Collagens in the aged human macula

Pachychoroid spectrum disease

Recent improvements in ophthalmic imaging have led to the identification of a thickened choroid or pachychoroid to be associated with a number of retinal diseases. The number of conditions linked to this phenotype has continued to widen with specific endophenotypes found within the pachychoroid spectrum. The spectrum includes choroidal features such as focal or diffuse choroidal thickening and thinning of the overlying inner choroid, and choroidal hyperpermeability as demonstrated by indocyanine green angiography. In addition, these diseases are associated with overlying retinal pigmentary changes and retinal pigment epithelial dysfunction and may also be associated with choroidal neovascularization. This article provides a comprehensive review of the literature looking at diseases currently described within the pachychoroid spectrum including central serous chorioretinopathy, pachychoroid pigment epitheliopathy, pachychoroid neovasculopathy, polypoidal choroidal vasculopathy/aneurysmal type 1 neovascularization, peripapillary pachychoroid disease and focal choroidal excavation. We particularly focus on clinical imaging, genetics and pathological findings in these conditions with the aim of updating evidence suggesting a common aetiology between diseases within the pachychoroid spectrum.

Exploring the choroidal vascular labyrinth and its molecular and structural roles in health and disease

The choroid is a key player in maintaining ocular homeostasis and plays a role in a variety of chorioretinal diseases, many of which are poorly understood. Recent advances in the field of single-cell RNA sequencing have yielded valuable insights into the properties of choroidal endothelial cells (CECs). Here, we review the role of the choroid in various physiological and pathophysiological mechanisms, focusing on the role of CECs. We also discuss new insights regarding the phenotypic properties of CECs, CEC subpopulations, and the value of measuring transcriptomics in primary CEC cultures derived from post-mortem eyes. In addition, we discuss key phenotypic, structural, and functional differences that distinguish CECs from other endothelial cells such as retinal vascular endothelial cells. Understanding the specific clinical and molecular properties of the choroid will shed new light on the pathogenesis of the broad clinical range of chorioretinal diseases such as age-related macular degeneration, central serous chorioretinopathy and other diseases within the pachychoroid spectrum, uveitis, and diabetic choroidopathy. Although our knowledge is still relatively limited with respect to the clinical features and molecular pathways that underlie these chorioretinal diseases, we summarise new approaches and discuss future directions for gaining new insights into these sight-threatening diseases and highlight new therapeutic strategies such as pluripotent stem cell‒based technologies and gene therapy.

Characterization of a tissue-engineered choroid

The choroid of the eye is a vascularized and pigmented connective tissue lying between the retina and the sclera. Increasing evidence demonstrates that, beyond supplying nutrients to the outer retina, the different choroidal cells contribute to the retina's homeostasis, especially by paracrine signaling. However, the precise role of each cell type is currently unclear. Here, we developed a choroidal substitute using the self-assembly approach of tissue engineering. Retinal pigment epithelial (RPE) cells, as well as choroidal stromal fibroblasts, vascular endothelial cells and melanocytes, were isolated from human eye bank donor eyes. Fibroblasts were cultured in a medium containing serum and ascorbic acid. After six weeks, cells formed sheets of extracellular matrix (ECM), which were stacked to produce a tissue-engineered choroidal stroma (TECS). These stromal substitutes were then characterized and compared to the native choroid. Their ECM composition (collagens and proteoglycans) and biomechanical properties (ultimate tensile strength, strain and elasticity) were similar. Furthermore, RPE cells, human umbilical vein endothelial cells and choroidal melanocytes successfully repopulated the stromas. Physiological structures were established, such as a confluent monolayer of RPE cells, vascular-like structures and a pigmentation of the stroma. Our TECS thus recaptured the biophysical environment of the native choroid, and can serve as study models to understand the normal interactions between the RPE and choroidal cells, as well as their reciprocal exchanges with the ECM. This will consequently pave the way to derive accurate insight in the pathophysiological mechanisms of diseases affecting the choroid. STATEMENT OF SIGNIFICANCE: The choroid is traditionally known for supplying blood to the avascular outer retina. There has been a renewed attention directed towards the choroid partly due to its implication in the development of age-related macular degeneration (AMD), the leading cause of blindness in industrialized countries. Since AMD involves the dysfunction of the choroid/retinal pigment epithelium (RPE) complex, a three-dimensional (3D) model of RPE comprising the choroid layer is warranted. We used human choroidal cells to engineer a choroidal substitute. Our approach takes advantage of the ability of cells to recreate their own environment, without exogenous materials. Our model could help to better understand the role of each choroidal cell type as well as to advance the development of new therapeutics for AMD.

