Three-dimensional structure of type IV collagen in the mammalian lens capsule

Tissue, cellular, and molecular level determinants for eye lens stiffness and elasticity

The eye lens is a transparent, ellipsoid tissue in the anterior chamber that is required for the fine focusing of light onto the retina to transmit a clear image. The focusing function of the lens is tied to tissue transparency, refractive index, and biomechanical properties. The stiffness and elasticity or resilience of the human lens allows for shape changes during accommodation to focus light from objects near and far. It has long been hypothesized that changes in lens biomechanical properties with age lead to the loss of accommodative ability and the need for reading glasses with age. However, the cellular and molecular mechanisms that influence lens biomechanical properties and/or change with age remain unclear. Studies of lens stiffness and resilience in mouse models with genetic defects or at advanced age inform us of the cytoskeletal, structural, and morphometric parameters that are important for biomechanical stability. In this review, we will explore whether: 1) tissue level changes, including the capsule, lens volume, and nucleus volume, 2) cellular level alterations, including cell packing, suture organization, and complex membrane interdigitations, and 3) molecular scale modifications, including the F-actin and intermediate filament networks, protein modifications, lipids in the cell membrane, and hydrostatic pressure, influence overall lens biomechanical properties.

Simulation of defects, flexibility and rupture in biopolymer networks

Networks of biopolymers occur often in nature, and are vulnerable to damage over time. In this work, a coarse grained model of collagen IV molecules is applied in a 2D hexagonal network and the mechanisms by which these networks can rupture are explored. The networks are stretched linearly in order to study their structural limits and mechanism of rupture over timescale of up to 100 μs. Metrics are developed to track the damage networks suffer over time, and qualitatively analyse ruptures that occur. Further simulations repeatedly stretch the networks sinusoidally to mimic the in vivo strains. Defects of increasing levels of complexity are introduced into an ordered network, and their effect on the rupturing behaviour of the biopolymer networks studied. The effect of introducing holes of varying size in the network, as well as strips of finite width to mimic surgical damage are studied. These demonstrate the importance of the flexibility of the networks to preventing damage.

Self assembly of model polymers into biological random networks

The properties of biological networks, such as those found in the ocular lens capsule, are difficult to study without simplified models. Model polymers are developed, inspired by "worm-like" curve models, that are shown to spontaneously self assemble to form networks similar to those observed experimentally in biological systems. These highly simplified coarse-grained models allow the self assembly process to be studied on near-realistic time-scales. Metrics are developed (using a polygon-based framework) which are useful for describing simulated networks and can also be applied to images of real networks. These metrics are used to show the range of control that the computational polymer model has over the networks, including the polygon structure and short range order. The structure of the simulated networks are compared to previous simulation work and microscope images of real networks. The network structure is shown to be a function of the interaction strengths, cooling rates and external pressure. In addition, "pre-tangled" network structures are introduced and shown to significantly influence the subsequent network structure. The network structures obtained fit into a region of the network landscape effectively inaccessible to random (entropically-driven) networks but which are occupied by experimentally-derived configurations.

Simplified computational model for generating biological networks

A method to generate and simulate biological networks is discussed. An expanded Wooten–Winer–Weaire bond switching methods is proposed which allows for a distribution of node degrees in the network while conserving the mean average node degree. The networks are characterised in terms of their polygon structure and assortativities (a measure of local ordering). A wide range of experimental images are analysed and the underlying networks quantified in an analogous manner. Limitations in obtaining the network structure are discussed. A “network landscape” of the experimentally observed and simulated networks is constructed from the underlying metrics. The enhanced bond switching algorithm is able to generate networks spanning the full range of experimental observations.

We discuss a Monte Carlo method to simulate biological networks and compare to the underlying networks in experimental images.

