suprachoroid densely

pigmented outer layer of

the choroid. (We will NOT

identify this as a separate

layer.)

Blood in the choriocapillaris supplies the

retinal pigment epithelium and photoreceptor

layer. The rest of the retina is supplied by

branches of the central retinal artery.

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Substantia

Propria

of theChoroid

Slide Set

You will need to find these layers of the choroid in the lab.

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In retinaldetachment,

the seal between the

retinal pigment

epithelium and theretinal photoreceptors

(double headed arrow)

is broken. This deprives

the photoreceptors of

their blood supply in the

choriocapillaris.

Slide Set

You will need to use

an oil objective todistinguish the

Choriocapillaris and

Bruchs membrane

as separate features.

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Layers of the Sclera

Episclera hasvessels, nerves and

lots of adipose tissue.

Substantia Propriahas thick bundles of

collagen.Episclera

S

U

BS

T

A

N

TI

A

P

R

OP

R

I

AScleraSlide Set

CuttingArtifact

Retina

C

H

O

R

O

I

D

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The collagenbundles are

not as well

ordered as in

the cornea.

Fibroblast

The retina sends

signals to the sclera of

the growing eye ball.

These signals regulate

scleral growth, andhence the shape of the

eyeball, in order to

maintain a size

appropriate for proper

focus.If the eyeball is too

short, the result is

hyperopia. If it is too

long, the result is

myopia. The near workhypothesis suggests

that myopia may result

from doing too much

close up focusing in

early life.

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Slide Set

You may see skeletal muscle fibers in the episclera. These belong

to the extraocular muscles that point the eye at targets.

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Inner Limiting Membrane

Nerve Fiber Layer

Ganglion Cell Layer

Inner Plexiform Layer

Inner Nuclear Layer

Outer Plexiform Layer

Outer Nuclear layer

Photoreceptor Layer

Pigment Epithelium

Vitreous

You will

need to

know the

layers of

the retina

fortodays

lab. The

contents

will be

handled

in a

fututre

lecture.

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The optic nerve

head or opticpapilla can be

visualized with

your ophthalmoscope. Its appearance

is important for diagnosis. The central

retinal artery enters here.

Cupping of the optic papilla is

indicative ofglaucoma.

Swelling of the nerve head,

papilledema, is indicative of increasedintracranial pressure.

This exam also shows the macula, the

thick central retina, which contains the

fovea, the area of best vision.

Slide Set

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If you look

at your slide

beforeplacing it on

the stage,

this is what

youll see.

The clever

student will

use this to

figure out

where they

should belooking

before they

use the

microscope.

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The End

Slides showing the developmental schemathat produces the layers of the eye ball and

related terminology are appended here.These may be of value for thecomprehensive exam.

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Tunics of the EyeInner Tunic

Pars Optica (seeing part)

Neural retina Pigment epithelium

Pars Caeca (blind part)

Pars Ciliaris

Unpigmented CiliaryEpithelium

Pigmented CiliaryEpithelium

Pars Iridica Anterior Iridial Epithelium

Posterior Iridial Epithelium

Vascular Tunic Pars Optica (seeing part)

Choroid


Pars Ciliaris

Stratum Vasculosum

Ciliary Muscle

Pars Iridica

Iridial Stroma

Outer orFibrous Tunic

Pars Optica (seeing part) Sclera


Pars Ciliaris

Sclera

Pars Anterior

Corneal Stroma

Optic erve

Sclera

Eye Li


CornealStroma

Conjunctiva

CiliaryBo y

Lens

Choroi

VitreousChamber

AnteriorChamber

euraletina

etinalPi mentEpithelium

onuleof inn

PosteriorChamber

Iris

Neural

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Lens Placode

NeuralTube

VascularTunic

Lens Vesicle

Lids

Lids

CornealEpithelium

Fibrous Tunic

Lens Placode

NeuralTube

Vascular

Tunic

Lens Vesicle

Lids

Lids

CornealEpithelium

FibrousTunic

Optic Stalk

Optic Cup

Lens Placode

NeuralTube

Ectoderm

VascularTunic

Lens Vesicle

Lids

Lids

Corneal

Epithelium

Fibrous Tunic

Eye development drawings

that you can do.

Future Inner Tunic

Pigment

Epithelium

Neural RetinaOpticN

rve

Sclera

Eye

id


CornealStroma

Conjunctiva

CiliaryBody

Lens

Choroid

VitreousChamber

AnteriorChamber

NeuralRetina

Retinal

igmentEpithelium

Zonuleof Zinn

osteriorChamber

Iris

1

2

3

4

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Ciliary

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Opt ic Nerve

Sclera

Eye Lid

Anterior

Corneal

Epithelium

Corneal

Stroma

Conjunc tivaBody

Lens

Choroid

VitreousChamber

Anterior

Chamber

Neural

Retina

Retinal# igment

Epithelium

Zonuleof Zinn

# osterior

ChamberIris

Optic Nerve

CiliaryBody

Lens

Iris

ars tica ars aeca

Choroid

StratumVasculosum

CiliaryMuscle

IridealStroma

Vascular TunicPars Optica (seeing part)

Choroid


Pars CiliarisStratum Vasculosum

Ciliary Muscle

Pars Iridica

Iridial Stroma

Optic Cup

Lens Placode

NeuralTube

Ectoderm

VascularTunic

Lens Vesicle

Lids

Lids

CornealEpithelium

Fibrous Tunic

The vascular tunic derives from the

mesenchyme and neural crest

that aggregates around the optic cup

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Citations for Materials Used

T.D. Pollard and W.C. Earnshaw,Cell

Biology, Saunders ElsevierInc. Philadelphia, 2004.

P.C. Cross and K.L. Mercer, CellandTissue Ultrastructure AFunctionalPerspective, Freeman and Co., New York, 1993.

L.C. Junqueira and J. Carneiro, BasicHistologyTextandAtlas,10th Edition, Lange Medical Books, McGraw-Hill, Chicago, 2003.

AB.M. Carlson, Human Embryology and Developmental Biology, 3rd

Edition, Mosby, Philadelphia, 2004.

F. Geneser, Textbook of Histology, Munfsgaard, Lea & Febiger,Philadelphia, 1986.

Documents

EyeLec.2010-11