Structure, Function, and Pathology of Bruch’s Membrane

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Chapter 20 Structure, Function, and Pathology of Bruch’s Membrane

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Introduction, history, embryology

Bruch’s membrane is a thin (2–4 µm), acellular, five-layered extracellular matrix located between the retina and choroid.1,2 It extends anteriorly to the ora serrata, interrupted only by the optic nerve. Tissue resembling Bruch’s membrane is visible anterior to the ora serrata extending forward to the pigmented epithelium of the ciliary body. Bruch’s membrane lies between the metabolically active retinal pigment epithelium (RPE) and a capillary bed (choriocapillaris) and thus serves two major functions as the substratum of the RPE and a vessel wall. It has major clinical significance because of its involvement in age-related macular degeneration (AMD) and other chorioretinal diseases.

Early history

Carl Ludwig Wilhelm Bruch first isolated the “lamina vitrea” that we now know as Bruch’s membrane, and described it in his 1844 doctoral thesis,3,4 where he also first described the tapetum found in many mammals. By light microscopy, Bruch’s membrane appeared transparent with little internal structure. Later studies by Smirnow5 divided this membrane into an outer elastic layer (first described by Sattler in 1877) and an inner cuticular layer, separated by a dense plexus of very fine elastic fibers.6,7

Development of Bruch’s membrane

The bipartite character of Bruch’s membrane arises from the embryology of its tissue. When the optic cup invaginates and folds, its inner layer forms the neural retina, and its outer layer forms the RPE. The RPE lies in contact with mesenchyme. At this apposition, Bruch’s membrane forms by 6–7 weeks’ gestation. Thus, its inner layer is composed of ectodermal tissue and its outer layer is composed of mesodermal tissue. At the border of two layers, the elastic layer forms last, becoming histologically visible by 11–12 weeks.810

The collagen that fills the extracellular space and the later-appearing elastin appear to be made by invading fibroblasts and the filopodia of endothelial cells lining the adjacent choriocapillaris. The two basal laminas are produced by their associated cell layers.11 In addition to collagen IV subunits specific to specialized basal lamina, RPE expresses genes for structural collagen III and angiostatic collagen XVIII in a developmentally regulated manner linked to photoreceptor maturation.12

By week 13, fenestrations are apparent in the endothelium facing Bruch’s membrane,10 indicating that, at this stage, transport across this tissue may be functional. Choroidal endothelial cells originate from paraocular mesenchyme. Development of the choroidal vasculature, and Bruch’s as part of it, depends on differentiated RPE and its production of inductive signals, including basic fibroblast growth factor and vascular endothelial growth factor (VEGF).13

Structure of Bruch’s membrane in the young adult eye

Hogan’s five-layer nomenclature for Bruch’s membrane14 is commonly used. Gass proposed a three-layer system that did not include the cellular basal laminas as part of Bruch’s proper.15 These layers are shown in Fig. 20.1 and their constituents are given in Table 20.1.

Table 20.1 Structural and molecular components of Bruch’s membrane

Layer (common abbreviation) Component; age change References
Basal laminar deposit (BlamD) + Fibronectin, laminin, IV α4–5, VI, endostatin, EFEMP1 164, 167, 206209
RPE basal lamina (RPE-BL) IV α1–5, V, laminins 1, 5, 10, and 11, nidogen-1, heparan sulfate, chondroitin sulfate 18, 19, 21, 66, 210, 211
Lipid wall/basal linear deposit (BlinD) + Lipoproteins 38, 39, 212
Inner collagenous layer (ICL) I, III, V, fibronectin, chondroitin sulfate, dermatan sulfate, lipoproteins ↑, apoE, heme, clusterin, vitronectin 34, 35, 38, 39, 50, 66, 146, 152, 210, 213215
Elastic layer (EL) Elastin ↑, calcium phosphate 14, 6668, 210, 216
Outer collagenous layer (OCL) I, III, V, fibulin-5, fibronectin, chondroitin sulfate, dermatan sulfate, lipoproteins ↑, apoE, clusterin 21, 39, 50, 152, 210, 215, 217
ChC-basal lamina IV α1, 2, V, VI, laminin, heparan sulfate, chondroitin sulfate, endostatin 18, 208, 210, 211, 218
Bruch’s, throughout or layer not specified I ↑, collagen solubility ↓, perlecan, MMP-2, MMP-9, TIMP-2; TIMP-3, pentosidine, CML ↑, GA-AGE ↑, RGR-d, apoB, oxidized apoB-100, 7-KCh, LHP, HHE, DHP-lys, C3d, C5b-9, pentraxin-3 ↑, thrombospondin-1, zinc 62, 66, 138, 139, 147, 218230