Nanoscale Topography and Poroelastic Properties of Model Tissue Breast Gland Basement Membranes

Basement membranes (BMs) are thin layers of condensed extracellular matrix proteins serving as permeability filters, cellular anchoring sites, and barriers against cancer cell invasion. It is believed that their biomechanical properties play a crucial role in determining cellular behavior and response, especially in mechanically active tissues like breast glands. Despite this, so far, relatively little attention has been dedicated to their analysis because of the difficulty of isolating and handling such thin layers of material. Here, we isolated BMs derived from MCF10A spheroids-three-dimensional breast gland model systems mimicking in vitro the most relevant phenotypic characteristics of human breast lobules-and characterized them by atomic force microscopy, enhanced resolution confocal microscopy, and scanning electron microscopy. By performing atomic force microscopy height-clamp experiments, we obtained force-relaxation curves that offered the first biomechanical data on isolated breast gland BMs to our knowledge. Based on enhanced resolution confocal microscopy and scanning electron microscopy imaging data, we modeled the system as a polymer network immersed in liquid and described it as a poroelastic material. Finite-element simulations matching the experimental force-relaxation curves allowed for the first quantification, to our knowledge, of the bulk and shear moduli of the membrane as well as its water permeability. These results represent a first step toward a deeper understanding of the mechanism of tensional homeostasis regulating mammary gland activity as well as its disruption during processes of membrane breaching and metastatic invasion.

Micromorphology analysis of the anterior human lens capsule

PURPOSE

This study aimed to quantify the three-dimensional micromorphology of the surface of the human lens capsule as a function of age.

METHODS

Imaging experiments were conducted on whole human lenses received from eight human cadavers (donor age range: 30-88 years). Imaging was performed with an atomic force microscope (AFM) in contact mode in fluid. The porosity and surface roughness were quantified from the height images obtained. A novel approach, based on stereometric and fractal analysis of three-dimensional surfaces developed for use in conjunction with AFM data, was also used to analyze the surface microtexture as a function of age.

RESULTS

The AFM images obtained depict a highly ordered fibrous structure at the surface of the lens capsule, although the overall structure visually changes with age. Porosity and roughness were quantified for each image and analyzed as a function of donor age. The interfibrillar spacing revealed an increasing trend with age, although this result was not significant (p = 0.110). The root mean square (RMS) deviation and average deviation significantly decreased with increasing age (p<0.001 for both). The fractal analysis provided quantitative values for 29 amplitude, hybrid, functional, and spatial parameters. All the hybrid parameters decreased with age, although not significantly. Of the functional parameters, the surface bearing index increased significantly with age (p = 0.017) and the summit height exhibited a decreasing trend with age (p = 0.298). Of the spatial parameters, the dominant radial wavelength trend moved toward an increase with age (p = 0.103) and the cross-hatch angle tended toward a decrease with age (p = 0.213).

CONCLUSIONS

Significant changes in the three-dimensional surface microtexture of the human lens capsule were found with age, although more experiments on a larger dataset are needed to conclude this with certainty. The analyzed AFM images demonstrate a fractal nature of the surface, which is not considered in classical surface statistical parameters. The surface fractal dimension may be useful in ophthalmology for quantifying human lens architectural changes associated with different disease states to further our understanding of disease evolution.

Finite element implementation of a multiscale model of the human lens capsule

An axisymmetric finite element implementation of a previously described structural constitutive model for the human lens capsule (Burd in Biomech Model Mechanobiol 8(3):217-231, 2009) is presented. This constitutive model is based on a hyperelastic approach in which the network of collagen IV within the capsule is represented by an irregular hexagonal planar network of hyperelastic bars, embedded in a hyperelastic matrix. The paper gives a detailed specification of the model and the periodic boundary conditions adopted for the network component. Momentum balance equations for the network are derived in variational form. These balance equations are used to develop a nonlinear solution scheme to enable the equilibrium configuration of the network to be computed. The constitutive model is implemented within a macroscopic finite element framework to give a multiscale model of the lens capsule. The possibility of capsule wrinkling is included in the formulation. To achieve this implementation, values of the first and second derivatives of the strain energy density with respect to the in-plane stretch ratios need to be computed at the local, constitutive model, level. Procedures to determine these strain energy derivatives at equilibrium configurations of the network are described. The multiscale model is calibrated against previously published experimental data on isolated inflation and uniaxial stretching of ex vivo human capsule samples. Two independent example lens capsule inflation analyses are presented.

