Ординатура / Офтальмология / Английские материалы / Shields Textbook of Glaucoma, 6th edition_Allingham, Damji, Freedman_2010

Schwann cells of ciliary nerves

Ciliary ganglion

Most orbital bones, cartilage, and connective tissue of the orbit

Muscular layer and connective tissue sheaths of all ocular and orbital vessels

The following regulatory genes have been grouped into large families of transcription factors: homeobox genes, zinc finger genes, and helix-loop-helix genes. Homeobox genes encode for a 60-amino acid DNA-binding element and specifically determine the target gene for a transcription factor. These genes are frequently involved in determining the regional identity of the embryo or individual fate and differentiation of cells (10). Examples of homeobox genes include the PAX family and POU domain family. The zinc finger family of genes is thought to be the most abundant of the transcription factors. These genes

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share a common motif of a zinc atom binding to a group of histidine and cysteine amino acids and holding together a small loop of amino acids. Examples of this gene family include the retinoic acid receptors (RAR) and retinoid × receptor (RAX), whic h direct the binding of retinoic acid. Mutations in these receptors have been associated with abnormal eye development (11). The helix-loop-helix family of genes is characterized by two helical DNA-binding domains held together by a special domain or region called as “leucine zipper” (12).

Table 1.2 Selected Genes Involved in Vertebrate Eye Development

nerve

a Skeletal muscle, heart, stomach, thyroid, trachea, bone marrow, thymus, prostate, small intestine, colon, lung, pancreas, testis, ovary, spinal cord, lymph node, and adrenal gland.

TGF-ß, transforming growth factor beta; IOP, intraocular pressure; COAG, chronic open-angle glaucoma; HLH, helix-loop-helix.

The role for these various structural, regulatory, and cellspecific genes in ocular development has been most extensively examined thus far in the retina, which is highly complex and only partially understood (12). Although not as extensively studied as retinal development, the anterior ocular segment, including the ciliary body and lens (13), also has important and complex roles in the development of the normal eye. The tissue origins of the ciliary epithelium, ciliary smooth muscle, and lens are listed in Table 1.1. The lens induces differentiation of ciliary epithelium at the edge of the optic cup (Fig. 1.3), and the iris develops later from the edge of the optic cup. The ciliary muscle and stroma differentiate after the ciliary epithelium is formed. It is not clear when during gestation the ciliary epithelium becomes active to secrete aqueous humor, but it is assumed to start very early after formation (14). As the IOP increases, the eye grows. It is also believed that the increase in IOP provides the force to generate ciliary folds in the ciliary body and to change the shape of the cornea (15).

Abnormalities in the development of the anterior chamber angle, or anterior segment dysgenesis, are exemplified in Axenfeld-Rieger syndrome (see Chapter 14). Thus far, genes that have been shown most frequently to cause anterior segment dysgenesis encode transcription factors that are important in early development. These transcription factors include PITX2, PITX3, PAX6, FOXC1, FOXC2, and FOXC3 (16). In transgenic mice, the cell signaling molecule, bone morphogenetic proteins, and related signaling molecules play an important role in normal development of the anterior segment (17).

An approach to study embryology of ocular structures is using data obtained through bioinformatics—a discipline that integrates the study of genes, pathways, and function. Gene expression data, also known as transcript or mRNA expression, may be gleaned in discrete ocular tissues and at various time points in development (18). Such a “global” overview of ge ne expression in these discrete ocular tissues enables us to hypothesize and to design studies to answer some fundamental cell biology questions about these ocular structures. By comparing and contrasting the gene expression profiles of these discrete ocular tissues at various stages of development and the impact of environmental exposures, we will understand the function of these eye structures at the cellular and molecular level (see further discussion in Chapter 8).