Two Bioactive Molecular Weight Fractions of a Conditioned Medium Enhance RPE Cell Survival on Age-Related Macular Degeneration and Aged Bruch's Membrane

PURPOSE

To characterize molecular weight fractions of bovine corneal endothelial cell conditioned medium (CM) supporting retinal pigment epithelium (RPE) cell survival on aged and age-related macular degeneration (AMD) Bruch's membrane.

METHODS

CM was subject to size separation using centrifugal filters. Retentate and filtrate fractions were tested for bioactivity by analyzing RPE survival on submacular Bruch's membrane of aged and AMD donor eyes and behavior on collagen I-coated tissue culture wells. Protein and peptide composition of active fractions was determined by mass spectrometry.

RESULTS

Two bioactive fractions, 3-kDa filtrate and a 10-50-kDa fraction, were necessary for RPE survival on aged and AMD Bruch's membrane. The 3-kDa filtrate, but not the 10-50-kDa fraction, supported RPE growth on collagen 1-coated tissue culture plates. Mass spectrometry of the 10-50-kDa fraction identified 175 extracellular proteins, including growth factors and extracellular matrix molecules. Transforming growth factor (TGF)β-2 was identified as unique to active CM. Peptides representing 29 unique proteins were identified in the 3-KDa filtrate.

CONCLUSIONS

These results indicate there is a minimum of two bioactive molecules in CM, one found in the 3-kDa filtrate and one in the 10-50-kDa fraction, and that bioactive molecules in both fractions must be present to ensure RPE survival on Bruch's membrane. Mass spectrometry analysis suggested proteins to test in future studies to identify proteins that may contribute to CM bioactivity.

TRANSLATIONAL RELEVANCE

Results of this study are the first steps in development of an adjunct to cell-based therapy to ensure cell transplant survival and functionality in AMD patients.

Investigating the choriocapillaris and choroidal vasculature with new optical coherence tomography technologies

The body of knowledge of in vivo investigation of the choroid has been markedly enhanced by recent technological advances in optical coherence tomography (OCT). New insights elucidating the morphological features of the choriocapillaris and choroidal vasculature, in both physiological and pathological conditions, indicate that the choroid plays a pivotal role in many posterior segment diseases. In this article, a review of the histological characteristics of the choroid, which must be considered for the proper interpretation of in vivo imaging, is followed by a comprehensive discussion of fundamental principles of the current state-of-the-art in OCT, including cross-sectional OCT, en face OCT, and OCT angiography using both spectral domain OCT and swept source OCT technologies. A detailed review of the tomographic features of the choroid in the normal eye is followed by relevant findings in prevalent chorioretinal diseases, focusing on major causes of vision loss such as typical early and advanced age-related macular degeneration, polypoidal choroidal vasculopathy, central serous chorioretinopathy, pachychoroid spectrum disorders, diabetic choroidopathy, and myopia.

Pachychoroid neovasculopathy

PURPOSE

To report 3 cases of pachychoroid neovasculopathy, a form of Type 1 (sub-retinal pigment epithelium) neovascularization, occurring over areas of increased choroidal thickness and dilated choroidal vessels.