Fractal analysis of AFM images of the surface of Bowman's membrane of the human cornea

The objective of this study is to further investigate the ultrastructural details of the surface of Bowman's membrane of the human cornea, using atomic force microscopy (AFM) images. One representative image acquired of Bowman's membrane of a human cornea was investigated. The three-dimensional (3-D) surface of the sample was imaged using AFM in contact mode, while the sample was completely submerged in optisol solution. Height and deflection images were acquired at multiple scan lengths using the MFP-3D AFM system software (Asylum Research, Santa Barbara, CA), based in IGOR Pro (WaveMetrics, Lake Oswego, OR). A novel approach, based on computational algorithms for fractal analysis of surfaces applied for AFM data, was utilized to analyze the surface structure. The surfaces revealed a fractal structure at the nanometer scale. The fractal dimension, D, provided quantitative values that characterize the scale properties of surface geometry. Detailed characterization of the surface topography was obtained using statistical parameters, in accordance with ISO 25178-2: 2012. Results obtained by fractal analysis confirm the relationship between the value of the fractal dimension and the statistical surface roughness parameters. The surface structure of Bowman's membrane of the human cornea is complex. The analyzed AFM images confirm a fractal nature of the surface, which is not taken into account by classical surface statistical parameters. Surface fractal dimension could be useful in ophthalmology to quantify corneal architectural changes associated with different disease states to further our understanding of disease evolution.

Primate lens capsule elasticity assessed using Atomic Force Microscopy

The purpose of this project is to measure the elasticity of the human and non-human primate lens capsule at the microscopic scale using Atomic Force Microscopy (AFM). Elasticity measurements were performed using AFM on the excised anterior lens capsule from 9 cynomolgus monkey (5.9-8.0 years), 8 hamadryas baboon (2.8-10.1 years), and 18 human lenses (33-79 years). Anterior capsule specimens were obtained by performing a 5 mm continuous curvilinear capsulorhexis and collecting the resulting disk of capsular tissue. To remove the lens epithelial cells the specimen was soaked in 0.1% trypsin and 0.02% EDTA for 5 min, washed, and placed on a Petri dish and immersed in DMEM. Elasticity measurements of the capsule were performed with a laboratory-built AFM system custom designed for force measurements of ophthalmic tissues. The capsular specimens were probed with an AFM cantilever tip to produce force-indentation curves for each specimen. Young's modulus was calculated from the force-indentation curves using the model of Sneddon for a conical indenter. Young's modulus of elasticity was 20.1-131 kPa for the human lens capsule, 9.19-117 kPa for the cynomolgus lens capsule, and 13.1-62.4 kPa for the baboon lens capsule. Young's modulus increased significantly with age in humans (p = 0.03). The age range of the monkey and baboon samples was not sufficient to justify an analysis of age dependence. The capsule elasticity of young humans (<45 years) was not statistically different from that of the monkey and baboon. In humans, there is an increase in lens capsule stiffness at the microscale that could be responsible for an increase in lens capsule bulk stiffness.

Significance of the lacunar hydration pattern after corneal cross linking

PURPOSE

The aim of the present study was to investigate the characteristic honeycomb hydration pattern after corneal cross linking using in vivo rabbit cornea.

METHODS

After removal of the central epithelium, the right corneas of 4 New Zealand white rabbits were cross-linked applying a photosensitizing 0.1% riboflavin-dextran solution and UV-A light of 370 nm wavelength with a surface irradiance of 3 mW/cm for 30 minutes. Two animals were euthanized 3 days postoperatively and another 2 were euthanized 6 weeks postoperatively. The corneas of the enucleated eyes were evaluated using 4-mum light microscopic sections with tangential en face and cross-sectional orientation.

RESULTS

By day 3 after treatment, complete apoptotic damage and loss of the stromal keratocytes and endothelial cells were found in the central irradiated area through the entire thickness of the stroma. There was marked lacunar edema in the former positions of the apoptotic keratocytes in the anterior 250 microm of the stroma and diffuse edema in the adjacent posterior and lateral zones. Lacunar edema was identified best on tangential sections. By week 6, the cytoarchitecture of the cornea appeared normal again, and complete resolution of both lacunar and diffuse corneal edema had occurred.

CONCLUSIONS

After riboflavin/UV-A cross linking of in vivo rabbit cornea, a characteristic lacunar hydration pattern can be observed in the anterior stroma with maximum cross linking, whereas diffuse edema is present in the adjacent areas without significant cross linking. The lacunar edema may explain the temporary demarcation of the anterior stroma after cross linking on biomicroscopy because of increased light scattering. The network pattern of cross linking may contribute to the elasticity of the cornea after cross linking.