Table shows definitely localized components. Most determinations were made in macula. Studies showing histochemical/immunohistochemical verification of biochemistry and ultrastructural validation of structures identified by light microscopy techniques were given greater weight. Localizations were assigned to specific layers if immunogold-electron microscopy or high-magnification confocal microscopy images were available. Roman numerals denote collagens. Components are ordered within each layer: structural components, lipoproteins, extracellular matrix and its regulation, modified lipids and proteins, complement/immunity, cellular response/activity, metals. Known changes with advancing age are bold with an arrow indicating direction of change. New additions with age are shown with a plus (+). Plain text means no change or not tested.

CML, carboxymethyl-lysine226; 7-KCh, 7 keto-cholesterol229; GA-AGE, glycolaldehyde-derived advanced glycation end products221; HHE, 4-hydroxyhexenal66,218; DHP-lys, dihydropyridine lysine.66

RPE basal lamina (RPE-BL)

This ∼0.15-µm-thick layer is a meshwork of fine fibers like other basal laminas in the body.16,17 The RPE-BL resembles that of the choriocapillaris endothelium but does not contain collagen VI. The RPE-BL contains collagen IV α3–5,18 like that of kidney glomerulus, another organ with specialized filtration and transport functions. The RPE synthesizes specific laminins that preferentially adhere Bruch’s membrane to the RPE through interaction with integrins.19

Inner collagenous layer (ICL)

The ICL is ∼1.4 µm thick and contains 70-nm-diameter fibers of collagens I, III, and V in a multilayered criss-cross, parallel to the plane of Bruch’s membrane.1 The collagen grid is associated with interacting molecules, particularly the negatively charged proteoglycans chondroitin sulfate and dermatan sulfate.20,21

Elastic layer (EL)

The EL consists of stacked layers of linear elastin fibers, crisscrossing to form a 0.8-µm-thick sheet with interfibrillary spaces of ∼1 µm. This sheet extends from the edge of the optic nerve to the ciliary body pars plana.1 In addition to elastin fibers, the EL contains collagen VI, fibronectin, and other proteins, and collagen fibers from the ICL and outer collagenous layer (OCL) can cross the EL. Some EL elastin fibers are said to cross the tissue space between the choriocapillaris and join bundles of choroidal elastic tissue.22 The EL confers biomechanical properties, vascular compliance, and antiangiogenic barrier functions. It is more discontinuous in the macula, perhaps explaining why choroidal neovascularization (CNV) is more prominent there.23 This concept is supported by the extensive laser-induced neovascularization in mice deficient in lysyl oxidase-like 1, an enzyme required for elastin polymerization.24

Bruch’s membrane in an aged eye

Aging is the largest risk factor for developing AMD,28 and Bruch’s membrane undergoes significant age-related changes. Identification of factors predisposing to disease progression is a priority. This task has been challenged by difficulty imposed by the thinness of the tissue, and the closely integrated functions of RPE, Bruch’s, and choriocapillaris. Current opinion holds that RPE and Bruch’s membrane age in concert, and normal Bruch’s membrane aging transforms insidiously into AMD pathology.1,16,17,29 This section covers aging, to inform the following section on function.

Lipid accumulation: Bruch’s membrane lipoproteins

Early electron microscopists described aged Bruch’s membrane as being filled with debris, including amorphous electron-dense material, membrane fragments, vesicles, and calcification.1,25 Debris deposition in ICL and OCL begins in the second decade in the macula and is delayed in equatorial regions, a regional lag also reported for individual components.30 Identifying this material has been a fruitful approach to understanding antecedents of disease.

