Anatomy and Aplied anatomy of Uvea

Presenter : Dr. Darshan S MModerator : Dr. K Kanthamani

• Uvea is the middle vascular coat of eye ball.• From anterior to posterior, the uvea or uveal

tract can be divided into three parts - Iris, - Ciliary body - choroid.

• The name “uvea” has originated from Latin word grape.

• Why a grape ? If the stem is removed from a grape, the hole

looks like the pupil and the grape the eyeball.

Iris

• Iris is the anterior most part of the uveal tract. It is a thin and circular structure which forms a diaphragm like structure in front of the crystalline lens.

• The word “iris” has originated from a Greek word. In Greek mythology the iris is the name of Greek goddess of rainbow -

• The diaphragm formed by iris contains a central aperture known as pupil.

• The location of the pupil is not exactly central, its little nasal to the center. The pupil determines the amount of light entering the eye.

• The normal size of pupillary aperture is 3-4mm.

• Iris is attached to the middle of anterior surface of ciliary body.

• The iris divides the space in front of the lens into anterior chamber and posterior chamber.

Topography of IRIS:

> Average diameter of the iris is 10 to 11 mm. It is thickest at collarette, which is located approximately 1.5 mm from the pupillary margin and thinnest at iris root,

• Root is the part of iris which joins with the ciliary body.

• The thickness of iris root is approximately 0.5 mm.

• Anterior surface of the iris is divided into a pupillary zone and a ciliary zone by a circular ridge, located 1.5 mm away from pupillary margin, called collarette

• also known as iris frill.

Aplied anatomy

• During blunt trauma, damage to iris occurs most commonly at the iris root, where the iris rips away from the ciliary body

• Anterior surface of the iris is divided into a pupillary zone and a ciliary zone by a circular ridge, located 1.5 mm away from pupillary margin, called collarette (also known as iris frill).

Aplied anatomy

• Collarette is the site of foetal pupillary membrane attachment

Pupillary Zone

• Pupillary zone extends from pupillary margin to collarette.

• Pupillary zone is relatively flat. • Pupillary margin is marked by a dark border,

known as pupillary ruff.• Pupillary ruff is the anterior termination of

the pigmented layer.

Ciliary Zone

• Ciliary zone of iris extends from collarette to iris root.

• There are some depressions or pit arranged in rows present in this area known as crypts.

• Crypts are found in two locations. Those present near collarette are relatively larger and known as Fuchs’s crypt and few are seen in periphery of the iris

Aplied anatomy

• In laser iridotomy, the opening is created in areas of iris crypts, as it requires less amount of energy in these thinnest areas of iris thickness.

Microscopic structure of Iris1. Anterior limiting layer

• Anterior limiting layer lines the iris and is the anterior most condensations of iris stroma.

• The layer consist of mainly fibroblasts and melanocytes.

• These cells are arranged in a meshwork

• -fibroblasts are located on the surface and melanocyte beneath them.

• The colour of iris depends on the thickness of the layer and melanocyte dispersed in anterior limiting layer.

• The color of the iris is largely determined by three main variables:

(1) the density and structure of the iris stroma, (2) the pigment epithelium and

(3) the pigment content (granules) within the melanocytes of the iris stroma

• Heterochromia of iris (Greek: heteros 'different' + chroma 'color'):

• it is of two kinds. • In complete heterochromia, one iris is a different

color from the other. • In partial heterochromia or sectoral

heterochromia, part of one iris is a different color from its remainder.

Aplied anatomy

• Neovascularisation of iris occurs in anterior limiting layer.

2. Iris stroma

• Iris stroma forms the main bulk of iris tissue and contains sphincter pupillae, dilator pupillae muscles, vessels and nerves

• Cells in iris stroma:• Pigmented cells = melanocytes + clump cells• Non-pigmented cells = fibroblasts +

lymphocytes + macrophages + mast cells

Aplied anatomy • Pigment dispersion syndrome• It is a bilateral condition characterized by the

liberation of pigment granules from the iris pigment epithelium.

• It Is caused by the mechanical rubbing of the posterior pigment layer of the iris against lens zonules as a result of excessive posterior bowing of the mid-peripheral portion of the iris.

• sometimes pigment epithelium itself may be abnormally susceptible to pigment shedding.

Muscles in iris stroma:• The sphincter pupillae muscle is a circular muscle,• 0.75 to 1 mm wide, composed of smooth-muscle cells.• The muscle is 0.1 to 1.7 mm in thickness and is considerably

thicker than the dilator papillae.• It encircles the pupil and is located in the pupillary zone of

the stroma. • The sphincter muscle is firmly adherent to the surrounding

stroma of iris. • Sphincter muscle is composed of spindle-shaped cells that

are oriented parallel to the pupillary margin so contraction of the sphincter causes the pupil to constrict (a process known as miosis).

• The muscle is innervated by the parasympathetic system.

Aplied anatomy

• the sphincter pupillae retains its function even if severed radially because of its unique distribution of fibres.

• the dilator pupillae muscle extends from the iris root to a point in the stroma below the midpoint of the sphincter.

• A dense band of connective tissue separates the sphincter and dilator muscles from each other.