METHODS

A retrospective observational case series of three patients who underwent comprehensive ophthalmic examination and multimodal imaging with fundus photography, fundus autofluorescence, spectral domain optical coherence tomography, enhanced depth imaging optical coherence tomography, fluorescein angiography, and indocyanine green angiography.

RESULTS

In all 3 eyes of 3 patients, aged 55 years to 63 years, there was Type 1 neovascularization overlying a localized area of choroidal thickening and dilated choroidal vessels seen with enhanced depth imaging optical coherence tomography. With indocyanine green angiography, there were large choroidal veins and choroidal hyperpermeability seen beneath the area of the neovascular tissue in all three eyes. No eyes had evidence of submacular exudative detachment or autofluorescence changes to suggest antecedent acute or chronic central serous chorioretinopathy. No eyes had drusen or degenerative changes to suggest age-related macular degeneration or other degenerative diseases. In one patient, the fellow unaffected eye demonstrated retinal pigment epithelium abnormalities, best seen with fundus autofluorescence, overlying focal dilated choroidal vessels seen with enhanced depth imaging optical coherence tomography and associated choroidal hyperpermeability seen with indocyanine green angiography, consistent with the diagnosis of pachychoroid pigment epitheliopathy. All three eyes showed the appearance of polypoidal structures within the neovascular tissue.

CONCLUSION

Pachychoroid neovasculopathy falls within a spectrum of diseases associated with choroidal thickening that includes pachychoroid pigment epitheliopathy, central serous chorioretinopathy, and polypoidal choroidal vasculopathy and should be considered as a possible diagnosis in eyes with features of Type 1 neovascularization and choroidal thickening in the absence of characteristic age-related macular degeneration or degenerative changes. Pachychoroid neovasculopathy may occur as a focal abnormality within the macula, even in myopic eyes with normal subfoveal choroidal thickness. Pachychoroid neovasculopathy can ultimately progress to the development of polypoidal choroidal vasculopathy.

Mice that produce ApoB100 lipoproteins in the RPE do not develop drusen yet are still a valuable experimental system

PURPOSE

Mice typically produce apolipoprotein B (apoB)-48 and not apoB100. Apolipoprotein B100 accumulates in Bruch's membrane prior to basal deposit and drusen formation during the onset of AMD, raising the possibility that they are a trigger for these Bruch's membrane alterations. The purpose herein, was to determine whether mice that predominantly produce apoB100 develop features of AMD.

METHODS

The eyes of mice that produce apoB100 were examined for apoB100 synthesis, cholesteryl esterase/filipin labeling for cholesteryl esters, and transmission electron microscopy for lipid particles and phenotype.

RESULTS

Apolipoprotein B100 was abundant in the RPE-choroid of apoB100, but not wild-type mice by Western blot analysis. The apolipoprotein B100,(35)S-radiolabeled and immunoprecipitated from RPE explants, confirmed that apoB100 was synthesized by RPE. Apolipoprotein B100, but not control mice, had cholesteryl esters and lipid particles in Bruch's membrane. Immunoreactivity of ApoB100 was present in the RPE and Bruch's membrane, but not choroidal endothelium of apoB100 mice. Ultrastructural changes were consistent with aging, but not AMD when aged up to 18 months. The induction of advanced glycation end products to alter Bruch's membrane, did not promote basal linear deposit or drusen formation.

CONCLUSIONS

Mice that produce apoB100 in the RPE and liver secrete lipoproteins into Bruch's membrane, but not to the extent that distinct features of AMD develop, which suggests that either additional lipoprotein accumulation or additional factors are necessary to initiate their formation.

Appearance of regressing drusen on optical coherence tomography in age-related macular degeneration

OBJECTIVE

To describe and interpret a multilaminar sub-retinal pigment epithelium (RPE) intense hyper-reflectivity observed in vivo in eyes clinically diagnosed with regressing drusen.

DESIGN

Observational case series.

PARTICIPANTS

Twenty-three consecutive patients clinically diagnosed with regressing calcific drusen due to nonneovascular age-related macular degeneration (AMD).