Most prominent among the changes in Bruch’s membrane is a profound accumulation of lipids. Clinical observations on fluid-filled RPE detachments in older adults led to Bird and Marshall’s hypothesis that a lipophilic barrier in Bruch’s blocked a normal, outwardly directed fluid efflux from the RPE31 (as opposed to leakage from CNV). This hypothesis motivated a seminal histochemical study by Pauleikhoff et al.32 that demonstrated oil red O-binding material (esterified cholesterol (EC), triglyceride (TG), fatty acid) localized exclusively to Bruch’s membrane, unlike other stains. This lipid was absent <30 years, variably present at 31–60 years, and abundant at ≥61 years.33,34 A specific fluorescent marker, filipin, which binds the 3β-hydroxy group of sterols to reveal unesterified (free) cholesterol (UC) or EC depending on tissue pretreatment,35 indicated that EC is a prominent component of the oil red O-binding deposition.35 Macular EC rose linearly from near zero at age 22 years to reach high and variable levels in aged donors. EC was detectable in periphery at ∼1/7 macular levels and increased significantly with age. Hot-stage polarizing microscopy34 similarly demonstrated prominent age-related increases in EC in Bruch’s membrane, manifest as liquid crystals (“Maltese crosses”) when examined through a polarizing filter. Few birefringent crystals signifying the neutral lipid TG were found.

Histochemical, ultrastructural, biochemical, gene expression, and cell biological evidence now indicate that the EC-rich material accumulating with age in Bruch’s membrane is a lipoprotein-containing apolipoprotein B, assembled by the RPE.36 This process, ongoing throughout life yet first revealed by aging, has implications for the formation of AMD-specific lesions, intraocular transport, RPE physiology, nutrition of outer retina, and maintenance of photoreceptor health. In 1926, Verhoeff and Sisson speculated that lipid deposition might precede Bruch’s membrane basophilia and fragmentation, common in older eyes due to “lime salts [calcification] in the elastic layer.”37

Ultrastructural studies described in Bruch’s membrane of older eyes36 numerous small (<100 nm), round, electron-lucent vesicular profiles, implying aqueous interiors. Lipid-preserving preparation techniques together with extraction studies show that these so-called vesicles are actually solid, lipid-containing particles, now considered lipoproteins (Fig. 20.3B). These methods include postfixation in osmium paraphenylenediamine (OTAP)35 and, most strikingly, quick-freeze/deep-etch (QFDE), a freeze fracture method with an etching step to remove frozen water.3840 Particles vary in size from 60 to 100 nm but could be as large as 300 nm, occasionally appearing to coalesce (Fig. 20.3).

Lipoprotein particles are first seen among fibrils of the elastic layer in early adulthood, extending inward ultimately to fill most of the open space of the ICL by the seventh decade of life.40 Most fatefully, a new layer, the lipid wall,38 then forms with solid particles stacked 3–4 deep occupying nearly 100% of a space between RPE basal lamina and OCL of many older eyes. The lipid wall displaces ICL collagen fibrils that anchor the RPE basal lamina (Fig. 20.3). It is considered a precursor to basal linear deposits, a specific lesion of AMD (see below).

Lipoprotein composition can provide clues to sources of its components.41 When isolated (Fig. 20.4A), Bruch’s membrane lipoproteins are found to be EC-enriched (EC/total cholesterol = 0.56; EC/TG = 4–11; Fig. 20.4B). For comparison, hepatic very-low-density lipoprotein (VLDL), of similar diameter, is TG-rich. An early report of TG-enriched Bruch’s membrane neutral lipid42 was not replicated. Abundant EC points to the only mechanism by which neutral lipids are released directly from cells, an apoB-containing lipoprotein, like hepatic VLDL or intestinal chylomicrons. Significantly, RPE expresses the apoB gene and protein, along with microsomal triglyceride transfer protein (MTP), required for apoB lipidation and secretion. Lack of functional MTP is the basis of abetalipoproteinemia, a rare inherited disorder that includes a pigmentary retinopathy.43,44 The combination of apoB and MTP within native RPE marks these cells as constitutive lipoprotein secretors.45 Secretion of full-length apoB has been demonstrated in rat-derived and human-derived RPE cell lines.46,47 Consistent with an RPE origin, particles first appear in the elastic layer of Bruch’s membrane and fill in towards the RPE.39

