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Lecture 26: Ocular Anatomy
Timothy Beer (Modified by Dave Reilly 2013)
Very Brief Overview of the Three Tunics Of the Eye
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Fibrous tunic
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Vascular tunic (“Uveal Tract”)
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Forms a capsule enclosing and protecting the other components of the eye
Made up of the sclera and the cornea
 Sclera has primarily structural functions
 Cornea is part of the optic apparatus
Made up of the choroid, ciliary body, ciliary muscle and iris
 Choroid has primarily nutrient
 Ciliary body generates the aqueous humor
 Ciliary muscle functions in the optic apparatus
 Iris functions as a contractile diaphragm (aperture) in the optic apparatus
Neural tunic (Retina = neural layer and pigmented layer)
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Made up of the retina
 Retina proper forms the photo-receptive layer of the eye
 Is a double-layered epithelium that also covers the ciliary process and the posterior surface of the iris
 Here, it has both nutritive and structural functions
Chambers: Anterior Chamber, Posterior Chamber, Vitreous Chamber
Fibrous Tunic: Cornea and Sclera
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Cornea (1/6 of globe)
o MAIN focusing structure.
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Forms the anterior surface of the eye
Occupies an area that largely corresponds to the pigmented iris, which is visible behind the cornea
Formed by three cellular layer, which are separated from each other by two thin, acellular layers
Avascular (NO blood vessels)
NO pigmentation
Anterior surface of the cornea
 Lined by stratified squamous epithelium
 The basement membrane of this epithelium rests on the first acellular layer, Bowman’s membrane
(anterior limiting lamina)
 Bowman’s membrane separates the epithelium from the corneal stroma
 Bowman’s membrane consists of densely packed collagen fibrils embedded in ground substance
 Consists of > 200 layers of regularly organized collagen fibrils (mostly type I collagen)
 Keratocytes (flattened fibrocytes) are located between the layers of collagen fibers
 The regular arrangement of the collagen fibers and their small diameter account for the transparency of
the cornea
Posterior surface of the cornea
 Lined by endothelium called the posterior endothelium
 The posterior endothelium and the corneal stroma are separated from each other by the Descemet’s
membrane (limiting lamina)
 Descemet’s membrane corresponds to the basement membrane of the posterior endothelium
Lateral margins of the cornea
 Anterior corneal epithelium is continuous with the conjunctiva at the lateral margins
 Corneal stroma is continuous with the sclera at the lateral margins
Sclera (5/6 of globe)
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Tough layer of dense CT consisting of collagenous fibers and networks of elastic fibers
Melanocytes are present in the deep parts of the sclera in addition to the usual complement of CT cells
Distended by the intraocular pressure, the sclera maintains the shape of the eyeball
The sclera is also the site of attachment for the ocular muscles
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Anteriorly, the sclera forms a slight protrusion into the eyeball it merges with the cornea – the scleral spur
 The scleral spur provides a point of insertion for part of the ciliary muscle
 The sclero-corneal junction houses the canal of Schlemm, through which aqueous humor is drained in
ciliary veins (Largest of the trabecular meshwork sinuses)
Vascular tunic (“uveal layer”): Choroid, Ciliary Body and Iris
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Choroid
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Ciliary Body
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Consists of loose CT, which houses a dense network of blood vessels
CT cells and melanocytes are numerous
 Melanocytes give the choroid its dark color
Small blood vessels are especially frequent in the innermost part of the choroid (called the choriocapillary layer)
 The choriocapillary layer supplies the retina with nutrients
Located between the choroid and the retina is Bruch’s membrane
 Bruch’s membrane consists of two layers of collagen fibers and a network of elastic fibers between them
An inward extension of the choroid at the level of the lens
A small amount of loose CT, similar to that of the choroid, is located between smooth muscle cells forming the bulk
of the ciliary body
 These smooth muscle cells form three bundles, collectively called the ciliary muscle
The inner surface of the ciliary body and its processes are lined by two layers of columnar cells which belong to the
retinal layer
 These two layers comprise the ciliary epithelium, which is formed by the pars ciliaris of the retina
 The outer cell layer is pigmented
 The inner cell layer (i.e. the layer that faces the posterior chamber of the eye) is NOT pigmented
Ciliary processes
 Short extension of the ciliary body toward the lens
 Contain a dense network of capillaries
The cells of the inner layer of the ciliary epithelium generate the aqueous humor of the eye
 Tight junctions between the cells form the blood-aqueous humor barrier
Fibers, which consist of fibrillin, extend from the ciliary processes toward the lens and form the suspensory
ligament of the lens
 These fibers are also called “zonule fibers”
Two of the bundles of the ciliary muscles attach to the sclera and stretch the ciliary body when the contract
 In this way, the ciliary muscle regulates the tension of the zonule fibers
 The reduced tension results in thickening of the lens, which focuses the lens on close objects
 This process is referred to as accommodation and is under parasympathetic control
 Contraction of the ciliary body decreases tension on the lens allowing it to become rounder and
refract light more.