BIOLOGY OF AQUEOUS HUMOR INFLOW

The regulation of IOP is a complex physiologic trait that depends on (a) production of aqueous humor,

(b) resistance to aqueous humor outflow, and (c) episcleral venous pressure. P.8

To reduce this highly complex and only partially understood situation to its simplest form, IOP is a function of the rate at which aqueous humor enters the eye (inflow) and the rate at which it leaves the eye (outflow). When inflow equals outflow, a steady state exists, and the pressure remains constant. The remainder of this chapter deals with these inflow and outflow parameters and their complex interrelationships with the IOP.

Cellular Organization of the Ciliary Body and the Ciliary Processes

The ciliary body is one of three portions of the uveal tract, or vascular layer of the eye; the other two structures in this system are the iris and choroid. The ciliary body is composed of (a) muscle, (b) vessels,

(c) epithelia lining the ciliary processes, and (d) nerve terminals from the autonomic nervous system (Fig. 1.4).

Ciliary Body Muscle

The ciliary muscle consists of two main portions: the longitudinal and the circular fibers (Fig. 1.4). The longitudinal fibers attach the ciliary body to the limbus at the scleral spur. This portion of muscle then runs posteriorly to insert into the suprachoroidal lamina (fibers connecting choroid and sclera) as far back as the equator or beyond. The circular fibers occupy the anterior and inner portions of the ciliary body and run parallel to the limbus. One-third portion of the ciliary muscle has been described as radial fibers, which connect the longitudinal and circular fibers. The physiologic function and pharmacologic action of parasympathomimetic agents as they relate to the ciliary muscle are discussed in Chapter 32. Ciliary Body Vessels

On the basis of studies in primate and human eyes, the vessels of the ciliary body appear to have a complex arrangement with collateral circulation on at least three levels (19, 20): (a) The anterior ciliary arteries on the surface of the sclera send out lateral branches that supply the episcleral plexus and anastomose with branches from adjacent anterior ciliary arteries to form an episcleral circle, (b) The anterior ciliary arteries then perforate the limbal sclera. In the ciliary muscle, branches of these arteries anastomose with each other as well as with branches from the long posterior ciliary arteries to form the intramuscular circle. Divisions of the anterior ciliary arteries also provide capillaries to the ciliary muscle and iris and send recurrent ciliary arteries to the anterior choriocapillaris. (c) The major arterial circle lies near the iris root anterior to the intramuscular circle and is actually the least consistent of the three collateral systems. Although the primate studies reveal a contribution from perforating anterior ciliary arteries, microvascular casting studies of human eyes, as well as several nonprimate animals, indicate that this “circle” is formed primarily, if not exclusively, by paralimbal branches of the long posterior ciliary arteries, which begin dividing in the anterior choroid. In any case, the major arterial circle is the immediate vascular supply of the iris and ciliary processes.

Figure 1.4 Schematic of the three major components of the ciliary body: (1) the ciliary muscle, composed of longitudina [LCM), radial, and circular (CCM) fibers; (2) the vascular system, formed by branches of the anterior ciliary arteries (ACA) and long posterior ciliary arteries (LPCA), which form the major arteria circle (MAC); and (3) the ciliary epithelium (CE), composed of an outer pigmented and an inner nonpigmented layer.

Each ciliary process in primates is supplied by two branches from the major arterial circle: the anterior and posterior ciliary process arterioles (20) (Fig. 1.5). Anterior ciliary process arterioles supply the anterior and marginal (innermost) aspects of the major ciliary processes. These arterioles have luminal constrictions before producing irregularly dilated capillaries within the processes, suggesting precapillary arteriolar sphincters. This may represent the anatomic site of adrenergic neural influence on aqueous humor production by regulation of blood flow through the ciliary processes. The posterior ciliary process arterioles supply the central, basal, and posterior aspects of the major ciliary processes, as well as all portions of the minor processes. These arterioles are of larger caliber than the anterior arterioles and lack the constrictions seen in the latter vessels. Both populations of arterioles have interprocess anastomoses.