image

Fig. 20.4 Bruch’s membrane lipoprotein composition. (A) Lipoprotein particles isolated from Bruch’s membrane are large and spherical; negative stain.153 (Source: Li C-M, Chung BH, Presley JB, et al. Lipoprotein-like particles and cholesteryl esters in human Bruch’s membrane: initial characterization. Invest Ophthalmol Vis Sci 2005;46:2576–86). Bar = 50 nm. (B) Bruch’s membrane lipoprotein composition inferred from direct assay,50,153 (Sources: Wang L, Li C-M, Rudolf M, et al. Lipoprotein particles of intra-ocular origin in human Bruch membrane: an unusual lipid profile. Invest Ophthalmol Vis Sci 2009;50:870–7) (Li C-M, Chung BH, Presley JB, et al. Lipoprotein-like particles and cholesteryl esters in human Bruch’s membrane: initial characterization. Invest Ophthalmol Vis Sci 2005;46:2576–86), druse composition, and retinal pigment epithelium gene expression.139,154 (Sources: Malek G, Li C-M, Guidry C, et al. Apolipoprotein B in cholesterol-containing drusen and basal deposits in eyes with age-related maculopathy. Am J Pathoi 2003;162:413–25) and (TG, Li C-M, Clark ME, Chimento MF, et al. Apolipoprotein localization in isolated drusen and retinal apolipoprotein gene expression. Invest Ophthalmol Vis Sci 2006;47:3119–28) TG, triglyceride; EC, esterified cholesterol; UC, unesterified cholesterol; PL, phospholipid; Apo, apolipoproteins. The question mark signifies that not all apolipoproteins are known.

Indirect evidence that Bruch’s membrane lipoproteins are of intraocular origin also emerges from the epidemiologic literature. If the EC deposition in Bruch’s membrane and AMD-associated lesions were a manifestation of systemic perifibrous lipid and atherosclerosis, then a strong positive correlation between disease status and plasma lipoprotein levels, like that documented for coronary artery disease,48 might be expected but has not emerged.49

Identifying the upstream sources of Bruch’s membrane lipoprotein constituents is essential for understanding the biological purpose of this pathway and the prospects for eventual clinical exploitation. Studies using isolated lipoproteins from Bruch’s membrane50 and Bruch’s membrane choroid EC51 report a high mole percentage of linoleate (>40%) and low docosahexaenoate (<1%) for all lipid classes.52 This composition strongly points away from photoreceptor outer segments (35% docosahexaenoate in membrane phospholipids) as an upstream source, as long postulated,53,54 and towards plasma lipoproteins (45–55% linoleate in all lipid classes). These data have been interpreted to signify that plasma lipoproteins are major contributors upstream to an apoB lipoprotein of RPE origin. In contrast, the sources of UC in Bruch’s lipoproteins are not yet known and could be outer segments, plasma lipoproteins, endogenous synthesis, or a combination.

Lipoproteins may thus be assembled from several sources, including outer segments, remnant components from the photoreceptor nutrient supply system, and endogenous synthesis. According to this model,52 plasma lipoproteins serve as vehicles for delivery of lipophilic nutrients (carotenoids,55 vitamin E, and cholesterol56) to photoreceptors by RPE, which has functional receptors for low-density lipoprotein (LDL) and high-density lipoprotein.57,58 Nutrients are stripped from these lipoproteins by the RPE for delivery to the photoreceptors, and the remnants are repackaged for secretion into Bruch’s membrane as part of apoB-containing lipoproteins, where they begin to accumulate during age and become toxically modified to instigate inflammation in AMD.

Other aging changes

Bruch’s membrane thickens throughout adulthood (20–100 years) two- to threefold under the macula and becoming more variable between individuals at older ages.25,59,60 Equatorial Bruch’s membrane changes little while Bruch’s membrane near the ora serrata increases twofold during this time.25 In the macula, the OCL thickens more prominently than the ICL.61 A large ultrastructural study of 121 human donor eyes demonstrated that the macular EL is 3–6 times thinner than peripheral EL23 at all ages.