METHODS

Patients were submitted to confocal scanning laser ophthalmoscopy (cSLO) fundus imaging and "eye-tracked" spectral-domain optical coherence tomography (SD-OCT).

MAIN OUTCOME MEASURES

Localization and possible origin and composition of the multilaminar sub-RPE hyperreflectivity.

RESULTS

Thirty eyes of 23 consecutive patients (8 male and 15 female; mean age, 82.7±10.1 years) showing on SD-OCT an intense multilaminar sub-RPE hyperreflectivity, which matched with regressing calcific drusen as visualized by cSLO infrared (IR) and MultiColor (Heidelberg Engineering, Heidelberg, Germany) images, were included in this study. The multilaminar hyperreflectivity was found to localize to beneath the RPE and above the outer Bruch's membrane (oBM) layer. A mean of 1.2 multilaminar sub-RPE hyperreflectivities per SD-OCT scan were identified by 2 readers. The SD-OCT analysis allowed the 2 readers to describe 3 different types of sub-RPE hyperreflectivity. "Type 1" laminar/multilaminar hyperreflectivity (found in 24 scans of 12 eyes) was characterized by an intense signal originating from what we interpreted as the inner Bruch's membrane (iBM) layer. "Type 2" multilaminar hyperreflectivity (found in 130 scans of 27 eyes) was characterized by an intense signal originating from the oBM layer. "Type 3" multilaminar fragmented hyperreflectivity (found in 22 scans of 11 eyes) was characterized by an intense signal originating from what we interpreted as both the iBM and the oBM, showing different degrees of fragmentation.

CONCLUSIONS

We describe a novel SD-OCT finding appearing as multilaminar sub-RPE intense hyper-reflectivity observed in vivo in eyes with regressing drusen. This multilaminar sub-RPE hyperreflectivity could be interpreted as layers of lipid mineralization (membranous debris also called "lipoprotein-derived debris" developing calcification), internal and external to the basement membrane, with different degrees of fragmentation.

Molecular responses of choroidal endothelial cells to elastin derived peptides through the elastin-binding protein (GLB1)

PURPOSE

Neovascular AMD involves the activation of choroidal endothelial cells to increase their inflammatory and angiogenic behaviors. Elastin derived peptides (EDPs) can elicit some of these phenotypic changes in endothelial cells. This investigation was performed to follow up on those findings by determining a receptor for these peptides in the human eye as well as evaluating the effects of elevated EDPs on choroidal cells in vitro and in vivo.

METHODS

The expression of elastin receptor genes including GLB1 was analyzed using reverse transcription PCR. Migration of choroidal endothelial cells was quantified in the presence of inhibitors to different EDP binding proteins. C57BL6 mice were injected with EDPs and studied by electroretinography, transmission electron microscopy, and microarray analysis.

RESULTS

An alternatively spliced form of beta-galactosidase (GLB1) is present on human choroidal endothelial cells and acts as a receptor for EDPs. Elevated levels of circulating EDPs do not affect retinal function in the mouse, but do increase the expression and deposition of collagen IV in the RPE/choroid complex.

CONCLUSIONS

EDPs may play a role in neovascular AMD by binding to and inducing neovascular phenotypes in choroidal endothelial cells through their receptor, GLB1. These peptides also cause an increased mRNA expression and deposition of collagen IV in the RPE/choroid, which may alter diffusion properties between the retina and choriocapillaris.

Age-related macular degeneration: genetic and clinical findings

Age-related macular degeneration (AMD) is a sight threatening eye disease that affects millions of humans over the age of 65 years. It is considered to be the major cause of irreversible blindness in the elderly population in the developed world. The disease is prevalent in Europe and the United States, which has a large number of individuals of European descent. AMD is characterized by a progressive loss of central vision attributable to degenerative and neovascular changes that occur in the interface between the neural retina and the underlying choroid. This location contains the retinal photoreceptors, the retinal pigmented epithelium, a basement membrane complex known as Bruch's membrane and a network of choroidal capillaries. AMD is increasingly recognized as a complex genetic disorder where one or more genes contribute to an individual's susceptibility to development of the condition, while the prevailing view is that the disease stems from the interaction of multiple genetic and environmental factors. Although it has been proposed that a threshold event occurs during normal aging, the sequelae of biochemical, cellular, and molecular events leading to AMD are not fully understood. Here, we review the clinical aspects of AMD and summarize the genes which have been reported to have a positive association with the disease.

