Ciliary Body

• Circular structure
• 6mm wide
• Numerous functions
• Accommodation
• Production of aqueous
• Production of zonules

• Production of extracellular vitreous components: collagen and hyaluronic acid (main GAG in vitreous)

• Age related changes:

• Atrophy of the CB affects the longitudinal and reticular muscle fibres, less so the circular fibres

• Migrates anteriorly

Embryology

• Derived from a combination of neuroectoderm and mesenchyme

• Neuroectoderm bilayer forms the ciliary epithelium which also has two layers and is continuous with the neural retina and RPE (also formed from neuroectoderm)

• Mesenchyme provides the stroma and ciliary muscle
• Develops at 11-12 weeks
• 1. Outer pigmented layer of neuroectoderm is invaded by capillaries
• 2. The vessels enlarge and push towards inner non-pigmented layer creating folds (future ciliary processes)
• 3. Folds become more numerous and complex
• 4. Flat area forms posteriorly (pars plana)
• 5. Aqueous production commences at 20 weeks (earliest)
• Scleral spur and CB muscle fibres are not fully developed at term.

Macroscopic structure

• Scleral spur anteriorly
• Pars plicata is 2mm wide, sits anteriorly. Comprises about 70 ciliary processes
• Pars plana is 4mm wide, sits posteriorly
• Ora serrata further posteriorly (contains non-pigmented ciliary epithelium)
• Wider temporally than nasally

Epithelium

• Bilayer
• Two layers of cells lie apex to apex, hence:

• Inner basal lamina facing the posterior chamber: continuous with the ILM of the neural retina

• Outer basal lamina against the stroma/muscle: continuous with Bruch’s membrane
• The inner non-pigmented layer is rich in Golgi and mitochondria and secretes aqueous.
• Contains carbonic anhydrase
• Contributes to production of zonules and vitreous components
• Tight junctions between the non-pigmented epithelial cells form a blood-aqueous barrier
• The main protein present in tight junctions is occludin

• The outer pigmented layer is rich in melanocytes. Permeable zonula adherens and macula adherens connect cells

• The pigmented epithelial cells are therefore more leaky
• Continuous with the RPE

Aqueous production

• Relatively constant rate of 1-3 microlitres per minute (the entire volume is replaced every 1-2 hours)

• Produced by the non-pigmented ciliary epithelium
• Produced by a combination of:
• Passive diffusion of electrolytes down a concentration gradient

• Ultrafiltration of plasma dependent on BP, IOP and blood osmotic pressure across the leaky fenestrated capillaries in the ciliary body

• Active transport across the epithelial plasma membrane including carbonic anhydrase II and Na/K ATPase activity

• Note: the oncotic pressure of the ciliary stroma is greater than the hydrostatic pressure across the epithelium so active transport is the main mechanism of production.

• Also under adrenergic control (see below)

The composition of aqueous

• Electrolytes and low molecular weight compounds
• Slightly higher chloride than plasma
• Similar potassium as plasma

• Slightly lower sodium, bicarbonate and glucose than plasma (glucose levels increase in diabetes with consequences for the lens)

• Much lower calcium than plasma
• Lactate is the most abundant anion and at higher levels than plasma
• Slightly lower levels of urea: passes quite readily across membranes
• Oxygen: from the blood supply of the CB and iris (oxygen cannot pass through the cornea)
• Supplies the corneal endothelium, TM endothelium and lens
• Steroid sex hormones 
• Ascorbic acid: 

• Unique constituent, much higher levels than plasma. Possibly has a role as an antioxidant

• Glutathione is also present in aqueous and acts as a stabiliser of ascorbate as well as for enzymatic processes involved in cellular detoxification ie. protection of tissue from oxidative stress

• Lysozyme: provides antibacterial protection
• Carbonic anhydrase
• Plasminogen and plasminogen activator
• Hyaluronidase: may help regulate flow through the TM 
• Cytokines and growth factors including VEGF

• Growth factors may regulate the endothelium and epithelium of the cornea, hence the hyperplastic changes in these tissues in chronic inflammatory conditions

• Insulin-like growth factor and its binding protein are elevated in patients with diabetes

• VEGF-A is induced by hypoxia and is a potent regulator of angiogenesis and vascular permeability. It is increased in patients with active neovascularisation in PDR and CRVO

• Higher molecular weight compounds may be added in the AC

• Very low protein content (allows optical clarity): mainly albumin but some immunoglobulin and fibronectin, due to exclusion by the blood-aqueous barrier

• Some proteins do enter, possibly through the iris root but others may have been synthesised directly into the aqueous by the CB

• Complement C4

• Alpha2-macroglobulin: a carrier protein and proteinase inhibitor present in aqueous. Imbalance between these may lead to disease eg. glaucoma

• Hyaluronic acid

• AC volume: 0.2ml
• PC volume: 0.06ml
• pH: 7.5-7.6

Stroma/muscle

• The ciliary muscle comprises the anterior two-thirds of the CB

• It is smooth muscle comprising three groups of fibres in the vascular connective tissue stroma

• Outer longitudinal fibres (aka Brucke’s muscle) are continuous with the choroid (hence this also moves with accommodation) and attach to the sclera spur

• Middle oblique/radial fibres attach to scleral spur
• Continuous with the corneoscleral trabeculae
• Inner circular fibres: these are the most anterior (aka Muller’s muscle)

• Contraction produces an inwards and forwards movement of the CB which slackens the zonules and allows the lens to adopt a spherical shape, thus increasing its refractive index.

Blood supply

• Long posterior ciliary arteries travel anteriorly in the choroid and then divide in the CB

• They anastomose with the anterior ciliary arteries forming the major circle of the iris
• Venous drainage via vortex veins

Nerve supply

• Parasympathetic supply: preganglionic fibres originate in the Edinger-Westphal nucleus and travel in the oculomotor nerve. They synapse in the ciliary ganglion and postganglionic fibres travel in the short ciliary nerves to produce a plexus in the CB

• Sympathetic innervation via the sympathetic chain and superior cervical ganglion