Computational model of evolving lens capsule biomechanics following cataract-like surgery

Cataract surgery is an invasive procedure whereby lens fibers are removed through a permanent central hole, or capsulorhexis, in the surrounding lens capsule and replaced with an artificial intraocular lens (IOL). Remnant lens epithelial cells subsequently transdifferentiate to a more contractile and synthetic wound-healing phenotype, which causes significant structural and mechanical adaptations of the residual lens capsule. The goal of this study is to present a computational model capable of capturing salient features of the biomechanical evolution of the lens capsule following cataract-like surgery. The model is shown to predict marked long-term increases in thickness and stiffness of the lens capsule nearest the edge of the capsulorhexis comparable to reported measurements. Such models represent a first step toward understanding better the long-term interactions between the residual lens capsule and implanted IOL, thus initiating a new paradigm for the design of improved IOLs, including those having an accommodative feature.

Bilateral spontaneous rupture of the anterior capsule

We report an unusual case of bilateral anterior capsule rupture in a healthy 8-year-old child.

A structural constitutive model for the human lens capsule

Published data on the mechanical performance of the human lens capsule when tested under uniaxial and biaxial conditions are reviewed. It is concluded that two simple phenomenological constitutive models (namely a linear elastic model and a Fung-type hyperelastic model) are unable to provide satisfactory representations of the mechanical behaviour of the capsule for both of these loading conditions. The possibility of resolving these difficulties using a structural constitutive model for the capsule, of a form that is inspired by the network of collagen IV filaments that exist within the lens capsule, is explored. The model is implemented within a rectangular periodic cell. Prescribed stretches are imposed on the periodic cell and the network is allowed to deform in a non-affine manner. The performance of the constitutive model correlates well with previously published test data. One possible application of the model is in the development of a multi-scale analysis of the mechanics of the human lens capsule.

Pediatric anterior capsulotomy preferences of cataract surgeons worldwide: comparison of 1993, 2001, and 2003 surveys

We compared the pediatric anterior capsulotomy preferences of members of the American Society of Cataract and Refractive Surgery (ASCRS) and the American Association of Pediatric Ophthalmology and Strabismus (AAPOS) reported in 3 surveys (1993, 2001, and 2003). In 1993 and 2001, more than 50% of ASCRS respondents preferred manual anterior capsulotomy techniques; in 2001 and 2003, AAPOS respondents preferred manual and vitrector techniques. The ASCRS preferences remained unchanged when subdivided into domestic and international, as did AAPOS domestic preferences; however, more than 50% of AAPOS international preferences changed from manual alone in 2001 to a manual-vitrector combination in 2003. In 2003, more than 50% of AAPOS respondents worldwide preferred this combination: the vitrector for the very young patient and the manual anterior capsulotomy for the older child.

Altered multiaxial mechanical properties of the porcine anterior lens capsule cultured in high glucose

Hyperglycemia can alter the mechanical properties of tissues through the formation of advanced glycation endproducts in matrix proteins that have long half-lives. We used a custom experimental system and subdomain finite element method to quantify alterations in the regional multiaxial mechanical properties of porcine lens capsules that were cultured for 8 or 14 weeks in high glucose versus control media. Findings revealed that high glucose significantly stiffened the capsules in both the circumferential and the meridional directions, but it did not affect the known regional variations in anisotropy. Such information could be important in the design of both improved clinical procedures and intraocular implants for diabetic patients.

Extensibility and scanning electron microscopy evaluation of 5 pediatric anterior capsulotomy techniques in a porcine model

PURPOSE

To compare the extensibility and scanning electron microscopy (SEM) of 5 currently used pediatric anterior capsulotomy techniques: vitrectorhexis, manual continuous curvilinear capsulorhexis (CCC), can-opener, radio frequency diathermy, and plasma blade in a porcine model.

SETTING

Storm Eye Institute, Medical University of South Carolina, Charleston, South Carolina, USA.

METHODS

Extensibility was determined by stretching each capsulotomy until it ruptured and measuring it by calculating the mean stretch-to-rupture circumference of each capsulotomy (20 eyes per technique) as a percentage of its baseline circumference. Edge characteristics were evaluated using SEM.

