Ultrastructure of Reichert's membrane, a multilayered basement membrane in the parietal wall of the rat yolk sac

Developmental and mechanical roles of Reichert's membrane in mouse embryos

Embryonic development and growth in placental mammals proceeds in utero with the support of exchanges of gases, nutrients and waste products between maternal tissues and offspring. Murine embryos are surrounded by several extraembryonic membranes, parietal and visceral yolk sacs, and amnion in the uterus. Notably, the parietal yolk sac is the most outer membrane, consists of three layers, trophoblasts and parietal endoderm (PaE) cells, and is separated by a thick basal lamina termed Reichert's membrane (RM). RM is composed of extracellular matrix (ECM) initially formed as the basement membrane of the trophectoderm of pre-implanted embryos and followed by the heavy deposition of ECM mainly produced in PaE cells of post-implanted embryos. In addition to the physiological roles of RM, such as gas and nutrient exchange, it also plays a crucial role in cushioning and dispersing intrauterine pressures exerted on embryos for normal egg-cylinder morphogenesis. Mechanistically, such intrauterine pressures generated by uterine smooth muscle contractions appear to be involved in the elongation of the egg-cylinder shape, along with primary axis formation, as an important biomechanical element in utero. This review focuses on our current views of the roles of RM in properly buffering intrauterine mechanical forces for mouse egg-cylinder morphogenesis. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Adaptive F-Actin Polymerization and Localized ATP Production Drive Basement Membrane Invasion in the Absence of MMPs

Matrix metalloproteinases (MMPs) are associated with decreased patient prognosis but have failed as anti-invasive drug targets despite promoting cancer cell invasion. Through time-lapse imaging, optical highlighting, and combined genetic removal of the five MMPs expressed during anchor cell (AC) invasion in C. elegans, we find that MMPs hasten invasion by degrading basement membrane (BM). Though irregular and delayed, AC invasion persists in MMP- animals via adaptive enrichment of the Arp2/3 complex at the invasive cell membrane, which drives formation of an F-actin-rich protrusion that physically breaches and displaces BM. Using a large-scale RNAi synergistic screen and a genetically encoded ATP FRET sensor, we discover that mitochondria enrich within the protrusion and provide localized ATP that fuels F-actin network growth. Thus, without MMPs, an invasive cell can alter its BM-breaching tactics, suggesting that targeting adaptive mechanisms will be necessary to mitigate BM invasion in human pathologies.

Renal Tubular- and Vascular Basement Membranes and their Mimicry in Engineering Vascularized Kidney Tubules

The high prevalence of chronic kidney disease leads to an increased need for renal replacement therapies. While there are simply not enough donor organs available for transplantation, there is a need to seek other therapeutic avenues as current dialysis modalities are insufficient. The field of regenerative medicine and whole organ engineering is emerging, and researchers are looking for innovative ways to create (part of) a functional new organ. To biofabricate a kidney or its functional units, it is necessary to understand and learn from physiology to be able to mimic the specific tissue properties. Herein is provided an overview of the knowledge on tubular and vascular basement membranes' biochemical components and biophysical properties, and the major differences between the two basement membranes are highlighted. Furthermore, an overview of current trends in membrane technology for developing renal replacement therapies and to stimulate kidney regeneration is provided.

Evaluation of the topographical influence on the cellular behavior of human umbilical vein endothelial cells

Adhesion and proliferation of vascular endothelial cells are important parameters in the endothelialization of biomedical devices for vascular applications. Endothelialization is a complex process affected by endothelial cells and their interaction with the extracellular microenvironment. Although numerous approaches are taken to study the influence of the external environment, a systematic investigation of the impact of an engineered microenvironment on endothelial cell processes is needed. This study aims to investigate the influence of topography, initial cell seeding density, and collagen coating on human umbilical vein endothelial cells (HUVECs). Utilizing the MultiARChitecture (MARC) chamber, the effects of various topographies on HUVECs are identified, and those with more prominent effects were further evaluated individually using the MARC plate. Endothelial cell marker expression and monocyte adhesion assay are examined on the HUVEC monolayer. HUVECs on 1.8 μm convex and concave microlens topographies demonstrate the lowest cell adhesion and proliferation, regardless of initial cell seeding density and collagen I coating, and the HUVEC monolayer on the microlens shows the lowest monocyte adhesion. This property of lens topographies would potentially be a useful parameter in designing vascular biomedical devices. The MARC chamber and MARC plate show a great potential for faster and easy pattern identification for various cellular processes.

