Ciliary Body & Choroid
Uvea – Consists of the iris, ciliary body, choroid
Ciliary Body
Two Functions: using blood for the production of aqueous humor / accommodation of
lens
Divisions: Pars plicata and Pars plana
a. Pars plicata - Includes ciliary processes and parts of muscle.
b. Pars plana - Smooth, lacking processes, and merges with retina at the ora serrata.
Ciliary processes: collectively called the corona ciliaris
-Within Pars plicata
-Filled with many blood vessels (branches from major arterial circle)
-Processes increase surface area
-Aqueous humor produced here
-Capillaries of processes are highly fenestrated, not overlapping, and VERY LEAKY
Source: Major arterial circle that is fed by the Long Posterior Ciliary arteries
(LPCA)
Two layers of epithelium – pigmented and non-pigmented
-Blood vessels lie next to the pigmented epithelium; Unpigmented next to posterior
chamber.
-Blood and aqueous separated by only 3 cell layers:
Endothelium
Pigmented Epithelium
Non-pigmented Epithelium
Aqueous Composition:
VERY LITTLE protein – (Remember, optical clarity!)
Oxygen
HIGH ascorbate
Glucose
High lactate – (Metabolic by-products dumped into aqueous)
What drives aqueous production?
Diffusion (Small, uncharged and WATER molecules)
Ultrafiltration (NOT a factor)
Active transport (molecules transported against gradient)
Blood-aqueous barrier:
Fluid and solutes must move through cells when traveling from the blood to the aqueous.
Junctions in epithelium: Desmosomes, Gap Junctions, Tight Junctions, Macula Adherens
Aqueous flow:
2X the rate when we are awake
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UAB School of Optometry
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Highest at age 10 – 20; all downhill from there!
Pars Plana:
-Epithelium from plicata is continous- unpigmented (inner vitreal) and pigmented (outerscleral) The epithelium in the pars plana is more columnar and irregular in shape.
-Lies next to vitreous
-Bruch’s membrane between epithelium and capillaries (It is NOT present in par plicata)
This is a continuation of Bruch’s membrane behind RPE
-Water DOES moves into vitreous via osmosis and ULTRAFILTRATION
Ciliary muscle:
a.
meridional (Brucke’s)
b.
radial (Bowman’s)
c.
circular (Muller’s)
Attachment points: Scleral spur, choroid (muscle stars), pars plana, pars plicata
Innervated by parasympathetic nerves using ACH, from the ciliary ganglion
Zonules: fine elastic strands
Sites of attachmentregions between ciliary processes and lens capsule
Muscle acts as a sphincter.
Relaxation of muscle…zonules tightened…lens stretched to max diameter…large radius
of curvature (Flat lens) …no accomodation
Contraction of muscles…zonules less tension…diameter of lens becomes
smaller…greater curvature…more accomodation
Accomodation is a result of ciliary muscle contraction.
Presbyopia is loss of accommodation. By the age of forty five years, the maximum
accommodative response is less than 1 D.
Emmetropia is lack of refractive error.
Hyperopia is a refractive error in which the focal point of the optical system lies behind
the retina.
Ciliary Body Development: The ciliary epithelium forms first (from the rim of the optic
cup). Around the 4th month the muscles begin to form from mesenchyme cells outside the cilliary
epithelium. Muscles develop from the outside in, meridonal -> radial -> circular. Attachments to
the scleral spur at about 7 months.
Zonules – that attach the lens to the ciliary body are produced by the ciliary epithelium
The Choroid
Choriocapillaries:
This is the capillary bed. It’s a single layer of anastomosing fenestrated capillaries with wide
lumens (3-4 times normal), with occasional pericytes. Two or three red blood cells can pass
abreast, but they are usually moving in single file. The density of choriocapillaries varies from
region to region; it is densest in the macular area where it is the sole blood supply for a small
region of the retina (see Fig 11-32). Choriocapillaries is unique to the choroid, does not continue
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UAB School of Optometry
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into ciliary body. If the arterioles feeding into the capillary anastomosis out number the venules,
then the redundancy minimizes the effect of small blockages (Fig 11.31).
Bruch’s membrane –multilayered barier between the choroid and RPE layer is Bruch’s
membrane. Runs from the optic nerve to the ora serrata, changes slightly and continues under the
pars plana of the ciliary body. It’s a multilayer sheet with 5 recognized divisions from outer to
inner.
