**2. The outer retina and choroid**

late under the RPE (Fig. 2) [14-16]. Ultimately, early AMD can progress to the late form of the disease; geographic atrophy (commonly known as advanced dry AMD) (Fig. 1C) or neovas‐ cular AMD (commonly known as wet or exudative AMD) (Fig. 1D). Geographic atrophy is

**Figure 1.** Fundus Photographs in health, early age-related macular degeneration (AMD), late forms of atrophic AMD and neovascular AMD. The ocular fundus of a healthy eye, showing normal pigmentation and retinal blood vessels (A). Drusen (thick arrows), seen as multiple discrete round yellow sub-retinal pigment epithelium (RPE) deposits, are the first sign of early AMD (B). Atrophic AMD (C) is characterized by a window defect (thin arrows) with loss of RPE and overlying photoreceptors. Neovascular AMD (D) is characterized by choroidal neovascularization (CNV), which is prone to fluid exudation, hemorrhage, and fibrosis. The late-stage dry form of AMD, known as geographic atrophy. Note large regions of depigmentation, especially in the macula, which is at the center of the image.(C) In wet AMD,

Neovascular AMD is characterized by the growth of new abnormal blood vessels, with leaky walls, under the RPE from the subjacent choroid, resulting in choroidal neovascularization (CNV) and subsequent dysfunction or death of the overlying neurosensory retina [14,15]. Neovascular AMD progresses much more rapidly than early AMD and leads to a greater loss of central vision. What both forms have in common, however, is pathology at the RPE/choroid interface, which includes a thickening of BrM, due to the deposition of extracellular material between the RPE and BrM (sub-RPE deposits and drusen) (Fig. 2). This review will focus on the pathobiology of the early AMD by exploring the role of cigarette smoking and hypertension

**Figure 2.** Schematic image of the RPE-Bruch's membrane-choriocapillaris interface in AMD. Basal laminar deposits (BLD; \*\*) appear between the RPE cell and the RPE basement membrane, while basal linear deposits (BlinD; \*) localize at the inner collagenous layer beneath the RPE basement membrane. Arrowhead indicates endothelial cell basement

characterized by death of RPE and photoreceptors (Fig. 1C) [14,15].

40 Age-Related Macular Degeneration - Etiology, Diagnosis and Management - A Glance at the Future

leaky blood vessels from the choroid invade the overlying retina.

in the onset and development of the disease.

membrane.

As mentioned above, the pathology at the RPE/choroid interface, which includes deposition of extracellular material between the RPE and BrM, is what both AMD forms have in common. BrM undergoes several biochemical and anatomical changes with aging, including collage‐ nous thickening, calcification, and lipid infiltration, in the absence of apparent retinal dys‐ function [17,18]. The accumulation of specific deposits under the RPE is the hallmark histopathological feature of eyes with early AMD, when visual function is still not irreversibly impaired [14,19].

The RPE is a monolayer of hexagonally arranged, highly pigmented cells, located between the neural retina and the choroid, and forming part of the blood-retina barrier (Fig. 3). Its many functions include; the absorption of light that did not get captured by the photoreceptor outer segment pigments; epithelial transport of molecules (nutrients, ions, water, and metabolites) between the subretinal space and the choroidal blood supply; spatial ion buffering; reisomerization of the chromophore 11-cis-retinal from all-trans retinal; the daily removal of photoreceptor outer segments by phagocytosis; the secretion of molecules such as growth factors, proteases, and others that control the stability of the photoreceptor cells, BrM and the choroid; and finally, the modulation of the immune response, since the RPE participates in control of the immune privilege in the healthy eye or the mounting of an immune response in the diseased eye [20]. Abnormalities in any of these processes might participate in RPE cell pathology.