Multiplex analysis of age-related protein and lipid modifications in human Bruch's membrane

Aging of the human retina is characterized by progressive pathology, which can lead to vision loss. This progression is believed to involve reactive metabolic intermediates reacting with constituents of Bruch's membrane, significantly altering its physiochemical nature and function. We aimed to replace a myriad of techniques following these changes with one, Raman spectroscopy. We used multiplexed Raman spectroscopy to analyze the age-related changes in 7 proteins, 3 lipids, and 8 advanced glycation/lipoxidation endproducts (AGEs/ALEs) in 63 postmortem human donors. We provided an important database for Raman spectra from a broad range of AGEs and ALEs, each with a characteristic fingerprint. Many of these adducts were shown for the first time in human Bruch's membrane and are significantly associated with aging. The study also introduced the previously unreported up-regulation of heme during aging of Bruch's membrane, which is associated with AGE/ALE formation. Selection of donors ranged from ages 32 to 92 yr. We demonstrated that Raman spectroscopy can identify and quantify age-related changes in a single nondestructive measurement, with potential to measure age-related changes in vivo. We present the first directly recorded evidence of the key role of heme in AGE/ALE formation.

Hypoxia/reoxygenation induces CTGF and PAI-1 in cultured human retinal pigment epithelium cells

Early age-related macular degeneration (AMD) is characterized by thickening of Bruch's membrane due to the accumulation of extracellular matrix (ECM). This finding could be related to hypoxia of the retinal pigment epithelium (RPE). In the present study, we investigated the effects of hypoxia and reoxygenation on the expression of connective tissue growth factor (CTGF), plasminogen activator inhibitor-1 (PAI-1), collagen type IV (Col IV) and fibronectin (Fn) in cultured human RPE cells. Cultured human RPE cells were kept for 12-36h under hypoxic conditions (1% O(2)). Reoxygenation was conducted for 24h. Hypoxia-mediated CTGF and PAI-1 expression were analyzed by using immunohistochemistry, Northern and Western blot analysis. Actinomycin D was added to examine whether hypoxia induces the transcription of CTGF and PAI-1 mRNA. Furthermore, cells were transfected with siRNA against hypoxia-inducible factor-1alpha (HIF-1alpha) and kept under hypoxic conditions. The effects of antioxidants on hypoxia/reoxygenation-mediated CTGF and PAI-1 expression were tested by real-time PCR analysis. Production of Col IV and Fn were investigated by real-time PCR and Western blot analysis. Both hypoxia and hypoxia/reoxygenation increased the expression of CTGF, PAI-1, Col IV and Fn. Actinomycin D prevented the new transcription of CTGF and PAI-1 mRNA by hypoxia. Using siRNA against HIF-1alpha, the hypoxia-mediated increase of CTGF and PAI-1 was inhibited. Antioxidants attenuated the reoxygenation-mediated increase of CTGF and PAI-1. The process of hypoxia/reoxygenation in the RPE may lead to an increase of ECM in the RPE and thus may contribute to the accumulation of ECM in Bruch's membrane.

Retinal pigment epithelial detachment

Detachment of the retinal pigment epithelium is a prominent feature of many chorio-retinal disease processes, the most prevalent of which is age-related macular degeneration (AMD). Detachment of the retinal pigment epithelium may or may not be associated with choroidal neovascularization and may be caused by different types of pathogenesis, each associated with distinct angiographic features, natural course, visual prognosis, and response to treatment. The phrase "detachment of the retinal pigment epithelium" is used quite often, not always in the correct association and with no clear differentiation between its various types. It is important to identify the specific nature of detachment of the retinal pigment epithelium, and to establish an accurate diagnosis and treatment plan. Therefore, we present a review of the existing types of detachment of the retinal pigment epithelium with what we propose as being appropriate nomenclature and classification, and potential treatment recommendations.