Morphology, histochemistry and glycosylation of the placenta and associated tissues in the European hedgehog (Erinaceus europaeus)

INTRODUCTION

There are few descriptions of the placenta and associated tissues of the European hedgehog (Erinaceus europaeus) and here we present findings on a near-term pregnant specimen.

METHODS

Tissues were examined grossly and then formalin fixed and wax-embedded for histology and immunocytochemistry (cytokeratin) and resin embedded for lectin histochemistry.

RESULTS

Each of four well-developed and near term hoglets displayed a discoid, haemochorial placenta with typical labyrinth and spongy zones. In addition there was a paraplacenta incorporating Reichert's membrane and a largely detached yolk sac. The trophoblast of the placenta contained diverse populations of granule which expressed most classes of glycan. Intercellular membranes were also glycosylated and this tended to be heavier in the labyrinth zone. Fetal capillary endothelium had glycosylated apical surfaces expressing sialic acid and various other glycans. Glycogen was present in large cells situated between the spongy zone and the endometrium. Trophoblast cells in the placental disc and under Reichert's membrane, as well as yolk sac endoderm and mesothelium, were cytokeratin positive. Reichert's membrane was heavily glycosylated. Yolk sac inner and outer endoderm expressed similar glycans except for N-acetylgalactosamine residues in endodermal acini.

DISCUSSION

New features of near-term hedgehog placenta and associated tissues are presented, including their glycosylation, and novel yolk sac acinar structures are described. The trophoblast of the placental disc showed significant differences from that underlying Reichert's membrane while the glycan composition of the membrane itself showed some similarity to that of rat thereby implying a degree of biochemical conservation of this structure.

The glomerular basement membrane as a model system to study the bioactivity of heparan sulfate glycosaminoglycans

The glomerular basement membrane and its associated cells are critical elements in the renal ultrafiltration process. Traditionally the anionic charge associated with several carbohydrate moieties in the glomerular basement membrane are thought to form a charge selective barrier that restricts the transmembrane flux of anionic proteins across the glomerular basement membrane into the urinary space. The charge selective function, along with the size selective component of the basement membrane, serves to limit the efflux of plasma proteins from the capillary lumen. Heparan sulfate glycosaminoglycans are anionically charged carbohydrate structures attached to proteoglycan core proteins and have a role in establishing the charge selective function of the glomerular basement membrane. Although there are a large number of studies in the literature that support this concept, the results of several recent studies using molecular genetic approaches to minimize the anionic charge of the glomerular basement membrane would suggest that the role of heparan sulfate glycosaminoglycans in the glomerular capillary wall are still not yet entirely resolved, suggesting that this research area still requires new and novel exploration.

Basement membranes: cell scaffoldings and signaling platforms

Basement membranes are widely distributed extracellular matrices that coat the basal aspect of epithelial and endothelial cells and surround muscle, fat, and Schwann cells. These extracellular matrices, first expressed in early embryogenesis, are self-assembled on competent cell surfaces through binding interactions among laminins, type IV collagens, nidogens, and proteoglycans. They form stabilizing extensions of the plasma membrane that provide cell adhesion and that act as solid-phase agonists. Basement membranes play a role in tissue and organ morphogenesis and help maintain function in the adult. Mutations adversely affecting expression of the different structural components are associated with developmental arrest at different stages as well as postnatal diseases of muscle, nerve, brain, eye, skin, vasculature, and kidney.