Unbalanced regulation of extracellular matrix molecules and their modulator matrix are thought to result in Bruch’s membrane thickening. Increased histochemical reactivity for glycoconjugates, glycosaminoglycans (GAGs), collagen, and elastin is seen in the macula relative to equator and near the ora serrata.25 Collagen solubility declines with age.62 Matrix metalloproteinases MMP-2 and MMP-3 increase with age, as does a potent tissue inhibitor of metalloproteinases, TIMP-3. TIMP-3 immunoreactivity reaches adult levels at 30 years of age near vasculature in lung, kidney, and in Bruch’s membrane, signifying the end of developmental organogenesis.63 The reduction or absence of TIMP-3 is proangiogenic, as this protein not only regulates metalloproteinases during the normal turnover of Bruch’s membrane matrix components, but it also binds to VEGF.64,65

The EL thickens with age but decreases relative to overall thickening of Bruch’s membrane.23 Thus elastin referenced to other Bruch’s constituents, as detected by Raman spectroscopy, decreases with age.66 Similar arguments can be made for collagen III and IV. A prominent age change,67 noted early,37 is calcification and ensuing brittleness. This process involves fine deposition of electron-dense particulate matter,14 confirmed as calcium phosphate68 on individual elastin fibrils.

Long-lived proteins like collagens are modified in vivo by nonenzymatic Maillard and free radical reactions to yield advanced glycation end products (AGEs) and the formation of lipid-derived reactive carbonyl species like 4-hydroxyhexenal and linoleate hydroperoxide, collectively called age-related lipoperoxidation end products (ALEs). Accumulation of AGEs and ALEs, characteristic of diabetes and atherosclerosis, also occurs in aging Bruch’s membrane (Table 20.1). Finally, other components more prominent in aged eyes include complement components C3d, C5b-9, and pentraxin-3, a homolog of the acute-phase respondent C-reactive protein. Thus, at the molecular level, aging Bruch’s membrane contains evidence of many biological activities, including remodeling, oxidative damage, and inflammation, in addition to lipoprotein accumulation.

Function of Bruch’s membrane

As a vessel wall of the choroid, Bruch’s membrane’s primary function is structural, like other vessel walls. Its architecture is similar to vascular intima, with a subendothelial extracellular matrix and elastic layer corresponding to the internal elastic lamina. The abluminal surface of Bruch’s differs from other vessel walls in that it abuts a basal lamina, that of the RPE. The luminal surface faces a fenestrated vascular endothelium and basal lamina, making Bruch’s membrane structurally analogous to the renal glomerulus and providing a basis for commonality between retinal and kidney disease.6971 The importance of fluid and macromolecular transport across the renal glomerulus is well known.72 Transport is a second important function of Bruch’s membrane.

Structural role of Bruch’s membrane

Bruch’s membrane encircles more than half the eye and stretches with the corneoscleral envelope as intraocular pressure (IOP) increases. It therefore withstands this stretch and returns to its original shape when IOP decreases. This tissue also stretches to accommodate changes in choroidal blood volume. Finally, the choroid (and Bruch’s membrane with it) may act as a spring that pulls the lens during accommodation.73,74 For these reasons, then, Bruch’s membrane requires elasticity. Marshall and Hussain’s group estimated the modulus of elasticity in Bruch’s membrane choroid preparations to be 7–19 MPa.75 These values are similar to those of sclera (although sclera is much thicker and thus can support more load), consistent with the notion that Bruch’s membrane contributes to load bearing. After early adulthood, the modulus of elasticity of human Bruch’s membrane–choroid complex increases (P < 0.001) at a rate of ∼1% per year. Bruch’s membrane stiffness in AMD eyes does not differ from age-matched normals.76

Transport role of Bruch’s membrane

The choroid services the metabolic needs of the outer retina, facilitated in part by fenestrated endothelium. Oxygen, electrolytes, nutrients, and cytokines destined for the RPE and photoreceptors pass from the choriocapillaris and through Bruch’s membrane, and waste products travel back in the opposite direction for elimination. Vitamins, signaling molecules, and other factors needed for photoreceptor function are carried to the RPE by lipoprotein particles passing through Bruch’s membrane, as do the RPE-produced lipoproteins that are eliminated in the opposite direction. The RPE pumps water from the subretinal space to counter the swelling of the interphotoreceptor matrix GAGs. This fluid also flows across Bruch’s membrane to reach the circulation. Thus, many transport processes involve Bruch’s membrane, as reviewed here.