• Because of the radial arrangement of the fibres of the muscle, contraction of the dilator pupillae muscle pulls the pupillary portion toward the root, thereby enlarging the the size of pupil ( a process ka mydriasis).

• The dilator pupillae muscle is sympathetically innervated.

Blood vessels in iris stroma:• The iris arteries are branches of major circle of the iris,

located in the ciliary body near the iris root. • The iris vessels usually follow a radial course from the iris

root to the pupil margin. • These vessels are surrounded by a dense network of

collagenous fibrils which is embedded in to the collagen network of the stroma.

• Such arrangement of collagen network prevents the iris vessels from kinking and compression during the extensive iris movement during constriction and dilatation of pupil.

• Iris veins have very thin walls consisting of endothelium surrounded by a thin layer of collagen

Ciliary body• Ciliary body is the middle part of the uveal tract . It is a ring

shaped structure which projects posteriorly from the scleral spur.

• It is brown in colour due to melanin pigment. • Anteriorly it is confluent with the periphery of the iris (iris

root) and anterior part of the ciliary body bounds a part of the anterior chamber angle.

• Posteriorly ciliary body has a crenated periphery, known as ora serrata, where it is continuous with the choroid and retina.

• Ciliary body has a width of approximately 5.9 mm on the nasal side and 6.7 mm on the temporal side.

Parts of ciliary body• Ciliary body, in cross section, is a triangular structure .• Outer side of the triangle (O) is attached with the

sclera with suprachoroidal space in between.• Anterior side of the triangle (A) forms part of the

anterior & posterior chamber. In its middle, the iris is attached.

• The inner side of the triangle (I) is divided into two parts. The anterior part (2 mm) with finger like processes is known as pars plicata (corona ciliaris) and posterior smooth (4 mm) is known as pars plana (orbicularis ciliaris).

Pars plicata:• The pars plicata is the portion of ciliary body

which contains the ciliary processes.• Ciliary processes are the finger like projections ,

which extend into the posterior chamber.• The regions between ciliary processes are called

valleys of Kuhnt. • They are approximately 70 to 80 in numbers. • A ciliary process measures approximately 2 mm

in length, 0.5 mm in width, and 1 mm in height.

Pars plana• Pars plana is the flat or smooth part of the

ciliary body. • It terminates at the ora serrata, which is the

transitional zone between ciliary body and choroid.• Histologically, the pars plana consists of a double

layer of epithelial cells: the inner, nonpigmented epithelium, which is continuous with neurosensory retina

• and the outer, pigmented epithelium, which is continuous with the retinal pigment epithelium (RPE) .

Aplied anatomy

• The pars plana is a relatively avascular zone, which is important surgically in the pars plana approach to the vitreous space.

• The pars plana provides surgical access to the vitreous and retina

Ora serrata: the transition zone

• The ora serrata can be termed as the anterior border of the neurosensory retina

• Aplied anatomy Topographically, ora serrata corresponds to the

insertion of the medial and lateral rectus muscles.

Lens and ciliary body:

• The zonules course from ciliary body to the lens.

• Some of these zonular fibers insert into the internal limiting membrane of the pars plana region and travel forward through the valleys (valleys of Kuhnt) between the ciliary processes

Layers of ciliary body:

• From inside to outside (from sclerad to vitread), ciliary body consists of following four layers.

• 1. Ciliary epithelium:• 1A . Non pigmented epithelium of ciliary body NPE of

ciliary body extends from iris root to ora serrata.• It begins as a continuation of posterior pigmented

epithelium of iris near iris root.• At ora serrata, the NPE continues as sensory retina.• It gives origin to parts of the suspensory lens ligament.

• 1 B. Pigmented epithelium of ciliary body:• The cells of the pigmented epithelium are 8 to 10

micro meter wide and contain large pigment granules.

• These pigment granules are three to four times larger than those of the choroid and retina.

• These cells are rich in organelles and are very active metabolically .

•The cells of pigment epithelium secretes basement membrane which continues posteriorly with the retinal pigment epithelium (RPE).

Aplied anatomy

• Ciliary epithelium : metabolic activity• The cells of both the ciliary epithelium have a

greater number of mitochondria and thus they have a higher degree of metabolic activity, with a significant role in the active secretion of aqueous humor.

2. Ciliary stroma:

• Ciliary stroma contains blood vessels, nerves and ciliary muscle.

• Ciliary stroma continues anteriorly with iris stroma and continues posteriorly with choroidal stroma after thinning out at pars plana

• Blood vessels in ciliary stroma: Major arterial circle of iris

• Major arterial circle of iris, formed by the anastomosis of long posterior ciliary arteries and anterior ciliary arteries , is located in ciliary stroma near iris root just in front of circular portion of ciliary muscle.

• Ciliary stroma also consists of numerous capillaries which are fenestrated and large in size.