The regulation of basement membrane formation and cell-matrix interactions by defined supramolecular complexes

Several constituents of basement membranes, including type IV collagen, laminin, heparan sulphate proteoglycan and nidogen, form a defined supramolecular complex that is an obligatory intermediate in the formation of this matrix. We have named this defined supramolecular complex the 'basement membrane matrisome'. Matrisome structures composed of other collagens, proteoglycans and glycoproteins may participate in the formation of other extracellular matrices. Cells show specific interactions with components of the extracellular matrix. We discuss studies that indicate that melanoma cells can express receptors for both laminin and fibronectin. However, these receptors are expressed in a reciprocal fashion, depending on the exposure of the cell to these proteins. Binding of either fibronectin or laminin to the cells elicits a distinct phenotype. This represents a mechanism in which cellular activity can be regulated by extracellular matrix factors during development and in repair.

The ultrastructural organization and architecture of basement membranes

Basement membranes are ubiquitous complex, multicomponent structures having diverse functions. They are morphologically distinct and exhibit specific structural details including the lamina rara and lamina densa. In addition, the interstitial stroma abutting the lamina densa has a unique organization. While the composition of basement membranes is still incompletely known, several components have been identified, including collagen types IV and V, laminin and heparan sulphate proteoglycan. High resolution immunoelectron microscopic studies have allowed the development of various models of the organization and architecture of the basement membrane, suggesting specific localizations of the various collagen types and specific domains of the collagen molecules, laminin and other components. In addition, high resolution metal shadow casting techniques have allowed the development of molecular models of specific components of the basement membrane and methods of studying the domain structure and interactions of these components.

Biosynthesis and structure of the basement membrane proteoglycan containing heparan sulphate side-chains

Endothelial, epithelial and muscle cells produce a similar proteoglycan for deposition in basement membrane. This proteoglycan is initially synthesized as a low buoyant density proteoglycan containing 3-5 heparan sulphate side-chains (15,000-65,000 Mr each), along one half of a 400,000 Mr core protein. A portion of the population of these macromolecules is degraded to produce small high density proteoglycans containing a variable-sized core protein less than 100,000 in Mr. This biosynthetic and degradative process probably accounts for the variety of differently sized heparan sulphate proteoglycans that have been isolated from various basement membrane sources.

Inhibitors of collagenase IV and cell adhesion reduce the invasive activity of malignant tumour cells

The invasive activities of some malignant cells appear to be activated by contact with laminin. This protein occurs solely in basement membranes and the binding of malignant cells to the surface of this extracellular matrix initiates the invasion process. Passage of the cells across basement membrane requires degradative activity and laminin induces the production of collagenase IV which lyses the collagen IV network. The motility of the cells is enhanced by chemo-attractants and by matrix molecules. Peptides that inhibit the binding of tumour cells to laminin, inhibitors of collagenase IV, and inhibitors of specific pathways of arachidonic acid metabolism prevent invasion as well as the metastasis of malignant cells and could be employed to stop the spread of cancer.

Ultrastructural characterization of an artificial basement membrane produced by cultured keratinocytes

A recent study in our laboratories on the growth of keratinocytes at the culture medium/air interface has led to the identification of a novel thin sheet-like matrix that supports adherent cells. This novel matrix consists of components secreted by keratinocytes, including type IV collagen, and laminins 1 and 5, that self-assembled to a membrane structure. In the present study, a detailed ultrastructural characterization of this membrane was done with high-resolution electron microscopy after negative staining. The basic organization of the membrane was found to be a dense network of 8- to 10-nm-wide irregular rod-like elements. High-resolution examination and immunolabeling showed that type IV collagen filaments form the core of these elements, and other components including heparan sulfate proteoglycan in the form of 4.5- to 5-nm-wide ribbon-like "double tracks" are aggregated around it. These detailed features of the membrane strikingly resembled those of the basement membrane in vivo. These ultrastructural similarities indicate that the membrane may also have basement membrane-like functional properties, and suggest that it should be considered for testing in future medical applications.

Assembly, stability and integrity of basement membranes in vivo

Basement membranes are layered structures of the extracellular matrix which separate cells of various kinds from the surrounding stroma. One of the frequently recurring questions about basement membranes is how these structures are formed in vivo. Up to a few years ago, it was thought that basement membranes were formed spontaneously by a process of self-assembly of their components. However, it has now become clear that cell membrane receptors for basement membrane components are essential factors for the formation and stability of basement membranes in vivo. The present review highlights the modern concepts of basement membrane formation.