RESULTS

The mean extensibility of each technique tested (vitrectorhexis 161%, CCC 185%, can opener 149%, radio frequency 145%, plasma blade 170%) was significantly different (P<.001, 1-way analysis of variance). The SEM examination found that the vitrectorhexis had a scalloped edge with the whole edge rolled over, presenting a smooth surface toward the inside of the capsulotomy; the manual CCC produced the smoothest edge, with no irregularities noted; the can-opener edge was irregular, showing each puncture of the needle had created a small arc, with occasional regions of the edge rolled over in a "hit-and-miss" fashion; the radio-frequency diathermy capsulotomy edge was ragged, rough, and irregular; and the plasma blade capsulotomy edge was rougher than the manual CCC, but there were fewer irregularities than the radio-frequency diathermy edge had.

CONCLUSIONS

The manual CCC technique produced the most extensible porcine capsulotomy, followed by the plasma blade, vitrectorhexis, can-opener, and radio-frequency techniques, in a porcine model. The manual CCC technique also produced the smoothest anterior capsulotomy edge according SEM evaluation.

Redistribution of strain and curvature in the porcine anterior lens capsule following a continuous circular capsulorhexis

Cataract surgery is the most commonly performed surgical procedure in the US, it consists of three steps: introduction of a hole into the lens capsule, removal of the clouded lens through this access hole, and insertion of an artificial lens. We hypothesize that errant behavior by the residual epithelial cells of the lens capsule following surgery are due, in part, to surgically-induced changes of the native stress and strain fields in the lens capsule. Because the capsular bag can be regarded mechanically as a membrane, here we study changes in curvature and strains due to the most common means of introducing the initial access hole: a continuous circular capsulorhexis (CCC). We show that a modest sized CCC increases circumferential strains and decreases meridional strains by up to approximately 20% and that curvatures change by up to approximately 13%, particularly near the edge of the CCC. We submit that such changes can induce mechanobiological responses that are responsible, in part, for some of the long-term complications following cataract surgery.

Anterior lens capsule abnormalities in Alport syndrome

Alport syndrome is a hereditary, progressive disease characterized by progressive nephritis, sensorineural deafness, and ocular abnormalities, including anterior lenticonus. The ultrastructure of the lens capsule abnormalities in Alport syndrome is reported. Four anterior lens capsules from 31-year-old patient and 26-year-old patient with lenticonus who were affected by the Alport syndrome were obtained at capsulectomy. And all four anterior lens capsules were examined by transmission electron microscopy. The histopathologic findings showed that the thickness of the anterior lens capsules was decreased (4-13 microm) and that there were many vascular dehiscences localized at the inner part of the lens capsule. There were large numbers of capsular dehiscences containing fibrillar materials and vacuoles. The anterior capsules were clearly fragile in this disease, forming the basis for the progressive lenticonus and anterior polar cataract.

Tensile mechanical and creep properties of Descemet's membrane and lens capsule

Descemet's membrane (DM) and the lens capsule (LC) are two ocular basement membrane structures which in comparison with other basement membranes have exceptional thicknesses which increase with age. Both membranes are supposed to contain networks of type IV collagen and laminin linked together with nidogen/entactin and containing other glycoproteins and proteoglycans. DM is a unique basement membrane which in addition contains fine filaments of type VIII collagen arranged in a hexagonal lattice. The mechanical functions of the LC are in lens suspension and accommodation, and its mechanical properties, previously investigated, are of great interest from a surgical point of view. DM serves as an endothelial basement membrane. Otherwise, its physiological function is unknown but may be one of mechanical support, filtration, or fluid barrier. Data on the mechanical properties of DM or the supramolecular assembly of type VIII collagen are very scarce or absent. The aim of this study was to determine and compare the mechanical properties of the two ocular membranes in order to elucidate the properties of DM in the light of those of LC. The human eyes were from testamentary donors and rat, cow, and sow eyes were obtained from normal animals. The tensile mechanical properties were determined by a volume-strain procedure and creep properties by subjecting the membranes from the latter three species to a constant axial stress. In rat, cow, and sow, DM was less strained to obtain a fixed moderate stress value (0.5 MPa) and showed to be 3.4- to 5.2-fold stiffer and to attain 2.7- to 4.6-fold higher stress at a strain value of 0.10 when compared with LC. The maximal strain, stiffness and stress were found to be less than those of the LC. In humans, DM and LC showed very similar mechanical properties. The instantaneous creep of DM was found to be less than that of LC indicating a higher stiffness of DM in the axial direction. In conclusion, depending on the species, DM showed to possess from one-fourth to similar mechanical strength as that of LC, and, in rat, cow, and sow, DM appeared to be stiffer in both lateral and axial directions at moderate strain values when compared with LC.