Vascular casting studies of capillary networks in the ciliary processes of human eyes suggest three different vascular territories with discrete arterioles and venules (19). The first is located at the anterior end of the major ciliary processes and is drained posteriorly by venules without significant connections to other venules in the ciliary processes. The second is in the center of the major processes, whereas the third capillary network occupies the minor processes and posterior third of the major processes. Both of the latter territories are drained by marginal venules, which are situated at the inner edge of the major processes. It is thought that these three vascular territories may reflect a functional differentiation in the process of aqueous humor production. Venous drainage is into choroidal veins, either from the posterior aspects of the major and minor processes or by direct communication from the interprocess connections (Fig. 1.6).

Ciliary Processes

The functional unit responsible for aqueous humor secretion is the ciliary process, which is composed of

(a) capillaries, (b) stroma, and (c) epithelia (Figs. 1.4 and 1.6). The ciliary process capillaries occupy the center of each process. The thin endothelium has false “ porous” areas of fused plasma

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membranes with absent cytoplasm, which may be the site of increased permeability. A basement membrane surrounds the endothelium, and mural cells, or pericytes, are located within the basement membrane (21).

Figure 1.5 Schematic of vascular interconnections of two contiguous major ciliary processes. Lateral anterior arteriolar branches join to form interprocess capillary networks (arrowhead), which provide communication between major processes. Laterally directed posterior arterioles form posterior interprocess networks through which the minor ciliary processes receive blood. In addition, both anterior and posterior interprocess networks drain directly into the choroidal veins (arrows). MAC, major arterial circle. (From Morrison JC, Van Buskirk EM. Ciliary process microvasculature of the primate eye. Am J Ophthalmol. 1984;97:372-383, with permission.)

A very thin stroma surrounds the capillary networks and separates them from the epithelial layers. The stroma is composed of ground substance, consisting of mucopolysaccharides, proteins, and plasma solutes (except those of large molecular size); very few collagen connective tissue fibrils, especially collagen type III (22); and migrating cells of connective tissue and blood origin (21). Tubular microfibrils with and without elastin have been demonstrated in bovine ciliary body, especially in the stroma of the pars plana, in relation to zonules (23).

Figure 1.6 Light microscopic view of ciliary processes, sectioned perpendicular to radial ridges, showing major ciliary processes and minor ciliary processes from a human eye stained with toluidine blue.

Two layers of ciliary epithelium surround the stroma, with the apical surfaces of the two cell layers in apposition to each other (Fig. 1.7). The pigmented epithelium has numerous melanin granules in the cytoplasm and an atypical basement membrane on the stromal side.

In the nonpigmented epithelium, the basement membrane is composed of glycoproteins that are immunoreactive for laminin and collagen types I, III, and IV (24). This membrane, which faces the aqueous humor, is also called the internal limiting membrane and fuses with the zonules. The nonpigmented epithelium stains less intensely than the pigmented layer for cytokeratin 18 but more so for vimentin, with the predominant distribution in the crests of the pars plicata and the posterior pars plana (25). It also stains with antibodies against S-100 protein (22). Another molecule with restricted expression in the nonpigmented cells are the water channels aquaporin-1, which is also expressed in trabecular meshwork endothelium, and aquaporin-4 (26). In transgenic knockout mice, which do not express these water channels, IOP is significantly reduced compared within the wild-type mice, whose water channels are normally expressed. The mechanism of IOP lowering is through reduction in decreasing aqueous humor production, but not in outflow. Although these genetically modified mice have a

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phenotype of lower IOP, patients with aquaporin-1 mutations have normal IOP (27).

