The Uveal Tract

OPTOMETRY · SEMESTER 2

Ocular Anatomy and Physiology

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The Uveal Tract

CHAPTER 9. THE UVEAL TRACT

THE FOLLOWING TOPICS WILL BE COVERED BY THE END OF THIS

CHAPTER:

DEFINITION AND STRUCTURE OF THE UVEA

THE IRIS

THE CILIARY BODY

THE CHOROID

SOME DISORDERS AFFECTING THE UVEA

A RWONS

1. DEFINITION AND STRUCTURE.
The uvea is the vascular middle layer of the eye ball.
When the outer layer , the sclera ,is removed the eye ball appears like a black
grape hanging on its stalk( the optic nerve), hence the name.
The uvea is made up of three structures:
The iris.
The ciliary body.
The choroid
The is no clear demarcation between these three parts, they are continuous
with each other , lining the sclera from the anterior opening to the posterior
aperture for the optic nerve.
2. THE IRIS.
This is the most anterior part of the uvea, measuring about 12mm in
diameter.. The iris is a thin, circular structure located anterior to
the lens, often compared to a diaphragm of an optical
‘system. The center aperture
It is a thin, contractile, SdoUghnUt/shaped” structure that is found in the
anterior segment of the eye between the cornea and the lens.
It in turn divides the anterior segment into anterior and posterior chambers.
The anterior chamber is the space between the cornea and the iris, while
the posterior chamber is the space between the iris and the lens.
This division of the anterior segment is important for the Cataract/surgeons!)
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The central opening of the “doughnut” is called the pupil.
By varying the size of the pupil, the iris muscles ( sphincter and dilator) ,
control the amount of light entering the eye.
All light that enters the eye, passes through the pupil.
The aqueous humour , formed by the ciliary processes in the posterior
chamber , circulates through the pupil into the anterior chamber.
The iris is made up of : connective'tissue;muscles)/pigmented|cells
(melanocytes), nerves, blood vessels.
The colour of the iris varies from one individual to another, depending on the
amount of pigment contained by the melanocytes.
2.1. Surface anatomy of the iris.
a. The anterior surface.
The anterior surface of the iris contains crypts and crevices in contact with
the aqueous
in the anterior chamber.
The aqueous humour thus has direct communication with the tissue spaces
of the iris.
The center is the pupil, a black aperture with a varying diameteffrom=8
mm.
The pupil appears black because the interior of the eye is dark.
The anterior surface is divided into:
e acentral, pupillary zone and
e aperipheral , ciliary zone.
The iris is thickest at the demarcation zone between the pupillary and the
ciliary zones called collarette, which lies about 2mm from the pupillary zone.
At the pupillary margin , a darker zone is seen surrounding the pupil. This is
the so called pupillary ruff, representing the posterior pigmented layer of
the iris curving around the pupillary margin.
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b. The posterior surface.
The posterior surface of the iris is markedly pigmented and shows a
number of contraction folds.
2.2. Microscopic anatomy of the iris.
The iris consists of two layers:
a. the stroma;
b. the pigmented epithelial layers;
a. The iris stroma.
The stroma devoid of epithelium, represents the anterior border of the iris.
Embryologically itforiginates frommesenchymey)
It is composed of a highly vascular connective tissue containing collagen
fibres, fibroblasts, melanocytes, nerve fibres and the smooth muscles that
control the pupillary movements (the sphincter and the dilator pupillae)
Blood vessels form the bulk of the iris stroma.
Their course is mostly radial, arising from the major arterial circle passing
towards the centre.
The major arterial circle is located in the ciliary body, not the iris.
At the level of the collarette , anastomoses occur between arteries and veins,
to form the minor vascular circle , which is often incomplete.
The sphincter pupillae muscle is an 1mm wide circular band of smooth
muscle fibers, located in the posterior stroma near the pupillary margin.
Its innervation is by parasympathetic fibres fromthe Edinger Westphal
fiiclelis of cranial/nerve lll;ithese fibres synapse in the ciliary ganglion and
are distributed via the short posterior ciliary nerves to the sphincter muscle.
When the sphincter pupillae contracts , the pupil constricts ( miosis ).
The dilator pupillae is made up by myorpithelial cells that extend from the
iris root to the sphincter pupillae, lying parallel and anterior to the posterior
pigmented epithelium.
The nerve supply is from the sympathetic postganglionic fibres via the long
ciliary nerves.
When the dilator muscle contracts , the pupil enlarges ( mydriasis ).
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b. The pigmented epithelial layers.
There are 2 pigmented layers, the anterior and the posterior pigmented layer.
Embryologically derived from the neuroectoderm of the optic cup, the cells
of the two layers are apposed to each other apex to apex.
Between lies a potential space that can, under certain circumstances, fill _
with fluid and become real space.
The anterior epithelial layer , only slightly pigmented, lies in contact with the
iris stroma and is closely associated with the myoepithelial cells of the
dilator pupillae muscle.
It is continuous withithejouten layer of the ciliary epithelium:
The posterior epithelial layer , facing the posterior chamber, is made of cells
densely packed with melanin.
Anteriorly it extends to the pupillary ruff while posteriorly is continous with
the inner layer of the ciliary epithelium.
Anterior chamber angle
Trabecular meshwork Ciliary zone Pupillary zone
Schlemm's canal
SOS Collarette
[ — Pupil frill
Pupillary
27 margin
Anterior border
Iris root layer Sphincter
papillae
__ Stroma with blood
vessels and nerves
Anterior epithelium—
radial myopepithelial
dilator papillae
muscle
Ciliary process Posterior
epithelium pigmented
epithelium