The contribution of vascular basement membranes and extracellular matrix to the mechanics of tumor angiogenesis

The goal of this review is to highlight the contribution of extracellular matrix and vascular basement membranes to the regulation of angiogenesis and tumor progression. Here we present a new concept that vascular basement membrane influences endothelial cells and possibly other cell types in a solid state assembled form, and also in a degraded solution state form. Depending on the structural integrity, composition and exposure of cryptic sites, the vascular basement membrane proteome exerts functional influences on proliferating and resting endothelial cells. This review provides the reader with an appreciation of this newly evolved concept in the area of vascular biology.

Mechanical properties of the human lens capsule

The human lens capsule has recently been the subject of much attention in an attempt to understand its physiological function in relation to the accommodative function, its functional reserve in the elderly population, and its potential in relation to cataract surgery. This overview presents our current knowledge of the mechanical properties of the human lens capsule, discussed on basis of its structure and its role in accommodation and cataract surgery.

Basement membranes: structure, assembly and role in tumour angiogenesis

In recent years, the basement membrane (BM)--a specialized form of extracellular matrix (ECM)--has been recognized as an important regulator of cell behaviour, rather than just a structural feature of tissues. The BM mediates tissue compartmentalization and sends signals to epithelial cells about the external microenvironment. The BM is also an important structural and functional component of blood vessels, constituting an extracellular microenvironment sensor for endothelial cells and pericytes. Vascular BM components have recently been found to be involved in the regulation of tumour angiogenesis, making them attractive candidate targets for potential cancer therapies.

Morphologic alterations in the anterior lens capsule of canine eyes with cataracts

OBJECTIVE

To examine the morphologic changes in the anterior lens capsule and lens epithelium of canine eyes with cataracts.

SAMPLE POPULATION

Anterior lens capsules from the eyes of 25 dogs with cataracts and from an additional 10 canine globes with lenses subjectively assessed to be normal.

PROCEDURE

Thickness of each anterior lens capsule was measured by use of a digital microscopic camera and imaging software. All 25 capsules from eyes with cataracts were submitted for light microscopy; 4 were also submitted for electron microscopy.

RESULTS

Thickness of the anterior lens capsule increased with age for the normal lenses and the lenses with cataracts; the change with age was similar for both groups. Light microscopy revealed fibrous metaplasia of lens epithelial cells in 7 of 25 anterior lens capsules with focal thickenings of the posterior aspect of the capsule. Electron microscopy revealed deposition of collagen and basement membrane-like material by fibroblast-like cells.

CONCLUSIONS

Results indicate that thickness of the anterior lens capsule in dogs increases with age and that this increase in thickness is not significantly different between normal lenses and lenses with cataracts. In addition, epithelial cells from lenses with cataracts may undergo metaplasia to form plaques composed of fibrous tissue and ectopic basement membrane produced by epithelial cells.

Collagen packing in the dogfish egg case wall

The collagen which forms the egg case of the dogfish, Scyliorhinus canicula, is assembled in a three-dimensional network that results in a very resistant capsule. The capsule presumably accomplishes both a protective and a filtering role for the embryo contained within it. In the present work we have obtained electron micrographs of metal-shadowed, deep-etched replicas of the egg case and we have analyzed the micrographs using computer-based Fourier methods. The replicas provide details of the three-dimensional structure that have not been recorded before, allowing us to add particulars to the [1,0,0], [1,1,0], and [0,0,1] views previously obtained from plastic sections (C. Knupp et al., 1996, J. Struct. Biol. 117, 209-221). A molecular packing arrangement consistent with all of the present data is proposed. Although this collagen type is unique and is substantially shorter (45 nm) than other collagens, investigations into its structure may give an insight into related collagen types. The role of this collagen as both a protective structure and one with filtering properties controlling permeability is discussed in terms of other collagens with similar functions.