Figure 1.7 Schematic of the ciliary epithelium summarizing the histology and junctional complexes (A), physiology of ionic transport mechanisms (B), transmembrane signaling and enzymatic pathways and other paracrine functions (C). A: The ciliary epithelium is composed of two layers containing nuclei (A) with an outer pigmented layer (facing the stroma of the ciliary process) and inner nonpigmented layer (facing and lining the posterior chamber). Apical surfaces are in apposition to each other. Basement membrane (BM) lines the bilayer and constitutes the internal limiting membrane on the inner surface. The nonpigmented epithelium is characterized by mitochondria, zonula occludens (ZO), and lateral and surface interdigitations. The pigmented epithelium contains numerous melanin granules. Additional intercellular junctions include desmosomes (D) and gap junctions (G). B: Overall, there is a net

secretion (open arrows) of the cations (Na+, K+, and H+) and anions (Cl- and HCO3-), but there is also

some absorption (solid arrows) of these ions. The net effect is a negative charge (O) toward the posterior chamber relative to the ciliary body stroma (©). Th e transfer of these ions proceeds primarily through a transcellular route, or transport across the bilayer through some ion channels and transporters (black rectangles) Transfer also occurs to a lesser extent through the paracellular route, or between the cells. C: Aqueous humor secretion is highly regulated by multiple transmembrane receptor-mediated pathways (GPCR, G-protein coupled receptor; G, G-protein; AC, adenylate cyclase; ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; PLC, phospholipase C; PI, phosphatidyl inositol; DAG, diacyl glycerol; IP3, inositol trisphosphate), enzymatic-mediated pathways (GA, carbonic anhydrase type II

[and possibly type IV]), and specialized transporters, such as the aquaporin type I channel (AQP1), which has restricted expression in the nonpigmented ciliary epithelium. The precise localization to pigmented versus nonpigmented and orientation on apical versus basolateral surfaces are unknown for these pathways; thus, they are represented in a bilayer couplet. Other potential paracrine functions of the ciliary epithelium include secretion of small peptides (granules).

A variety of intercellular junctions connect adjacent cells within each epithelial layer, as well as the apical surfaces of the two layers (28). Such junctions include gap junctions, which are expressed by the pigmented cells, the nonpigmented cells and the pigmented-nonpigmented cells, and tight junctions or zonula occludens, which are expressed between the nonpigmented cells. It is primarily the zonula occludens in the nonpigmented ciliary epithelium that creates an effective barrier to intermediate and highmolecular-weight substances, such as proteins.

Electrophysiologic studies of rabbit ciliary epithelium suggest that all of the cells in the epithelium function as a syncytium (29). Tight junctions create a permeability barrier between the nonpigmented epithelial cells, which forms part of the blood-aqueous barrier. These tight junctions are said to be the “leaky” type, in contrast to the “nonleaky” type in the blood-retinal barrier, and may be the main diffusional pathways for water and ion flow. Microvilli separate the two layers of epithelial cells. In addition, “ciliary channels” have been described as spaces between the two epithelial layers. These channels may be related to the formation of aqueous humor in that they develop between the fourth and sixth months of gestation, corresponding to the start of aqueous humor production.

The Autonomic Innervation of the Ciliary Body

Both sympathetic and parasympathetic nerve endings innervate the ciliary body (30). The sympathetic fibers synapse in the superior cervical ganglion, and the postsynaptic fibers are distributed to the ciliary body vessels. Because the ciliary epithelium is not innervated, it is thought that the catecholamine neurotransmitters released from the sympathetic nerve endings “diffuse” to the adrenergic receptors on the ciliary epithelium. Stimulation of these receptors increases aqueous humor secretion by the ciliary epithelium (discussed further in the section on Molecular Mechanisms and Regulation of Aqueous Humor Production).

The parasympathetic fibers originate from the Edinger-Westphal nucleus to innervate the ciliary muscles. Stimulation of these nerve fibers releases acetylcholine, which then stimulates the cholinergic receptors on the ciliary muscle. These activated receptors cause the ciliary muscle to contract, causing accommodation by changing the shape of the crystalline lens. In addition, ciliary muscle contraction reduces resistance to conventional aqueous humor outflow, or trabecular outflow, and may also affect unconventional aqueous humor outflow, or uveoscleral outflow. The effect of the cholinergic pathway on the trabecular outflow pathway is used pharmacologically in the treatment of glaucoma and is discussed in Chapter 32.