Fig. 9.1. Diagram of iris and ciliary body anatomy.
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2. THE CILIARY BODY.
The ciliary body is aring shaped structuréthat bridges the iris'and! choroid.
It is 6mm wide extending from the scleral spur (anteriorly ) to the ora
serrata of the retina (posteriorly ).
Triangular in cross section , the apex is directed posteriorly to become
continuous with the choroid, while the base is anterior, giving rise to the iris.
2.1. Macroscopic anatomy of the ciliary body.
The ciliary body consists of two parts:
a. pars plana;
b. pars plicata.
a. Pars plana.
The pars plana represents the posterior 4 mm of the ciliary body.
It has a smooth, flat surface. Some of the zonular fibres attach here.
The safest posterior surgical approach to the vitreous cavity is through the
pars plana, located 3-4 mm from the corneal limbus.
b. Pars plicata.
This is the richly vascularised anterior 2 mm of the ciliary body, consisting of
approximately 70 ciliary processes.
The pars plicata is functionally the most important zone of the ciliary body ,
since aqueous humour is formed in the ciliary processes.
The majority of the zonular fibres which support the lens, arise from the
valleys between the ciliary processes.
2.2. Microscopic anatomy of the ciliary body.
The ciliary body can be divided microscopically in three parts:
a. the ciliary epithelium;
b. the ciliary muscle;
c. the ciliary stroma.
a. The ciliary epithelium.
The ciliary body is lined by a double layer of epithelial cells lying apex to apex,
the nonpigmented and the pigmented epithelium.
Embryologically , they represent the two layers of the optic cup
(neuroectoderm ).
The inner, nonpigmented epithelium facing the posterior chamber,
constitutes the anterior continuation of the neurosensory retina.
The outer , pigmented epithelium facing the stroma, is continuous
posteriorly with the retinal pigment epithelium.
The cells packed with pigment, contain numerous organelles also, indicating
some metabolic activity.
The structure of the two layers of ciliary epithelium appears to suggest that
both layers are involved in aqueous humour formation.
The presence of numerous cell attachments between the two layers
suggests that the activities of the two cell layers may be coordinated.
b. The ciliary muscle.
The ciliary muscle is a smooth muscle that has three layers of fibres:
e longitudinal;
e radial;
e circular.
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Innervation is by the postganglionic parasympathetic fibres derived from
c. The ciliary body stroma consists of loose connective tissue rich in blood
vessels and melanocytes.
2.3. The functions of the ciliary body.
The ciliary body has three functions:
e aqueous formation by the ciliary epithelium;
e aqueous dranaige (trabecular and uveoscleral );
e accommodation: when the ciliary muscle contracts the zonules relax
and the lens becomes more spherical, increasing its refractive power
(accommodates).
3. THE CHOROID.
The choroid , the posterior portion of the uveal tract, is athin, pigmented ,
extremely vascular layer, lining the inner surface of the sclera.
Between the sclera and choroid there is a potential space occupied by a thin,
pigmented connective tissue called suprachoroid.
3.1. The structure of the choroid.
The choroid has three layers :
a. The vessel layer consists of a :
e outer, large vessel layer (Haller’s)
e inner, medium vessel layer (Sattler’s)
The vessels are embedded in a loose connective tissue which also contains
abundant melanocytes and many immune cells such as mast cells and
macrophages.
b. The capillary layer ( choriocapillaris ).
It consists of a single layer of wide diameter capillaries, supported by
delicate connective tissue containing melanocytes.
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It has a lobular pattern and is most dense at the macula.
c. Bruch’s membrane is a thin acellular membrane resulting from the fusion
of the basal laminae of the retinal pigment epithelium and of the
choriocapillaris; thus, the connection between the retinal pigment epithelium
and choroid is strong.
The function of this membrane is not exactly known, although it is believed
it plays a role in the passage of tissue fluid from the choroidal capillaries to
the retina.
3.2. The blood supply and innervation of the choroid.
The choroid receives its blood mainly via the posterior ciliary arteries from
the ophthalmic artery. There is a minimal supply also, via the anterior ciliary
arteries.
Blood flow in the choroid is very high ( similar to the renal glomerular
capillaries) ; it is controlled by the sympathetic fibres of the short ciliary
nerves.
The 4 vortex veins drain the choroid into the ophthalmic veins.
The innervation is by the /ong and short ciliary nerves.
The long ciliary nerves origin is the trigeminal nerve (cranial nerve V ). They
carry sensory and sympathetic fibres.
The short ciliary nerves arise from the ciliary ganglion and carry
parasympathetic and sympathetic fibres.
3.3. The functions of the choroid.
a. nutrition of the outer retina is the principal function of the choroid.
b. helps in heat dissipation from the retina.
c. assists in regulating intraocular pressure.
d. the large number of pigment cells absorb excess light .
e. has immunological role, as it contains many immune cells and as there is
no intraocular lymphatic system.
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4. SOME DISORDERS AFFECTING THE UVEA.
Because of its extreme vascularity, the uveal tract is commonly involved in
general systemic diseases and may be a site for circulatory metastases.
From the uveal melanocytes malignant melanomas may arise; these are the
most common malignant intraocular tumours.
The iris, ciliary body and anterior part of the choroid share a common blood
supply , and as such, are often together involved in inflammatory processes.
Uveitis is broadly defined as inflammation of the uvea.
Lesions of the choroid may interfere with retinal nutrition and cause atrophy
of the retina.
With age, Bruch’s membrane may show thickened yellow patches, referred
to as drusen.