**2. Ultrastructure of the ciliary processes**

Each ciliary process is composed of a central stroma and capillaries, covered by a double layer of epithelium. (FIGURE 3)

The ciliary process capillaries occupy the center of each process [10]. The capillary endothelium is thin and fenestrated, representing areas with fused plasma membranes and no cytoplasm, which may have an increased permeability. A basement membrane surrounds the endotheli‐ um and contains mural cells or pericytes.

The stroma is very thin and surrounds the vascular tissues, separating them from the epithelial layers. The stroma is composed of ground substance (mucopolysaccharides, proteins and plasma of low molecular size), collagen connective tissue (especially collagen type III) and cells of connective tissue and the blood [11].

Ciliary process epithelia consist of two layers, with the apical surfaces in apposition to each other.

The pigmented epithelium is the outer layer, and the cuboidal cells contain numerous melanin granules in their cytoplasm. This layer is separated from the stroma by an atypical basement membrane, a continuation of Bruch`s membrane which contains collagen and elastic fibers [15].

The nonpigmented epithelium is composed of columnar cells with numerous mitochondria, well-developed endoplasmic reticulum seen in the cytoplasm, extensive infoldings of the membranes and tight junctions between the apical cell membranes. The basement membrane faces the aqueous humor, is composed of fibrils in a glycoprotein with laminin and collagens I, III and IV [16]. The apical cells of this membrane are connected by tight junctions (zonulae occludentae), creating a permeability barrier, which is an important component of the bloodaqueous barrier called the internal limiting membrane.

Adjacent cells within each epithelial layer and between the apical cells of the two layers are connected by gap junctions, tight junctions and desmosomes. The apical membranes of the nonpigmented epithelium are also joined by tight junctions [12,13,14]

These tight junctions are permeable only to low-molecular-weight solutes.

The anterior portion of the nonpigmented ciliary epithelium has the morphologic features of a tissue involved in active fluid transport, i.e., evidence of abundant sodium-potassium adenosine triphosphatase ( Na+ K+ ATPase), glycolytic enzymes activity, and incorporation of labeled sulfate into glycolipids and glycoproteins [17]. There are many indications that the aqueous humor is produced in the anterior portion of the nonpigmented epithelia of ciliary processes [17,18,19].

There is a potential space between the two epithelial layers, called "ciliary channels". The aqueous humor may be secreted into this space after beta-adrenergic agonist stimulation, but this notion requires additional studies [20].