1.
Basement membrane of the choroi- capillaries.
2.
Outer collaqeuous zone
3.
elastic layer
4.
inner collaqenous zone
5.
basement membrane of RPE cells
Tight adhesion between choroid and RPE filaments from RPE basement membrane merge with
inner collagenous zone. At the ora serrata, this membrane is continuous with the basement
membrane of the ciliary pigmented epithelium. The collagenous and elastic layer disappears into
the ciliary stroma.
Excess accumulation of lipofuscin within the inner collagenous zone of Bruch’s membrane is
called DRUSEN (Fig 11.34). Single druse, plural drusen associate with macular degeneration –
very hot topic of research now. What exactly is in these and where does it come from?
Choroid: Vascular coat provides nutrients into outer retina and is a route for metabolites to
leave. They are transported through the RPE, then diffuse through Bruch’s membrane into
choroicapillaries. These vessels are leaky, and the RPE provides the tight junctions to keep the
retina clear. Choroid is a darkly pigmented and absorbs excess light that penetrates RPE.
Subchoroidal space allows passage of vessels and nerves on the way to the anterior segment.
With age, excessive basement membrane (basal lamina) is deposited in the inner collagenous
zones of Bruch’s membrane. Called drusen, they appear as small yellow white spots. They are
located below the RPE basement membrane and push the retina inward
Blood supply to uveal tract. Short post ciliary arteries enter the globe around the optic nerve.
Branches form the choroid vessels. Long post ciliary arteries form the major arterial circle and
the anterior ciliary arteries form the intramuscular circles (inside the ciliary muscle). Venous
return via vortex veins.
Most choroidal blood vessels are branches of the short post, ciliary arteries or tributaries of the
vortex veins. Largest vessels enter (or exit) and the outersurface get smaller as we move in.
Capillary bed – choriocapillaries lay next to RPE and provide for photoreceptors.
Outermost part of stroma next to sclera is suprachoroides, melanocytes from this area can
migrate into the lamina fusca.
Suprachoroidea is the outermost part of the stroma next to the sclera. It has no particular
functional role that differs from the rest of the stroma.
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UAB School of Optometry
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Haller’s layer is the outer layer with the larger blood vessels.
Sattler’s layer is the inner layer with smaller blood vessels.
Anastomoses are rare in the short posterior ciliary artery system, and the supply to the
choriocapillaris is of the end-arterial type; the short posterior ciliary arteries are not
interconnected to provide alternative routes of supply in the event that one fails. The regions of
the capillary bed supplied by the separate short posterior ciliary arteries have little if any overlap;
the supply is segmental. The venous drainage by the vortex veins is almost the opposite,
however; anastomoses are common at all levels, and the drainage fields overlap.
Accumulations of material in the inner collagenous layer of Bruch’s membrane from inwardly
bulging regions called drusen. There are various degrees of drusen size, but they all have the
effect of pushing the photoreceptors away from their blood supply in the choriocapillaris. If the
separations becomes too large as the drusen increase in size, the overlying pigment epithelium
and photoreceptors maybe degenerate. Accumulations of large drusen that become confluent
may therefore produce sizable scotomas in the visual field.
Tapetium: choroidal reflecta – dogs, cats
Adaptation for nocturnal life. Guanine crystals are reflective and give the photoreceptors a
second chance to catch photons. The down side is the image degradation due to scattering of the
light by the random orientation of the reflective crystals.
Cycloplegia – paralysis of the ciliary muscle NO ACCOMMODATION
Cycloplegic drugs (ACH Blockers, muscarinic blockers)
Atropine – 7-10 days of mydriasis two weeks of cycloplegia (given for anterior
uveitis)
Tropicamide 4-6 hours of both (refraction)
Cyclopentoiate & Homatropine: both of these result in 1-2 days of both (anterior uveitis)
Development: The choriocapillaris development starts early (at 3 weeks) and follows that of the
RPE. As the pigment cells differentiate from posterior pole out to the rim of the optic cup, the
capillary beds also develop. The larger blood vessels of Sattler’s layer and Haller’s layer are not
present until 4-6 mos. The capillaries of the ciliary processes form independently from the
choroids, and they are present early and the LPCA which feeds them
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UAB School of Optometry
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