• The capillaries are more in numbers in ciliary processes, making them the most vascular organ of the eye

Muscle in ciliary stroma: ciliary muscle

• Ciliary muscle is a nonstriated or smooth muscle primarily situated in the anterior two thirds of the ciliary body stroma. The muscle has three parts –

> Outer longitudinal (Brücke's muscle) > Middle oblique portion (also called reticular

or radial) > inner circular portion (Müller's muscle)

• Contraction of the ciliary muscle, especially of the longitudinal and circular fibers, pulls the ciliary body forward during accommodation.

• This forward movement of ciliary body relieves the tension in the suspensory lens ligament (zonules), making the elastic lens more convex and there by helps the eye in accommodation by increasing the refractive power of the lens

• Ciliary muscle is innervated by the autonomic nervous system, parasympathetic postganglionic fibres derived from the oculomotor nerve.

• The nerve fibres reaches the muscle via short ciliary nerve.

• The parasympathetic stimulation activates the muscle for contraction, whereas sympthetic innervation likely has an inhibitory effect.

Supraciliary lamina:

• supraciliary lamina is the outermost layer of ciliary body which lies adjacent to the sclera.

• Because of the lamellar arrangement of connective tissue in this area, supra ciliary lamina acts as a potential space.

• Thus it also helps aqueous humor to exit by the unconventional pathway .

Aplied anatomy

• Ciliary body detachment occurs through supraciliary lamina

Ciliary process :

• Ciliary processes are finger like projections seen in pars plicata of ciliary body.

• Ciliary processes are approximately 70 to 80 in numbers in each eye and extend into the posterior chamber, and the regions between these ciliary processes are called valleys of Kuhnt.

• Zonules of lens (suspensory ligaments of lens) are inserted in these valleys.

• The ciliary processes are white whereas the valleys of Kuhnt are dark in colour.

• Ciliary processes increase the surface area of pars plicata, which is approximately 6 square centimetre, approximately five times the surface area of corneal endothelium

Blood supply of ciliary process:• Long posterior ciliary artery, a branch of

ophthalmic artery, pierce the globe near the optic nerve and run up to ciliary body to form major arterial circle.

• Several branches from major arterial circle supply ciliary processes.

• These are mainly pre capillary arterioles and they divide in to network of capillary plexuses in each of the ciliary processes.

• These vessels drain in to choroidal and intrascleral veins

Aplied anatomy

• The pre capillary arterioles supplying the ciliary processes have sphincters which may be responsible for the auto regulation of blood supply to the tissue.

Choroid

• Choroid is a thin but highly vascular membrane lining the inner surface of sclera.

• It extends from anteriorly ora serrata to the optic nerve posteriorly.

• Choroid becomes continuous with pia and arachnoid at the optic nerve.

• Choroid is normally 100-220 µm thick ; thickness is highest at macula 500- 1000 µm.

Aplied anatomy

• Choroidal thickness increases in intraocular inflammation.

• Choroid can be divided into the following layers histologically

• 1. Suprachoroid lamina (lamina fusca):• 2. Choroidal stroma:• 3. Layer of Choriocapillaris (Choriocapillaries

are thickest and most abundant in submacular area )

Aplied anatomy

• choroidal circulation:• Choroidal circulation constitutes 85% of the

blood circulation of the eye.• Choroidal blood flow is higher than that in

tissues like retina and brain.• Choroidal blood-flow ranges from 800 to 2000

mL/min/100 g of tissue.• In embryonic life, choroid serves as an

additional site for the erythropoiesis.

4. Bruch's membrane:

• Bruch's membrane is the innermost layer of choroid

• Bruch's membrane is composed of 5 layers and from internal to external,

• these are

Aplied anatomy

• Outer blood–retinal barrier The outer blood–retina barrier (BRB) is composed of three structural entities,

> the fenestrated endothelium of the choriocapillaris,

> Bruch's membrane and > the retinal pigment epithelium (RPE).

Blood supply of uveal tract:

• The blood supply of the uveal tract is mainly from three arteries namely short posterior ciliary arteries, long posterior ciliary arteries and anterior ciliary arteries. The posterior ciliary arteries are branches of the ophthalmic artery.

SHORT POSTERIOR CILIARY ARTERIES

• 15 to 20 short posterior ciliary arteries arise → form 10 to 20 branches → enter the sclera in a ring around the optic nerve → anastomose with other branches from the short posterior ciliary arteries to form the circle of Zinn (Zinn-Haller) which encircles the optic nerve at the level of the choroid → they run in suprachoroidal space between sclera and choroid, branch and

• supply the choroid.

LONG POSTERIOR CILIARY ARTERIES

• two long posterior ciliary arteries enter the sclera: one lateral and one medial to the ring of short ciliary arteries → run between the sclera and the choroid to the anterior globe → enter the ciliary body and branch superiorly and inferiorly → anastomose with each other and with the anterior ciliary arteries to form a circular blood vessel,

• the major arterial circle of the iris

ANTERIOR CILIARY ARTERIES :

• 7 anterior ciliary arteries are derived from muscular branches of ophthalmic artery

• two each from arteries of superior rectus, medial rectus, inferior rectus and only one from lateral rectus muscle → reaches episclera, form plexus and give branches → pierce sclera near the limbus to enter the eye → anastomoses with long posterior ciliary arteies to form major arterial circle of iris

• Thank you !!