Transplantation of the RPE in AMD

The retinal pigment epithelium (RPE) maintains retinal function as the metabolic gatekeeper between photoreceptors (PRs) and the choriocapillaries. The RPE and Bruch's membrane (BM) suffer cumulative damage over lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals. Unlike palliative pharmacologic treatments, replacement of the RPE has a curative potential for AMD. This article reviews mechanisms leading to RPE dysfunction in aging and AMD, laboratory studies on RPE transplantation, and surgical techniques used in AMD patients. Future strategies using ex vivo steps prior to transplantation, BM prosthetics, and stem cell applications are discussed. The functional peculiarity of the macular region, epigenetic phenomena leading to an age-related shift in protein expression, along with the accumulation of lipofuscin may affect the metabolism in the central RPE. Thickening of BM with age decreases its hydraulic conductivity. Drusen are deposits of extracellular material and formed in part by activation of the alternative complement pathway in individuals carrying a mutant allele of complement factor H. AMD likely represents an umbrella term for a disease entity with multifactorial etiology and manifestations. Presently, a slow progressing (dry) non-neovascular atrophic form and a rapidly blinding neovascular (wet) form are discerned. No therapy is currently available for the former, while RPE transplantation and promising (albeit non-causal) anti-angiogenic therapies are available for the latter. The potential of RPE transplantation was demonstrated in animal models. Rejection of allogeneic homologous transplants in patients focused further studies on autologous sources. In vitro studies elucidated cell adhesion and wound healing mechanisms on aged human BM. Currently, autologous RPE, harvested from the midperiphery, is being transplanted as a cell suspension or a patch of RPE and choroid in AMD patients. These techniques have been evaluated from several groups. Autologous RPE transplants may have the disadvantage of carrying the same genetic information that may have led to AMD manifestation. An intermittent culturing step would allow for in vitro therapy of the RPE, its rejuvenation and prosthesis of BM to improve the success RPE transplants. Recent advances in stem cell biology when combined with lessons learned from studies of RPE transplantation are intriguing future therapeutic modalities for AMD patients.

Characterization of collagen fibers in Bruch’s membrane using chemical force microscopy

Bruch's membrane is a layer composed of collagen fibers located just beneath the retina. This study validates a strategy used to map the morphological and adhesion characteristics of collagen fibers in Bruch's membrane. Atomic force microscopy tips were functionalized with different chemical groups and used to map the hydrophilic and hydrophobic regions on the surface of the eye tissue. The largest adhesion forces were observed when tips functionalized with NH(2) groups were used. The trend in the adhesion forces was rationalized based on the distribution of different functional groups in the triple-helical structure of the collagen fibers. The results of this study can be used to design more effective strategies to treat eye diseases such as age-related macular degeneration.

Study of the Morphological and Adhesion Properties of Collagen Fibers in the Bruch's Membrane

The Bruch's membrane is located beneath the retina in vertebrate eyes. We have used atomic force microscopy to examine the morphological and adhesion properties of collagen fibers located in different portions of the membrane. The D-periodicity of the fibers was 62.54 +/- 4.25 nm and 63.78 +/- 4.14 nm for regions away from the optic nerve and close to it, respectively. The adhesion properties of the collagen fibers were evaluated using force volume imaging on a number of different eye samples. The adhesion force we recorded in regions away from the optic nerve was different compared to regions close to the optic nerve. The reported results allow us to understand the nanoscopic properties of connective tissues in the eye and are important for the design of new and improved biomaterials.

