16 What Roles Do Posterior Ciliary Processes and Pars Plana Play in Aqueous Formation?

Glaucoma is the most common cause of irreversible blindness worldwide.1 Although the pathophysiology of glaucoma is multifactorial, the treatment strategy essentially depends on lowering the intraocular pressure (IOP). This is accomplished either by increasing aqueous outflow or decreasing aqueous production. Aqueous humor is secreted by ciliary processes of the ciliary body pars plicata region.² Aqueous then exits the eye through the trabecular meshwork or the uveoscleral outflow pathways. Because the ciliary body produces aqueous humor, it is a major target of medical and surgical interventions for glaucoma. The ciliary processes are not uniform in structure and function along their anteroposterior length, and if they are to be treated, there may be value in selectively targeting treatment to different regions of the processes.

The aqueous humor is a clear fluid filling the anterior and posterior chambers of the eye. It is one of the fundamental components of the eye’s optical system in providing a transparent and colorless medium between the cornea and the lens. Additionally, it provides nutrition, removes excretory products from metabolism, transports neurotransmitters, stabilizes the ocular structure, and contributes to homeostasis of the avascular ocular tissues such as the posterior cornea, trabecular meshwork, lens, and anterior vitreous. The aqueous humor also permits inflammatory cells and mediators to circulate in the eye in pathological conditions, and drugs to be distributed to different ocular structures.^3,4

The ciliary body is an annular structure on the inner wall of the globe, positioned just behind the posterior surface of the iris. On cross section, its shape is that of a right-angled triangle, ~ 6 mm in length. The base of the ciliary body is home to the ciliary muscle, the contraction and relaxation of which causes the lens to become thicker or thinner for near or distance vision. The ciliary body is highly vascular and supplied by the anterior ciliary and long posterior ciliary vessels anastomosing as a major arterial circle in the ciliary body.^5,6 The surface of the ciliary body is elaborated into a series of ridges named ciliary processes.⁷ The ciliary processes start anteriorly where they are contiguous with the posterior iris, and taper posteriorly to merge into the pars plana. Anterior and posterior ciliary processes are not separate structures but refer, respectively, to the part of the process lying in front of the equator of the lens and to the part lying behind it.

The ciliary processes consist of a central core of loose connective tissue stroma covered by a specialized double-layered epithelium of nonpigmented ciliary epithelia (NPCE) and pigmented ciliary epithelia (PCE). The ciliary epithelium extends from just posterior to the iris, where it is a highly convoluted structure termed the pars plicata, toward the retina, where the ciliary epithelium flattens and becomes the pars plana.² The NPCE is a monolayer of columnar cells located internally facing the posterior chamber in direct contact with aqueous, and the PCE is a monolayer of cuboidal cells containing numerous melanin granules located more externally facing the ciliary body stroma. The two monolayers interact with each other along their apical surfaces and rest on basement membranes along their basal surfaces. The NPCE and PCE work in concert during aqueous formation.⁸ Tight junctions between NPCE cells form a barrier between vascularized and avascular ocular tissues that is extremely important for maintaining optical transparency of the anterior segment.⁷

It is assumed that elaboration of the ciliary body into ridges in the form of ciliary processes is a strategy to pack more capillaries into the tissue to serve the ciliary epithelial cells, optimize aqueous production, and nourish avascular tissues of the anterior segment.⁷ Studies in rabbits show that blood supply to different regions of the ciliary body and processes occurs through separate vascular territories.⁹ Although interspecies ciliary body anatomic variation is seen,⁹ it represents a conceptual framework for more selectively treating the tissue.¹⁰ Vascular territories may be divided as those supplying (1) anterior iridial ciliary processes, (2) prelenticular major ciliary processes, and (3) postlenticular minor ciliary processes between posterior extensions of the major ciliary processes. These morphologically separate vascular territories also respond differently to pharmacological probing, indicating region-specific vascular supply and vasoregulation of the ciliary processes along their anterior-posterior axis.¹¹ The pars plana has a vascular network of its own that anastomoses with the peripheral choriocapillaris.

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