ANATOMY OF IRIS : SURFACE and LAYERS

In this article we discuss in detail about the anatomy of the iris. If you are an audio visual learner you can visit my video on the anatomy of iris. Also you may refer to the text below as a read along notes for the same.

What is Uvea?

  • The eye ball has three coats; sclera, uvea and retina.
  • The middle vascular coat of the eye is called the Uvea/uveal tract.
  • From anterior to the posterior, the uvea is arbitrarily divided into three parts:-
    1.Iris
    2.Ciliary body
    3.Choroid

Anatomy Of Iris

  • The iris is the most anterior part of the uvea.
  • It is thin circular disc which is  analogous to a diaphragm of a camera.
  • It has a central aperture ~ PUPIL which regulates the amount of light entering the eye.
  • Pupil is situated slightly nasal in the iris .(not in centre)

IRIS ROOT :

  • The Iris is attached at the middle of the anterior surface of the ciliary body.
  • This surface is called the “face” of the ciliary body.
  • The attachment of iris is known a the Root Of The Iris
  • It is the thinnest part of the iris.
  • Detachment of iris drom its root is known as IRIDODIALYSIS .This changes the shape of pupil from circular to a D Shaped pupil.
  • The iris root form s the posterior limit of the angle of eye. For more visit anatomy of angle of eye.
image showing the structure of iris and iris root
IMPORTANT MEASUREMENTS: 
  • Diameter of Iris : 12mm
  • Thickness is 0.5 mm
  • Pupil Diameter : 3-4 mm
image showing the iris diameter and pupil diameter

CLINICAL NUGGET : IRIDODONESIS 

  1. The vibrating/trembling movement of the iris is known as iridodonesis.
  2. Normally the lens supports the iris from behind.
  3. Loss of support from the lens can cause iridodonesis.
  4. This is  seen in
    •Aphakia
    •Subluxation of lens

Anterior V/s Posterior chamber

  • The iris divides the space between the lens and the cornea into two chambers-
  • The chamber situated anterior to the iris is known as the ANTERIOR CHAMBER
  • The Chamber situated posterior to the iris is known POSTERIOR CHAMBER
image depicting the anterior chamber and posterior chamber of eye

Anterior Segment V/s Posterior Segment

  • Anterior chamber + Posterior Chamber together form the  ANTERIOR SEGMENT (consists of the aqueous humour)
  • Part of eyeball between the lens and the retina is known as the  POSTERIOR SEGMENT( contains the Vitreous humour)
  • We can study the anatomy of iris as its macroscopic/surface anatomy and its microscopic/layer wise anatomy.
image depicting the anterior segment and posterior segment of eyeball

Surface Anatomy Of Iris (Macroscopic Anatomy)

Iris has two surfaces ; an anterior surface and a posterior surface.

Anterior Surface of Iris : Pupillary and Ciliary Zones

  • The collarette is a zig zagged line situated at 2 mm from the pupillary margin
  • It represents the point of attachment of the pupillary membrane. The pupillary membrane is the part of anterior tunica vasculosa lentis of fetal vasculature. 
  • This usually regresses after birth , but if persists it forms a Persistent pupillary membrane.
  • The anterior surface of the iris can be divided into two zones by the collarette.
    1. THE CILIARY ZONE : the zone outer to the collarette.
    2. THE PUPILLARY ZONE: the zone inner to the collarette.
Image showing the iris is divided into two zones by the collarette , namely the ciliary zone and the pupillary zone
Topographical Details Of Ciliary Zone

RADIAL STREAKS :

  • Radial streaks corresponds to the radial iris vessels.
  • These are straighter in miotic pupil.
  • The streaks become wavy when pupil is dilated.

CONTRACTION FORROWS:-

  • Contraction Furrows are concentric faint lines parallel to the collarette
  • More pronounced when pupil dilates
  • More marked near the outer part of the ciliary zone.

IRIS CRYPTS :

  • Iris crypts are the points where the anterior layer of the iris is absent.(discussed below)
  • The crypts are arranged in two rows.
  • Peripheral crypts are present near the iris root.
  • Central crypts are present near the collarette.

What Is a PUPILLARY FRILL? 

  • Pupillary Frill is a fringe of black pigment present at the end of the pupillary margin.
  • It represents the anterior end of the optic cup embryologically
  • It is formed by the  extension of the posterior iris pigment around the edge of the pupil.
Anatomy of anterior surface of iris; crypts, radial streaks and contraction furrows

Posterior Surface of Iris

  • Posterior surface of the Iris is pigmented due to pigmented epithelium
  • It can be Brown or blue in colour.
  • Usually looks smooth to naked eye
  • However when microscopically seen, it also has some radial and concentric folds.

