This connective tissue matrix extends into the core of each ciliary process. The ciliary body stroma is directly continuous with the stroma of the iris anteriorly and with the choroidal stroma posteriorly.
Because the anterior surface of the iris has no epithelial covering, substances released into the ciliary body stroma have access to the anterior chamber by diffusing from the ciliary body stroma to the iris surface. Between the loose connective tissue stroma and the inner surface of the sclera is a complex smooth muscle called the ciliary muscle, also described later.
Although these steps are not independent, the first is related primarily to the ciliary body microvasculature and the second to the ciliary epithelium. Ciliary Body Microvasculature When viewed in cross section, each ciliary process contains a fibrovascular core that is continuous with the ciliary body stroma and covered by the bilayered ciliary epithelium Fig.
The arterioles that serve the ciliary body stroma, like those serving the iris, arise from the discontinuous major circle of the iris. The anterior arterioles supply the large diameter capillaries, near the crests of the processes, whereas the posterior arterioles supply the smaller caliber capillaries deep within each process.
The direction of blood flow in these systems is from anterior to posterior, toward a network of choroidal veins. The blood from the entire ciliary body ultimately leaves the eye via the vortex veins. Light micrograph of ciliary processes asterisks shows each process contains a fibrovascular core with large diameter capillaries arrow. Stuttgart, Germany: FK Schattauer, , p A cast of the vasculature within a single ciliary process. The major circle of the iris MAC gives rise to an anterior arrow and posterior arrowhead arteriole.
Double arrow denotes choroidal vein CV. Drainage from ciliary muscle to choroidal veins has been removed at asterisk. Diagrammatic representation of Fig. Blood flow in the ciliary body is most likely regionalized and under autonomic control. Scanning electron microscopic studies have revealed the presence of localized constrictions in casts of afferent arterioles in the ciliary body microvasculature that may reflect the presence of a sphincter-like system for controlling blood flow.
The capillaries derived from the posterior arterioles that serve the ciliary muscle are nonfenestrated and do not leak plasma proteins under normal conditions Fig. By contrast, the capillaries derived from the anterior arteriole, which pass within the stromal core of each ciliary process, lack tight junctions and are lined by fenestrated endothelial cells Fig.
Using tracers for plasma protein leakage such as horseradish peroxidase HRP , the capillaries of the ciliary body stroma are seen to be very permeable to macromolecules, as well as to ions and fluid Figs. As such, these vessels are limited in their capacity to serve as a selective permeability barrier. That function is one of several reserved for the ciliary epithelium. The ciliary body stroma is filled with black, HRP reaction product that has leaked from its fenestrated vessels.
The ciliary muscle shows relatively little staining resulting from the nonfenestrated vessels that serve it. The dark coloration of the iris is not HRP but the normal dark pigmentation imparted by the pigmented epithelial layers on its posterior surface. Transmission electron micrograph demonstrates the appearance of capillaries in the ciliary body stroma.
Numerous fenestrations arrowheads are evident along the circumference of the vessel wall. Transmission electron micrograph demonstrates that granular horseradish peroxidase HRP reaction product leaks through fenestrations arrowheads into the surrounding ciliary body stroma. The Ciliary Epithelium As noted earlier, the two cell layers that constitute the ciliary epithelium are named for their relative content of melanin pigment. The layer closest to the ciliary body stroma is the pigmented ciliary epithelium and that closest to the posterior chamber of the eye is the nonpigmented ciliary epithelium Fig.
The morphology of the ciliary epithelium varies along the surface of the ciliary body, in accord with the different demands placed on it in various locations. Both are known to be central to the production of aqueous humor and one of them, carbonic anhydrase, is regularly targeted for pharmacologic inhibition to reduce intraocular pressure in glaucoma.
Electron micrograph of the ciliary epithelium cynomolgus monkey, pars plicata. The posterior chamber is at the top and the ciliary body stroma, with its fenestrated capillaries C , is at the bottom. This tissue may be divided into three portions: a uveal meshwork, b corneoscleral meshwork and c juxtacanalicular tissue 6. By gonioscopy, the trabecular meshwork can be separated into two portions: an anterior named non-pigmented and a posterior pigmented.