**Figure 3.** Schematic image of the photoreceptors-RPE-Bruch's membrane-choriocapillaris interface and drusen in AMD.

BrM occupies a crucial interface between the RPE and choroid and contains the basement membrane of both the RPE and choroid (Fig. 3). BrM is traditionally considered to be a fivelayered stratified extracellular matrix that provides structural support to the overlying RPE and retina [6,18,21]. BrM also provides a semipermeable filtration barrier through which major metabolic exchange takes place between the RPE and the choriocapillaris. In the center of BrM is the elastic fiber layer (Fig. 3) sandwiched between the inner and outer collagenous layers. Finally, two basal laminas define the innermost and outermost layers. The innermost layer is the basal lamina of the RPE; the outermost layer is formed by the basement membrane of the endothelial cells that comprise the choriocapillaris [6,18,21]. The collagen layers contain striated collagen fibers mainly type I, III, and V [6,18]. Type I confers tensile strength to the tissue, type III is normally present in tissues with elastic properties, and type V acts to anchor basement membranes to stromal matrixes [17,18]. The interfiber matrix of BrM is comprised largely of glycosaminoglycans such as heparan sulfate (25%) and chondroitin/dermatan sulfate (75%). These provide an electrolytic barrier to diffusion and serve important regulatory roles by binding extracellular proteins and growth factors that are vital for cellular processes such as adhesion, migration, and differentiation [17,22,23].

associated with RPE cell function [32] as well as other poorly characterized components [24-27]. Further, low-grade monocyte infiltration within the choriocapillaris is often present

Cigarette Smoking and Hypertension Two Risk Factors for Age-Related Macular Degeneration

http://dx.doi.org/10.5772/53958

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Clinically, deposits of AMD are classified on fundoscopic features of morphology and size [24]. Although multiple classifications exist, most clinicians use size to classify drusen: small or "hard" (<63 μm) and soft, intermediate (>63 to <125 μm) and large (>125 μm) drusen [24]. When hard or soft drusen coalesce to the point of losing their boundaries they are then classified as "diffuse." Although diagnosis of AMD is typically made when intermediate or large drusen are present, the diagnosis can be also be made in the absence of drusen based on the presence of pigmentary changes indicative of RPE degeneration [3,29,30]. The specific contribution of drusen to AMD complications and progression are not well characterized, but the presence of macular drusen is considered a strong risk factor for the development of both forms of late AMD, geographic atrophy and neovascular AMD [3,34,35]. In general, eyes with clinical AMD have been found to express all three deposit subtypes [14,25,36]. Furthermore, histological, immunohistochemical, and ultrastructural studies of surgically-excised choroidal neovascular (CNV) membranes have shown that the cellular and extracellular constituents of CNV are the same regardless of the underlying disease, with the exception of the amount of BLD and the presence of BLinD, which is virtually exclusively found in CNV specimens from patients with

The cellular and molecular events involved in drusen formation have not been fully elucidated. Lack of scientific consensus exists regarding the origin of drusen, but at least five different paradigms are currently proposed to explain deposit formation in AMD; a) genetic hypothesis, b) lysosomal failure/lipofuscin hypothesis, c) choroidal hypoperfusion hypothesis, d) barrier hypothesis, and e) RPE injury hypothesis. Because our research is based on the RPE injury

There are a number of direct and/or indirect lines of evidence supporting a role for the RPE in drusen biogenesis. According to traditional models of drusen formation, any cellular material residing within drusen is predicted to be of RPE origin. Indeed, RPE-derived basal laminae, organelles and cellular fragments, and even entire cells can be detected in early "drusen". Some authors have described the appearance of RPE "debris" blebbing into drusen or pre-drusen sites [38]. RPE constituents, such as basal laminae, as well as lipofuscin and melanin granules, are observed within early drusen, where they likely

The theory that drusen were derived from damaged RPE was originally postulated by Donders, who first described drusen in a post-mortem eye, believed that drusen were derived from RPE nuclei, based on the supposition that the latter are relatively resistant to degradation [39]. Donders' theory was later modified by De Vicentis (1887) who proposed that degenerative change in the RPE cytoplasm, rather than in the nucleus, was the precipitating event. On the other hand, Muller (1856) proposed that drusen result from aberrant secretion of basement membrane components by the aged RPE [40].With the advent of electron microscopy, the substructural features of drusen were revealed, and new variants of the earlier theories were advanced. Some investigators have concluded, that drusen are formed when the RPE expels

hypothesis, in this section this theory is reviewed in light of the more recent finding.

underlying areas of deposits [25,27,33].

contribute to drusen volume and formation.

AMD [29,37].

The basement membrane of both the choriocapillaris and RPE are comprised of mostly type IV collagen [17,18]. The basement membranes additionally contain laminin, heparan sulfate, proteoglycans, and fibronectin. The outermost basal lamina also incorporates type VI collagen, which is associated with the choriocapillaris [17,18]. It has been suggested that this collagen may act to anchor fenestrated capillaries to the underlying choroid.