Layers Of Iris (Microscopic Anatomy)

The iris consists of 4 layers . From anterior to posterior these are:-

  1. Anterior limiting layer
  2. Iris Stroma
  3. Anterior Epithelium / (Anterior Pigmented epithelium)
  4. Posterior Pigmented Epithelium

CLINICAL NUGGET 

FUCHS HETROCHROMIC IRIDOCYCLITIS/ FUCHS UVEITIS

  • Progressive atrophy of the anterior layer and stroma of the iris is seen
  • This causes iris to lose color and development of hetrochromia.
  • Sometimes severe atrophy of the iris and make the underlying posterior pigment epithelium visible. In such a situation the iris looks darker . This is called as Inverted Heterochromia
  • Transillumination defects can also be seen  – advanced disease
  • In darker individuals, appreciation of heterochromia might be difficult.
  • In such cases blurring of iris crypts could point towards the progressive atrophy of fuchs uveitis.

Iris Stroma

  • The iris stroma forms the bulk of the iris tissue.
  • It is made up of a  network of collagen and ground substance.
  • It consists of :-
  1. Two muscles (Sphincter pupillae and Dilator Pupillae )
  2. Vessels And Nerves
  3. Pigmented cells with other cells like macrophages and  fibroblasts
image showing the four layers of iris : anterior epithelial layer, iris stroma, anterior epithelial layer and posterior epithelial layer

Anterior Limiting Layer of Iris

SPHINCTER PUPILLAE

  • Forms 1 mm circular band around the pupil
  • Derived from the ectoderm
  • Action:  Constriction of pupil (MIOSIS)
  • Nerve Supply :Innervated by the parasympathetics postganglionic fibres of third nerve via short ciliary nerve

DILATOR PUPILLAE

  • It is situated in the stroma of ciliary zone.
  • Has myoepithelium origins. It originated from the anterior epithelium near the iris root .
  • Action: Dilates the pupil (MYDRIASIS)
  • Nerve supply : Supplied by the cervical sympathetic fibres via long ciliary nerve.
image showing the dilator pupillae and sphincter pupillae in iris stroma

VESSELS IN IRIS STROMA 

  •  Iris vessels are radial in orientation.
  • These are derived as branches of the major arterial circle
  • The radial vessels are responsible for formation of the radial furrows.

PECULARITIES OF IRIS VESSES

  • They lack internal elastic lamina
  • Have non fenestrated endothelium.
  • This prevents leakage of the proteins from the Iris vessels into the iris stroma.

CLINICAL NUGGET 

RUBIOSIS IRIDIS ( NORMAL V/S ABNORMAL IRIS VESSELS )

  • Abnormal vascularisation is seen in cases of retinal and ocular ischemia, Chronic uveitis and Neovascular glaucoma
  • Abnormal vessels are not radial in orientation
  • They are dilated and have an irregular shape.
  • They usually show arborization.
image showing new vessels near pupillary margin and iris

CELLS IN IRIS STROMA 

  • Melanocytes : have melanosomes and branching filaments
  • Clump cells : round in shape without branching filaments
  • Other cells in iris are the fibroblasts, macrophages and lymphocyte

CLINICAL NUGGET 

Heterochromia in Horner Syndrome.

  • Iris pigmentation determined by the number of melanin granules in the melanocytes
  • Sympathetic innervation during fetal life determines the migration of the melanocytes
  • Patients with horner syndrome have lighter iris.
  • For video on horner syndrome.
Iatrogenic Increased Iris Pigmentation 
  • Prostaglandin analogues like latanoprost and bimatoprost, can cause increased iris pigmentation along with hypertrichosis of the eyelashes.

Anterior Epithelial Layer

  • Third layer of the iris.
  • It is also a pigmented layer (do not get confused ); although not as heavily pigmented than the Posterior Pigmented epithelium.
  • Itis considered an anterior continuation of pigmented layer of ciliary body 
  • It gives origin to the dilator pupillae muscle.

Posterior Pigmented Epithelial Layer

  • Posterior most layer of the iris
  • It is heavily pigmented
  • Derived from the inner layer of the optic cup.
  • It is an anterior continuation of the non pigmented epithelium  of the ciliary body which is a continuation of the neurosensory retina.
  • Consists of many melanocytes.
  • At the pupillary margin forms the frill and meets the anterior epithelium .

CLINICAL NUGGET 

ECTROPION UVEA :

  • Presence of iris pigment epithelium/ posterior pigment epithelium on the anterior surface of the lens is known as ectropion uvea.
  • It can  be seen in a variety of congenital and acquired conditions
image depicting ectropion uvea

CONCLUSION

So that was about the Iris anatomy. In our next post we shall discuss in detail about the anatomy of the ciliary body. If you are an audio visual learner you can check out our video on the anatomy of iris.

Stay tuned ! Dont forget to subscribe to our youtube channel “insight Ophthalmology”

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