Iris
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Posterior surface of the iris
 Covered by the retina
 The inner layer of the retina is called the posterior epithelium of the iris
 Both layers of the retina are pigmented, but pigmentation is heavier in the inner layer (kind of like the
retinal layer switching around… but they don’t actually switch, the inner layer just becomes pigmented)
In the area of the central opening of the iris (the pupil), the retina extends for a very short distance onto the
anterior surface of the iris
The stroma of the iris consists of a vascularized loose CT rich in melanocytes, in addition to macrophages and
fibrocytes
 All of this is surrounded by a loose meshwork of fine collagen fibers
Anterior surface of the iris
 NOT covered by epithelium, instead, there is a condensation of fibrocytes and melanocytes
 Collectively, this condensation makes up the anterior border layer of the iris
The iris forms the aperture of the eye
 Myoepithelial cells in the anterior layer of the retina have radially oriented muscular extension
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These extensions form a flat sheet immediately beneath the anterior layer of the retina
Derived from what used to be the pigmented layer of the retina… no unpigmented and become
myoepithelial cells.
o This flat sheet is referred to as the dilator pupillae muscle
o Under sympathetic control
 Embedded in the central portion of the stroma of the iris are smooth muscles
 These smooth muscle cells form the annular sphincter pupillae muscle
o In humans, this muscle surrounds the pupil as a thin band
o Under parasympathetic control
 The two muscles (pupillae muscle and sphincter pupillae muscle) regular the size of the pupil
 Pupillary constriction (Parasympathetic/muscarinic)
o Mediated by the sphincter pupillae muscle
o Clinically referred to as “miosis”
 Pupillary dilation (Sympathetic/Adrenergic)
o Mediated by the dilator pupillae muscle
o Clinically referred to as “mydriasis”
The color of the eyes is determined by the pigmentation of the cells in the stroma and anterior border layer of the
iris
 If cells are heavily pigmented, the eyes appear brown
 If cells have low pigmentation, the eyes appear blue
 Intermediate levels of pigmentation produce shades of green and grey
Neural Tunic: Retina (pigmented layer and neural layer)
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Derived from the Ectoderm evagination from the diencephalon forming the optic vesicle  optic cup. The optic cup
forms from the optic vesicle folding in on itself forming a double layered structure.
o Inner layer = neural layer
o Outer layer = pigmented layer.
Optic vesicle/cup induces the lens and cornea from ectoderm
Mesoderm invades forming the vascular, muscular and connective tissue elements.
Lens
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Provides fine focus.
Outer capsule of connective tissue
Epithelial cells differentiate into elongated fiber cells
Opacities = cataracts.
Eyelid
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Outer surface = thin skin
Inner surface = conjunctiva (stratified columnar epithelium w/ goblet cells.) reflects onto the sclera as well.
Tarsal plate: fibroelastic connective tissue
Orbicularis muscle
Sebaceous glands: Meibomium glands (large) and Zeis glands (small)
Sweat glands: glands of Moll.
Lacrimal gland
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Tubulo-alveolar serous glands: secrete tears
Large lumens and few ducts.
Looks like the pancreas w/o islets of Langerhans.
**Potential space between Retinal pigment epithelium and the Rod/cone photoreceptor
layer = location of separation in retinal detachment.