Normal Histology, Part 2

• Ciliary processes (pars
plicata): site of AH
formation; spp variable in
size and number
• Iridocorneal angle (ICA)
• Ciliary body musculature
which extends to flattened
posterior zone (pars plana)
Ciliary Body

Ciliary Body Processes
No. Size Form
Shrew Absent (ciliary roll)
Carnivores 74 – 76 Moderate Thin, Finely Convoluted
Cow, Horse 100, 102 Large Thick, Broadly Convoluted
Birds Numerous Small – Thin, Elongated
Moderate

Ciliary Process Microanatomy
• Bilayered epithelium
– Inner (next to posterior
chamber)
• Nonpigmented
– Outer (next to vasculature)
• Pigmented
– Both cuboidal, but columnar in
some instances (horse)
• Capillaries and/or venules
with fenestrated endothelium
• Stroma
– Loose c.t. between the
pigmented epithelium and
vasculature

Aqueous Production at Level of
• Passive diffusion and
ultrafiltration (via cell
membrane and tight
junctions)
• Active formation by
selective transport
systems by:
Carbonic anhydrase
Na/K ATPase
Bilayered Epithelium

• Fills the anterior and posterior
chambers
• Nutrient supply for lens and
cornea
• Waste depository
• Production and removal
results in IOP
• Provides clear medium
• Moves from posterior chamber
to anterior chamber to ICA &
uveoscleral pathway
Aqueous Humor

Composition Greatly
Differs from Plasma

• Consists of attenuating
smooth muscle, some
vasculature and
melanocytes
• Variable in width
• General widest dorsally
and temporally
Pars Plana

Aqueous Humor Outflow and the
Iridocorneal Angle (ICA)

Principal Components
• Pectinate ligament (PL)
• Corneoscleral trabecular
meshwork CSM)
• Uveoscleral trabecular
meshwork (UM)
• Angular aqueous plexus
(AAP)
Iridocorneal Angle

Iridocorneal Angle: Aqueous Outflow
Principal Components
• Pectinate ligament (PL):
attaches iris base to
fibrous tunic and keeps
ICA open
• Trabecular meshworks
(UM and CSM): hold &
filter AH
• Angular aqueous plexus
(AAP): drains AH
Apparatus

Pectinate Ligament: best developed in large
eyes with extensive cilioscleral sinuses
(ungulates, pinnipeds)
Cilioscleral Sinus
AC
Horse ,
Anterior ICA, x20
Hooded seal ,
Anterior ICA, x10

• Corneoscleral
(filtering
mechanism)
• Uveal (storage
area for AH)
Trabecular Meshwork

Angular Aqueous Plexus (AAP): Site of
greatest resistance to outflow and area
most responsible for IOP formation

Flow of Aqueous Humor:
Conventional and Unconventional
• Posterior chamber
• Anterior chamber
• ICA & beyond (US –
uveoscleral pathway:
3% in cats, 13% in
rabbits and 15% in
dogs)

porcine
Uveoscleral Outflow: little understood component of AH
dynamics among vertebrates – each species is different
bovine
Sedacca KK, Samuelson DA, Lewis PA. Examination of the Anterior Uveoscleral
Pathway in Domestic Species. Vet. Ophthalmol. 15 (Suppl 1): 1-7, 2012
equine
feline

N
o
r
m
a
l
G
l
a
u
m
o
a
Uveoscleral Outflow: Canine
Changes Associated with Glaucoma
UT
tangential sagittal elastic fibers
UT
Samuelson DA, Birkin-Streit A. Microanatomy of the Anterior Uveoscleral Outflow
Pathway in Normal and Primary Open-Angle Glaucomatous Dogs. Vet Ophthalmol 2011;
13 Sep no.943, p 1-7, Epub

Aqueous Humor Dynamics
• Outflow = inflow (18 – 30 mm Hg
among domestic spp: exs. 16-20
in the dog; 22-26 in the horse)
• Production mediated by:
– Na/K ATPase pump
– Carbonic anhydrase
– Hydrostatic pressure against
osmotic pressure gradient
• Facility (outflow) mediated by:
– Trabecular meshwork and AAP
– Ciliary body musculature
– Uveoscleral pathway

Aqueous Humor Dynamics Follow A
Diurnal Cycle
Canine

Ciliary Body Musclature

• Aqueous humor outflow
• Lenticular
accommodation
Functions of the CBM

Effectiveness among domestic species: moderate
in carnivores, weak in herbivores

Ciliary Body Musculature and Its
Development Among Mammals
Carnivore
Primate
Herbivore

Anterior portion of the CBM forms
tendinous endings to the TM and
facilitates AH Removal

Other paradigms of CBM exists within each major
mammalian group (carnivores, ungulates, rodents,
etc), each functioning to provide optimal ocular
refraction via changes in lens or globe shape and
position.
squirrel
pig
mongoose

Lenticular Accommodation
Contracted
Relaxed

• Ciliary body (CB) has little to no
musculature that will provide for lens
accommodation.
• Anterior CB and ICA possess
encapsulated corpuscles (EC),
believed to be pressure detectors,
which could detect changes in
intraocular pressure (IOP).
• Postulated that changes in eye shape
will result in shifting the lens axially,
i.e., lenticular accommodation and
cause changes in IOP detected by
ECs.
• IOP changes could result from
combination of eyelids’ muscles and
retractor bulbi muscle activity.
Aquatic condition
Bowhead whale;
Zhu et al, Mar Mam Sci
17:729, 2001
encapsulated corpuscle EC
within posterior ICA of pilot whale
- S. Miles, UF

Ex. baleen whales

• Variable ocular
refraction is
performed through a
combination of
extensive intraocular
muscles, including
iridal muscles, a
strong scleral
baseplate and a very
pliable lens.
• Muscles are skeletal
and may be used in
an uniocular manner.
Avian condition
Great Horned Owl

Summary
• Ciliary body functions primarily to produce and
remove aqueous humor, resulting in specific IOPs
• Possesses variably developed ciliary processes
and iridocorneal angles for AH dynamics
• Possesses variably developed musculature,
smooth in mammals, skeletal in non-mammals, for
lenticular accommodation and facilitation of AH
removal, or just facilitation of AH removal