Hydraulic conductivity of Bruch’s membrane

GAGs are concentrated in the interphotoreceptor matrix77,78 and corneal stroma.79 In both locations, these highly charged macromolecules maintain geometric fidelity essential for vision (periodic collagen spacing for corneal transparency, orderly photoreceptor spacing for visual sampling78,80,81). GAGs generate significant swelling pressure (up to 50 mmHg in cornea).82,83 Without a mechanism to maintain tissue deturgescence, GAGs would imbibe fluid, swell, destroy tissue geometry, and interfere with visual function. Corneal endothelium forestalls swelling by continuously pumping fluid out. This function is accomplished for retina by the RPE, and its failure can lead to retinal detachment. A driving force adequate to overcome the collective flow resistance of RPE, Bruch’s membrane, and choriocapillaris endothelium is provided by a gradient in fluid pressure and oncotic pressure (the osmotic pressure generated by plasma proteins). This balance is embodied by Starling’s law that characterizes the relationship between fluid flux (q = flow per unit area; positive when flow is out of the blood vessel) across a capillary vessel wall and the forces driving this flow:

image     (equation 1)

Lp is hydraulic conductivity, which characterizes the ease with which fluids flow cross the vessel wall. If the surface area of the blood vessel is A, then 1/(Lp A) is the flow resistance of the vessel wall. ΔP is the difference between the fluid pressure within the blood vessel (Pcc) and the pressure at the basal surface of the RPE (PRPE). ΔΠ is the difference between the oncotic pressure within the blood vessel (Πcc) and that at the basal surface of the RPE (ΠRPE). σ is the reflection coefficient that characterizes the extent to which the vessel wall rejects the plasma protein species generating ΔΠ. σ ranges from 0 for a freely permeable species to 1 when a species is completely rejected by the membrane.

We can estimate the magnitude of ΔP – σΔΠ using measured value of q and Lp. The fluid pumping rate by human RPE has been measured as q = 11 µL/h/cm2, similar to that in other animals (Table 20.2). The hydraulic conductivity of macular Bruch’s membrane/choroid of healthy young humans ranges from 20 to 100 × 10−10 m/s/Pa.84 Then, using q = 11 µL/h/cm2 and Lp = 50 × 10−10 m/s/Pa, we can calculate that the magnitude of (ΔP − σΔΠ) necessary to drive this flow through Bruch’s membrane is roughly 0.05 mmHg. (This does not include the flow resistance of choriocapillaris endothelium, which is not measured when Lp of a Bruch’s membrane/choroidal preparation is determined. For this highly fenestrated endothelium, Lp can be estimated as roughly 25 × 10−10 m/s/Pa,85 which does not affect our conclusions below.)

Table 20.2 Retinal pigment epithelium (RPE) fluid pumping rates

Species Fluid transport rate across RPE (µL/h/cm2) References
Frog 4.8–7.6 231, 232
Rabbit 12 ± 4 233, 234
Canine 6.4 235
Primate* 14 ± 3 236, 237
Human 11 238

RPE pumping rates were measured by readsorption of subretinal fluid or by direct measurement in culture.

* Cantrill and Pederson236 measured a much higher transport rate than that reported here, but used fluorescein as a tracer which likely does not track fluid flow due to its high diffusion coefficient.

σ can be roughly estimated by assuming that the fluid in the suprachoroidal space is in equilibrium with blood in the choroid. Using measurements of fluid pressure and of the plasma protein concentration (to estimate oncotic pressure) inside and outside the choriocapillaris,8688 equation (1) can be used to find σ ≈ 0.5.