Basement-membrane stromal relationships: interactions between collagen fibrils and the lamina densa

Collagens, the most abundant molecules in the extracellular space, predominantly form either fibrillar or sheet-like structures-the two major supramolecular conformations that maintain tissue integrity. In connective tissues, other than cartilage, collagen fibrils are mainly composed of collagens I, III, and V at different molecular ratios, exhibiting a D-periodic banding pattern, with diameters ranging from 30 to 150 nm, that can form a coarse network in the extracellular matrix in comparison with a fine meshwork of lamina densa. The lamina densa represents a stable sheet-like meshwork composed of collagen IV, laminin, nidogen, and perlecan compartmentalizing tissue from one another. We hypothesize that the interactions between collagen fibrils and the lamina densa are crucial for maintaining tissue-tissue interactions. A detailed analysis of these interactions forms the basis of this review article. Here, we demonstrate that there is a direct connection between collagen fibrils and the lamina densa and propose that collagen V may play a crucial role in this connection. Collagen V might also be involved in regulation of collagen fibril diameter and anchoring of epithelia to underlying connective tissues.

Secondary conformational structure of type IV collagen in different conditions determined by Fourier-transform infrared microscopic spectroscopy

The IR spectra of Type IV collagen in a crystalline state, in a solid film and in aqueous solution were investigated by FT-IR microscopic spectroscopy using either the transmission method or attenuated total reflection method. The effect of compression pressure on the secondary structure of Type IV collagen in different states was also studied. The results indicate that amide I, containing triple helix (1636 cm-1), alpha-helix (1655 cm-1) and random coil (1666 cm-1) peaks, for Type IV collagen in crystalline state had almost the same IR spectral position as that in solid film but the peak areas of amide I, II, III and the proteoglycan region was larger for Type IV collagen in solid film. The IR spectrum of amide I (alpha-helix: 1647 cm-1, triple helix: 1633 cm-1) for Type IV collagen in aqueous solution was different from that in the crystalline state or solid film. The random coil structure existed predominantly in the dry state of collagen. After compression pressure was applied to the crystal of type IV collagen, its IR spectrum transformed significantly. The peak area of amide I was clearly enlarged but the peak area of amide II decreased sharply. When the compression was applied to the solid film, its IR spectral position did not change but both peak areas of amide I and II decreased significantly although the peak area in amide III and the proteoglycan region were almost the same. We also found that two new additional peaks at 1054 and 1023 cm-1 appeared in the proteoglycan region for both of the above compressed samples. Pressure seemed to significantly influence the amide I, II and proteoglycan region, leading to the deformation of collagen.

High resolution platinum-carbon replication of freeze-dried basement membrane

High angle platinum/carbon (Pt/C) replication has proved to be a valuable tool in analyzing basement membrane structure in human amnion, bovine lens capsule, and the Engelbreth-Holm-Swarm (EHS) tumor. High resolution replicas for transmission electron microscopy (TEM) have been achieved by depositing 1.0 +/- 0.1 nm thick Pt/C films backed with rotary deposited 12.5 +/- 2.5 nm thick carbon films. The basement membrane collagen IV network was observed to consist of fine branching filaments containing globular domains intrinsic to the filaments. A second quasi-regular network is formed by laminin. Unidirectional 45 degree angle Pt/C replication was used for most of this work. The merits and deficiencies of unidirectional vertical replication (80 degree angle), unidirectional 45 degree angle, and 20 degrees low angle rotary replication are discussed. Vertical replication produces the highest resolution replicas and has the potential for revealing the overall pattern of basement membrane structural assembly if basement membrane preparations freeze-dried in low salt can faithfully maintain their in vivo structure.

A comparative study of the elastic properties of continuous tear curvilinear capsulorhexis versus capsulorhexis produced by radiofrequency endodiathermy

Capsulorhexis using radio-frequency endodiathermy may confer some advantages over continuous tear curvilinear capsulorhexis (CTCC) in certain clinical situations. It is unclear whether a capsulorhexis produced in this fashion has the clinically advantageous elasticity and resistance to tearing that a CTCC has been demonstrated to have. To investigate this, a test of capsular elasticity was carried out on pairs of eyes obtained from an eye bank, 42 eyes of 21 patients in total, using modified digital vernier calipers. One eye of each pair had a CTCC, the other a diathermy capsulorhexis (DC). The elasticity of the capsule in both groups was expressed by comparing the circumference of the capsulotomy at rest with its circumference at rupture. The mean capsular elasticity of the CTCC group was significantly greater than that of the DC group (p << 0.001). The capsular edge in both groups was examined using scanning electron microscopy, and the difference in morphology appears to be the source of the difference in elasticity.