Distribution of the collagen IV isoforms in human Bruch's membrane

AIMS

To determine the distribution of the alpha1 to alpha6 chains of type IV collagen in Bruch's membrane of the human posterior pole.

METHODS

Cryosections (10 micro m) from 18 human eyes (20 months to 83 years old) were acid treated, blocked with 10% normal goat serum, incubated for 1 hour with monoclonal antibodies against type IV collagen isoform specific peptides at 1:75 dilution, and visualised with an ABC staining kit.

RESULTS

In Bruch's membrane, the alpha1(IV) and alpha2(IV) chains were identified in retinal pigment epithelial (10/18 = 55%) and choriocapillaris basement membranes (18/18 = 100%); the alpha3(IV), alpha4(IV), and alpha5(IV) chains were also found in the retinal pigment epithelial basement membrane (13/18 = 72%). In the choroid, the alpha1(IV) and alpha2(IV) chains were detected in the blood vessels (18/18=100%). The alpha6(IV) chain was not identified in any sections.

CONCLUSION

The heterogeneous distribution of alpha1-2(IV) and alpha3-5(IV) in Bruch's membrane could give insights into the function of this structure in health, ageing, and diseases such as age related macular degeneration.

Structural variations of collagen in normal and pathological tissues: role of electron microscopy

The spectrum of ultrastructural appearances assumed by collagen in normal and pathological tissues is illustrated using techniques of thin section transmission electron microscopy and computer-assisted analysis. The normal fibrillar collagen types are described in order to provide a basis for comparing other normal and abnormal forms. In normal tissues, the anchoring fibril and basal lamina (basement membrane) represent tissue structures largely containing collagen but differing significantly in organisation from normal types I to III fibrillar collagen. In pathological tissue, deviations from normal fine structure are reflected in abnormal aggregates of collagen fibrils (amianthoid and skeinoid fibres) and abnormalities in fibril diameter and cross-sectional profile. Fibrous and segment long-spacing collagen represent two further organisational variants of collagen, the former found widely in pathological tissues, the latter very rarely. Much remains to be discovered about these abnormal collagen variants-their mode of formation, the cells that produce them, and their roles. They also present a challenge for the collagen biologist formulating hypotheses of collagen fibril assembly and molecular organisation.

In the mammalian eye type VI collagen tetramers form three morphologically different aggregates

The organization of the aggregates occurring in the stroma: (1) of the murine and human cornea after incubation in an ATP acidic solution; (2) of surgically excised epiretinal membranes (ERM); and (3) of the trabecular meshwork of monkey eyes was investigated morphologically and immunocytochemically on thin section electron microscopy. Morphology. The aggregates in the cornea appeared as cross-banded fibrils. The bands were uniformly electron dense (single banded form); they were separated from each other by interbands consisting of a bundle of filaments emerging in cross section as small areas of randomly assembled dot-like structures. In the ERM, most of the aggregates stood out as heteromorphic cross-banded bodies showing dense bands with electron denser borders (double banded form) and interbands composed of longitudinally oriented, parallel sheets or laminae of amorphous material enclosing thin, similarly oriented filaments. These extended, thinner and double in number (since interlacing with similar components of the opposite sheet), into the pale central zone of the dense band. The aggregates of the trabecular meshwork were heteromorphic, had uniformly dense bands (single banded form as in the cornea), but their interbands displayed longitudinal sheets (as the ERM aggregates). Immunocytochemistry revealed type VI collagen in the three eye aggregates with gold particles preferentially localized at the interbands. The specificity of the antibodies used was tested by Western blot analysis of type VI collagen samples extracted from human placenta and on homogenates of human cornea. In conclusion, the results indicate that the tetramers of type VI collagen may aggregate differently into structures with distinct supramolecular arrangements. These are illustrated in schematic drawings.