Molecular Mechanisms and Regulation of Aqueous Humor Production

Aqueous humor is a dynamic intraocular fluid that is vital to the health of the eye. The precise localization of aqueous humor production appears to be in the anterior portion of the pars plicata along the tips or crests of the ciliary processes (Fig. 1.2). This region has increased basal and lateral interdigitations, mitochondria, and rough endoplasmic reticulum in the nonpigmented ciliary epithelium; more numerous fenestrations in the capillary endothelium; a thinner layer of ciliary stroma; and an

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increase in cell organelles and gap junctions between pigmented and nonpigmented epithelia (30). Aqueous humor is derived from plasma within the capillary network of the ciliary processes. The circulating aqueous humor enters the posterior chamber and flows around the lens and through the pupil into the anterior chamber. Within the anterior chamber, a temperature gradient (cooler toward the cornea) creates a convection flow pattern, which may occasionally be visualized clinically when a patient has inflammation with circulating inflammatory cells. Initially, to reach the posterior chamber, the various constituents of aqueous humor must traverse the three tissue components of the ciliary processes, that is, the capillary wall, stroma, and epithelial bilayer. The principal barrier to transport across these tissues is the cell membrane and related junctional complexes of the nonpigmented epithelial layer, and substances appear to pass through this structure by the following processes: (a) diffusion (lipid-soluble substances are transported through the lipid portions of the membrane proportional to a concentration gradient across the membrane), (b) ultrafiltration (water and watersoluble substances, limited by size and charge, flow through theoretical “micropores” in the protein o f the cell membrane in response to an osmotic gradient or hydrostatic pressure), or (c) secretion (substances of larger size or greater charge are actively transported across the cell membrane). The latter process is mediated by transporters, which are proteins in the membrane, and requires the expenditure of energy generated by adenosine triphosphate (ATP) hydrolysis (29).

Basic Physiologic Processes

The following simplified three-part scheme describes the basic physiologic processes involved in aqueous humor production.

Accumulation of Plasma Reservoir

First, tracer studies suggest that most plasma substances pass easily from the capillaries of the ciliary processes, across the stroma, and between the pigmented epithelial cells before accumulating behind the tight junctions of the nonpigmented epithelium (30). This movement takes place primarily by diffusion and ultrafiltration. Drugs that alter perfusion of the ciliary blood vessels may exert their influence on IOP at this level (20).

Transport across Blood-Aqueous Barrier

Second, as mentioned previously, active secretion is a major contributor to aqueous humor formation (29). This active transport takes place through selective transcellular movement of certain cations, anions, and other substances across the blood-aqueous barrier formed by the tight junctions between the nonpigmented epithelium (Fig. 1.7). The process of aqueous humor secretion is mediated by transferring NaCl from the ciliary body stroma to the posterior chamber with water passively following. This secretion occurs in three steps by uptake of NaCl from stroma to pigment epithelial cells by

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electroneutral transporters, by passage of NaCl from pigmented to nonpigmented cells through gap

junctions, and finally by release of Na+ and Cl- through Na+,K+-activated ATPase and Cl- channels, respectively.

At the first step of NaCl secretion, rabbit in vitro studies demonstrated that paired activity of Na+/H+ and Cl-/HCO-3 antiports may be the dominant mechanism in the pigmented epithelium. At the opposite

nonpigmented epithelial surface, release of Na+ through Na+,K+-activated ATPase with the accompanying release of CP through ion channels is enhanced by agonists of A3 adenosine receptors (A3ARs). These mechanisms were confirmed in vivo in a mouse model that showed that inhibitors of

Na+/H+ antiports lower IOP and that A3AR agonists and antagonists raise and lower IOP, respectively. Carbonic anhydrase mediates the transport of bicarbonate across the ciliary epithelium through a rapid

interconversion between HCO-3 and CO2 (see details in Chapter 31). Bicarbonate formation influences

fluid transport through its effect on Na+, possibly by regulating the pH for optimum active transport of

Na+(31).

Other transported substances (see “ Function and Composition of Aqueous Humor” ) include ascorbic