The inner layers of the trabecular meshwork can be observed in the anterior chamber angle and are referred to as the uveal meshwork. This portion is adjacent to the aqueous humor, is arranged in bands or rope-like trabeculae, and extends from the iris root and ciliary body to the peripheral cornea. These strands are a normal variant and are called by a variety names such as iris process, pectinated fibers, uveal trabeculae, ciliary fibers, and uveocorneal fibers.
The deeper layers of the uveoscleral meshwork are more flattened sheets with wide perforations. The outer layers, the corneoscleral meshwork, consist of 8 to 15 perforated sheets.
The corneoscleral trabecular sheets insert into the scleral sulcus and spur. These sheets are not visible gonioscopically. The perforations are elliptical and become progressively smaller from the uveal meshwork to the deep layers of the corneoscleral meshwork [ 28 ].
The ultrastructure of the trabecular, uveal and corneoscleral meshworks is similar. Each sheet is composed of four concentric layers. The trabecular beams have a central core of connective tissue of collagen fiber types I and III and elastin. There is a layer composed of elastic fibers that provides flexibility to the trabeculae. The core is surrounded by a glass membrane, which is composed of fibronectin, laminin, heparin, proteoglycan and collagen type III, IV and V.
The endothelial layer is a continuous layer and covers all the trabeculae. The endothelial cells are larger, more irregular than corneal endothelial cells.
They are joined by gap junctions and tight junctions and have microfilaments, including actin filaments and intermediate filaments vimentin and desmin [ 30 ]. It can be seen, by slit-lamp examination, as a fine white ridge, just anterior to the meshwork, and with an indirect contact gonioscopic lens, it is identified at the point where the anterior and posterior beams of the cornea converge parallelepiped method to identify the transition between the cornea and the meshwork.
Gonioscopically, it has an irregular roughened pigmented surface. The amount and distribution of the pigment deposition varies considerably with age and race. At birth, it has no pigment, and develops color with age from light to dark brown, depending on the degree of pigment dispersion in the anterior chamber angle. The scleral spur is just posterior to the pigmented trabecular band, and it is the most anterior projection of the sclera internally.
Gonioscopically, it is seen as a prominent white line between the ciliary body band and pigmented trabecular. It can be obscured by excessive pigment dispersion, and is not visible at variable degrees of narrow or occluded angles. The iris processes, thickenings of the posterior uveal meshwork, may be frequently seen crossing the scleral spur.
They have the appearance of a variable number of fine and pigmented strands. The ciliary body band is the portion of ciliary body that is visible in the anterior chamber. The width of the band depends on the point of the iris insertion on the ciliary body. Gonioscopically, it appears as a densely pigmented band, gray or dark-brown, posterior to the scleral spur and anterior to the root of the iris. This tissue consists of a layer of connective tissue types III, IV and V collagen, fibronectin and ground substance glycosaminoglycans and glycoproteins , and it is lined on either side by endothelium [ 31 , 32 ].
Generally, it has a single lumen, but occasionally it is like a plexus with multiple branches. These collector vessels can run like a direct system, draining directly into the episcleral venous system or like an indirect system of more numerous, fine channels, forming an intrascleral plexus before draining into the episcleral venous system [ 34 , 35 ].
The aqueous humor reaches the episcleral venous system by several routes [ 36 ]. Most aqueous vessels run posteriorly draining into episcleral and conjunctival veins. Some aqueous vessels run parallel to the limbus before heading posteriorly toward the conjunctival veins. The episcleral veins drain into the cavernous sinus by the anterior ciliary and superior ophthalmic veins.
The conjunctival veins drain into superior ophthalmic or facial veins via the angular or palpebral veins [ 37 ]. Licensee IntechOpen. They are well-developed in hypermetropic, but are rudimentary or absent in myopic eyes. The Ciliaris muscle is the chief agent in accommodation, i. When it contracts it draws forward the ciliary processes, relaxes the suspensory ligament of the lens, and thus allows the lens to become more convex.
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