Structure of abnormal molecular assemblies (collagen VI) associated with human full thickness macular holes

Transversely banded deposits with an approximately 100-nm periodicity have been seen in association with a number of eye pathologies (e.g., age-related macular degeneration). Recently such aggregates have also been discovered in the cortical vitreous of a patient suffering from full thickness macular holes. The aggregates in the vitreous were of sufficient size and regularity for us to attempt 3D ultrastructural studies in the electron microscope. The molecules forming this aggregate pack in a centered tetragonal unit cell of dimensions approximately 26 x 26 x 180 nm. A real-space (r-weighted back projection) 3D reconstruction was computed. The aggregate is discussed in terms of its possible protein constituents. Collagen VI has been singled out as the most likely protein to form the aggregate. Two alternative models for the molecular packing are proposed, comprising aggregates of molecular tetramers or octamers. Understanding the structure of these abnormal banded deposits in the eye should help to throw light on the pathophysiological mechanisms of the diseases, including age-related macular degeneration, in which they occur.

Changes in Bruch's membrane and related structures with age

Age-related macular disease is a major and growing public health burden in developed Caucasian societies, accounting for about 50% of blind registration. Evidence exists that this is an emerging problem in Eastern Asia, although the phenotype appears to differ from that seen in Western society. It is likely that several genes are involved, and that the genes or allelic variants conferring are common. Environment plays a major role in its pathogenesis, and it is believed that genetic susceptibility becomes apparent only if there are sufficient environmental pressures. There is no therapy currently available that will have an impact on the prevalence of blindness from age-related macular disease. It has been shown that visual loss occurs as a reaction to ageing changes in Bruch's membrane, which is interposed between the choriocapillaris and the retinal pigment epithelium. The age changes in all three structures have been partly characterised, and as a consequence, multiple putative pathogenic mechanisms have been proposed. Cross-sectional studies of populations with different genetic background and life styles would serve to prove the importance of inheritance and environment. Molecular genetic analysis of blood from affected sibling pairs from these sources may indicate the relevant genes, the prevalence of which may differ in different communities. Enquiries as to life styles may determine important environmental influences. Examination of donor eyes from these communities may reveal distinctive features that may reflect the variation in genetic predisposition and environmental pressures. It is hoped that the findings from such studies will lead to novel and potentially successful management strategies.

The microcirculation of choroidal and ciliary body melanomas

The microcirculation of ciliary body and choroidal melanomas is remodelled into patterns. The presence of microvascular networks, composed of back-to-back loops that encircle microdomains of tumour, and parallel vessels with cross-linking, are associated with death from metastatic melanoma. The formation of these complex vascular patterns may result from reciprocal interactions between the tumour cell and the extracellular matrix, and pattern formation may reflect an invasive tumour cell phenotype. Ciliary body and choroidal melanomas are among the few forms of cancer treated before a pathologist assigns a grade to indicate whether tumour is likely to follow a benign or aggressive course. There is evidence to suggest that prognostically significant microcirculatory patterns may be detectable by non-invasive imaging techniques that may provide a substitute for biopsy to guide the clinical management of patients with these sight- and life-threatening tumours.

Correlation of ultrasound parameter imaging with microcirculatory patterns in uveal melanomas

Previous studies demonstrated a correlation between acoustic backscatter parameters and survival in ocular melanoma. The histologic presence of microvascular networks in ocular melanoma is also associated with death from metastases. This study tests the hypothesis that melanomas grouped on the basis of these microvascular patterns are separable by ultrasound spectrum analysis. We scanned 40 melanomas using a 10-MHz ultrasound unit equipped for digitization of radio frequency data. After enucleation, tumors were sectioned in planes corresponding to the ultrasonographic examination and stained to demonstrate microcirculation. Acoustic spectral parameters were compared between 14 melanomas with a nevuslike microcirculation and 26 with foci of high-risk microvascular structures. Smaller scatterer size, lower acoustic concentration and greater spatial variability were found to correlate with high-risk microvascular patterns and areas of cystic degeneration. We suggest that nonvascular extracellular matrix components associated with microvessels may be responsible for the correlation of acoustic parameters with microvascular pattern and distribution.