Allowing that Πcc = 27 mmHg,86 Pcc = IOP + 8 mmHg,87 and assuming that ΠRPE = 0 mmHg (fluid pumped by the RPE is assumed protein-free) and PRPE = IOP (assuming no pressure is generated by the RPE above that necessary for crossing Bruch’s), we find that ΔP – σΔΠ is approximately –5.5 mmHg pulling fluid into the choroid. Thus, in normal young adults, oncotic pressure within the choroid is more than sufficient to adsorb all the fluid pumped by the RPE. We can also use equation (1) to calculate that the lowest value of Lp that still adsorbs fluid pumped by the RPE without generating an elevated pressure at the RPE basal surface is Lp > 0.4 × 10−10 m/s/Pa.

Experiments using laser ablation of Bruch’s membrane/choroid explants allowed Starita et al.89 to conclude that the ICL was responsible for most of the flow resistance in Bruch’s membrane. Attempts to localize further the flow resistance using morphometric methods are complicated by first, stereological issues90 and second, the loss of ultrastructural fidelity from connective tissue conventionally processed for electron microscopy.38 Failure to appreciate the former difficulty can lead to unphysiologically low estimates for tissue porosity and thereby hydraulic conductivity.1

Age-related changes in hydraulic conductivity and disease

Fisher was the first to measure Lp of human Bruch’s membrane,91 finding that Lp decreased significantly with age. However, his values for Lp of Bruch’s membrane and other tissues are much lower than those found by later investigators.85,92,93 Marshall and Hussain’s group carefully revisited these measurements using Bruch’s membrane/choroid with RPE removed, a preparation that was simpler to create. They showed using laser ablation that the flow resistance of these preparations was entirely due to Bruch’s membrane.89 They also found that flow rate increased linearly with driving pressure, indicating that Lp of Bruch’s membrane is relatively insensitive to pressure up to 25 mmHg.

They reported that Lp of macular Bruch’s membrane exhibited a dramatic, exponential decline throughout life (Fig. 20.5), dropping from 130 × 10−10 m/s/Pa in young children to 0.52 × 10−10 m/s/Pa in old age. Lp of macular Bruch’s membrane dropped more rapidly with age than did that of the periphery, consistent with an accelerated process occurring in the macula.1,84,94,95 Note that the lowest value measured for Lp of Bruch’s membrane in normal eyes is similar to the calculated minimum value of Lp that allows complete fluid resorption (0.4 × 10−10 m/s/Pa; see above). Marshall and Hussain’s group reached similar conclusions regarding this process.94

image

Fig. 20.5 Hydraulic conductivity (Lp) of Bruch’s membrane as a function of age. Dotted lines are exponential fits to data from macular and peripheral regions, respectively. Note that all of the data from eyes with age-related macular degeneration (AMD) (taken only in peripheral region) have lower values of Lp than the best fit to data taken from peripheral Bruch’s membrane of nondiseased eyes.

(Reproduced with permission from Hussain AA, Starita C, Marshall J. Transport characteristics of ageing human Bruch’s membrane: implications for age-related macular degeneration (AMD). In: Ioseliani OR, editor. Focus on macular degeneration research. Hauppauge, New York: Nova Biomedical Books; 2004.)

Determining Lp of Bruch’s membrane in isolated macular samples of AMD eyes is difficult due to scar formation and other changes.94 However, Marshall and Hussein’s group showed that, in the periphery, Lp of Bruch’s membrane is decreased in AMD eyes as compared to age-matched normal eyes (Fig. 20.5).94 Assuming that similar processes occur in macular Bruch’s membrane due to the profound lipid accumulation in this region, then in diseased eyes, the RPE must generate higher pressures at its basal surface to drive fluid into the choriocapillaris, with further pathological consequences.31 Above an unknown threshold level, higher pressure will cause the RPE-BL to separate from the ICL, leading to RPE detachment and fluid accumulation, as seen in 12–20% of AMD patients.94

What causes the dramatic age-related decrease in Lp of Bruch’s membrane? It is natural to suspect the age-related lipid accumulation. In fact, McCarty et al.96 showed that lipid particles trapped in an extracellular matrix can generate very significant flow resistance, more than would be expected based simply on their size and number. However, Marshall and Hussain’s group observed that most of the marked change in Lp occurred before age 40 (Fig. 20.6A) while the increase in Bruch’s membrane lipid content occurred largely after this age. They thus concluded that other age-related changes must be responsible for changes in Lp.1,84

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