The ciliary body, which is triangular in cross section, bridges the anterior and posterior segments of the eye (see Fig 2-14). The apex of the ciliary body is directed posteriorly toward the ora serrata. The base of the ciliary body gives rise to the iris. The only attachment of the ciliary body to the sclera is at its base, via its longitudinal muscle fibers, where they insert into the scleral spur.
The ciliary body has 2 principal functions: aqueous humor formation and lens accommodation. It also plays a role in the trabecular and uveoscleral outflow of aqueous humor.
Ciliary Epithelium and Stroma
The ciliary body is 6–7 mm wide and consists of 2 parts: the pars plana and the pars plicata. The pars plana is a relatively avascular, smooth, pigmented zone that is 4 mm wide and extends from the ora serrata to the ciliary processes. The safest posterior surgical approach to the vitreous cavity is through the pars plana, located 3–4 mm from the corneal limbus.
The pars plicata is richly vascularized and consists of approximately 70 radial folds, or ciliary processes. The zonular fibers of the lens attach primarily in the valleys of the ciliary processes but also along the pars plana (see Figs 2-21, 2-47).
The capillary plexus of each ciliary process is supplied by arterioles as they pass anteriorly and posteriorly from the major arterial circle; each plexus is drained by 1 or 2 large venules at the crest of each process. Sphincter tone within the arteriolar smooth muscle affects the capillary hydrostatic pressure gradient. In addition, sphincter tone influences whether blood flows into the capillary plexus or directly to the draining choroidal vein, bypassing the plexus completely. Neuronal innervation of the vascular smooth muscle and humoral vasoactive substances may be important in determining regional blood flow, capillary surface area available for exchange of fluid, and hydrostatic capillary pressure. All of these factors affect the rate of aqueous humor formation.
The ciliary body is lined by a double layer of epithelial cells: the inner, nonpigmented ciliary epithelium and the outer, pigmented ciliary epithelium (Fig 2-23). The basal lamina of the nonpigmented epithelium faces the posterior chamber, and the basal lamina of the outer pigmented epithelium is attached to the ciliary stroma and blood vessels. The nonpigmented and pigmented cell layers are oriented apex to apex and are fused by a complex system of junctions and cellular interdigitations. Along the lateral intercellular spaces, near the apical border of the nonpigmented epithelium, are tight junctions (zonulae occludentes) that maintain the blood–aqueous barrier. The basal lamina of the pigmented epithelium is thick and more homogeneous than that of the nonpigmented epithelium.
The pigmented epithelium is relatively uniform throughout the ciliary body. Each of its cuboidal cells has multiple basal infoldings, a large nucleus, mitochondria, an extensive endoplasmic reticulum, and many melanosomes. The nonpigmented epithelium tends to be cuboidal in the pars plana but columnar in the pars plicata. It also has multiple basal infoldings, abundant mitochondria, and large nuclei. The endoplasmic reticulum and Golgi complexes in these cells are important for aqueous humor formation.
The uveal portion of the ciliary body, the stroma, consists of comparatively large fenestrated capillaries, collagen fibers, and fibroblasts.
A, The 2 layers of the ciliary epithelium, showing apical surfaces in apposition to each other. Basement membrane (BM) lines the double layer and constitutes the internal limiting membane (ILM) on the inner surface. The nonpigmented epithelium is characterized by large numbers of mitochondria (M), zonula occludens (ZO), and lateral and surface interdigitations (I). The blood–aqueous barrier is established by the intercellular ZOs. The pigmented epithelium contains numerous melanin granules (MG). Additional intercellular junctions include desmosomes (D) and gap junctions (GJ).
B, Pars plicata of the ciliary body showing the 2 epithelial layers in the eye of an older person. The nonpigmented epithelial cells measure approximately 20 μm high by 12 μm wide. The cuboidal pigmented epithelial cells are approximately 10 μm high. The thickened ILM (a) is laminated and vesicular; such thickened membranes are a characteristic of older eyes. The cytoplasm of the nonpigmented epithelium is characterized by its numerous mitochondria (b) and the cisternae of the rough-surfaced endoplasmic reticulum (c). A poorly developed Golgi apparatus (d) and several lysosomes and residual bodies (e) are shown. The pigmented epithelium contains many melanin granules, measuring about 1 μm in diameter and located mainly in the apical portion. The basal surface is rather irregular, having many fingerlike processes (f). The basement membrane of the pigmented epithelium (g) and a smooth granular material containing vesicles (h) and coarse granular particles are seen at the bottom of the figure. The appearance of the basement membrane is typical of older eyes and can be discerned with a light microscope (×5700).
(Part A reproduced with permission from Shields MB. Textbook of Glaucoma. 3rd ed. Baltimore: Williams & Wilkins; 1992. Part B modified with permission from Hogan MJ, Alvarado JA, Weddell JE. Histology of the Human Eye. Philadelphia: Saunders; 1971:283.)
The main arterial supply to the ciliary body comes from the anterior and long posterior ciliary arteries, which join to form a multilayered arterial plexus consisting of a superficial episcleral plexus; a deeper intramuscular plexus; and an incomplete major arterial circle often mistakenly attributed to the iris but actually located posterior to the anterior chamber angle recess, in the ciliary body (see Chapter 1, Figs 1-19, 1-20, 1-22). The major veins drain posteriorly through the vortex system, although some drainage also occurs through the intrascleral venous plexus and the episcleral veins into the limbal region.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.