Elongational flow studies on type IV collagen: comparison with type I

Elongation flow techniques have been used to investigate the birefringent response of monodisperse type IV collagen in dilute solution and the results compared with type I collagen. A four-roll mill apparatus was used to characterize the solutions at low strain rates, epsilon < or = 300 s-1. The birefringence is nonlocalized and rises gradually to a plateau value, in accordance with rigid-rod behavior. The gradients of the tangent to the curves at zero strain rate are estimated for types IV and I collagen. The concentrations of the solutions used were in the dilute to semidilute regimes. Using a value of 300 nm for the length of type I collagen, values of 364-408 nm were calculated for the length of the type IV collagen molecule, depending on the concentration regime chosen, which is consistent with biochemical predictions based on a rigid molecule. The results imply that the behavior of type IV collagen molecules in solution is similar to type I collagen, despite the presence of several sequence interruptions in the type IV helix.

Molecular bending and networks in a basement membrane-like collagen: packing in dogfish egg capsule collagen

Low-angle X-ray diffraction data have been obtained from three mutually perpendicular axes through sheets of the collagenous egg capsule of the dogfish Scyliorhinus caniculus, a collagen that resembles type IV collagen. The data are interpreted in the light of the body of knowledge of the structure derived from transmission electron microscopy by Knight and Hunt. A model to account for the X-ray data is proposed incorporating the main dimensions of the Knight and Hunt model which are confirmed by the diffraction data. Several features of the diffraction patterns are not explained by the existing model however, and a new model is proposed to account for these features. This consists of antiparallel packed pairs of two mutually parallel molecules, each kinked and rotated so as to produce a four-fold helix resembling a crankshaft. This has the advantage of conferring intermolecular linkage in three dimensions throughout the structure with tetragonal symmetry and unit dimensions a = b = 22.6 nm, c (fibre axis direction) = 39.3 nm. The result is a fairly rigid open polygonal network or sponge-like architecture that is capable of accommodating large quantities of water and other molecules.

Cryofixation of basement membranes followed by freeze substitution or freeze drying demonstrates that they are composed of a tridimensional network of irregular cords

Since conventional chemical fixation may extract tissue components and thus alter structural organization, cryofixation was used to reexamine the ultrastructure of three thick basement membranes: lens capsule, Reichert's membrane, and Engelbreth-Holm-Swarm (EHS) tumor matrix, and two thin basement membranes, those of epididymis and semi-niferous tubules. Cryofixation was achieved by slam freezing followed by either freeze substitution in dry acetone containing 1% osmium tetroxide and 0.05% uranyl acetate or freeze drying in a molecular distillation dryer. The results by both procedures demonstrate that thick basement membranes and the lamina densa of thin basement membranes are composed of a network of anastomosing strands referred to as cords. The cords vary in density and distinctiveness, but their thickness averages 3 to 5 nm in every tissue examined. The spaces separating the cords vary within wide limits, but their mean diameter is approximately 15 nm in every case. Two other common features are 1) the presence within the network of a few 1.5-3.0-nm-thick filaments and 2) 4.5-nm-wide sets of parallel lines referred to as double tracks. When these results are compared with those previously described after conventional fixation, no significant difference is observed in either the cord network or the associated filaments and "double tracks." However, in the thin basement membranes processed by cryofixation, the lamina densa is in direct contact with epithelial cells, whereas, after conventional fixation, the lamina densa is separated from the epithelial cells by a pale layer referred to as lamina lucida or lamina rara. Immunogold labeling of three basement membranes after cryofixation and freeze substitution in acetone containing 0.3% glutaraldehyde yields strong reactions for laminin, type IV collagen, and heparan sulfate proteoglycan. Comparison with previous results indicates that conventional formaldehyde fixation adequately preserves laminin and type IV collagen but causes the loss of some proteoglycan. It is concluded that, except for this loss and the absence of lamina lucida in cryofixed thin basement membranes, the morphological and antigenic features obtained after cryofixation are similar to those observed in the past after conventional fixation.