Basement membrane and beta amyloid fibrillogenesis in Alzheimer's disease

High-resolution ultrastructural and immunohistochemical studies revealed that in situ beta amyloid fibrils of Alzheimer's disease were made up of a core consisting of a solid column of amyloid P component (AP) and associated chondroitin sulfate proteoglycan, and a heparan sulfate proteoglycan surface layer with externally associated fine filaments of beta protein. The main body of beta amyloid fibrils closely resembled that of microfibrils. Abundant microfibrils were reported to be present at the basement membrane of capillaries with "leaky" blood-urine or blood-air barriers. Similarly, abundant microfibril-like beta amyloid fibrils are formed at the microvascular basement membrane in cerebrovascular amyloid angiopathy with altered blood-brain barrier. Since AP is an indispensable major component of microfibrils and microfibril-like structures, the formation of microfibrils may depend on, among other factors, the availability of AP. Thus, in beta amyloid fibrillogenesis fibrils may be built around AP which continuously leaks out from circulation into vascular basement membrane, and beta amyloid fibrils may be regarded as pathologically altered basement membrane-associated microfibrils. With no source of AP around them, senile plaque fibrils may also be derived from perivascular amyloid.

Basement membrane of mouse bone marrow sinusoids shows distinctive structure and proteoglycan composition: a high resolution ultrastructural study

Venous sinusoids in bone marrow are the site of a large-scale traffic of cells between the extravascular hemopoietic compartment and the blood stream. The wall of the sinusoids consists solely of a basement membrane interposed between a layer of endothelial cells and an incomplete covering of adventitial cells. To examine its possible structural specialization, the basement membrane of bone marrow sinusoids has now been examined by high resolution electron microscopy of perfusion-fixed mouse bone marrow. The basement membrane layer was discontinuous, consisting of irregular masses of amorphous material within a uniform 60-nm-wide space between apposing endothelial cells and adventitial cell processes. At maximal magnifications, the material was resolved as a random arrangement of components lacking the "cord network" formation seen in basement membranes elsewhere. Individual components exhibited distinctive ultrastructural features whose molecular identity has previously been established. By these morphological criteria, the basement membrane contained unusually abundant chondroitin sulfate proteoglycan (CSPG) revealed by 3-nm-wide "double tracks," and moderate amounts of both laminin as dense irregular coils and type IV collagen as 1-1.5-nm-wide filaments, together with less conspicuous amounts of amyloid P forming pentagonal frames. In contrast, 4.5-5-nm-wide "double tracks" characteristic of heparan sulfate proteoglycan (HSPG) were absent. The findings demonstrate that, in comparison with "typical" basement membranes in other tissues, the bone marrow sinusoidal basement membrane is uniquely specialized in several respects. Its discontinuous nature, lack of network organization, and absence of HSPG, a molecule that normally helps to maintain membrane integrity, may facilitate disassembly and reassembly of basement membrane material in concert with movements of adventitial cell processes as maturing hemopoietic cells pass through the sinusoidal wall: the exceptionally large quantity of CSPG may represent a reservoir of CD44 receptor for use in hemopoiesis.

Regulation of the differentiation and behaviour of extra-embryonic endodermal cells by basement membranes

Both the extracellular matrix and parathyroid hormone-related peptide (PTHrP) have been implicated in the differentiation and migration of extra-embryonic endodermal cells in the pre-implantation mammalian blastocyst. In order to define the individual roles and interactions between these factors in endodermal differentiation, we have used embryoid bodies derived from Lamc1(-/-) embryonic stem cells that lack basement membranes. The results show that in the absence of basement membranes, increased numbers of both visceral and parietal endodermal cells differentiate, but they fail to form organised epithelia. Furthermore, although parietal endodermal cells only migrate away from control embryoid bodies in the presence of PTHrP, they readily migrate from Lamc1(-/-) embryoid bodies in the absence of PTHrP, and this migration is unaffected by PTHrP. Thus, the basement membrane between epiblast and extra-embryonic endoderm is required for the proper organisation of visceral and parietal endodermal cells and also restricts their differentiation to maintain the population of primitive endodermal stem cells. Moreover, this basement membrane inhibits migration of parietal endodermal cells, the role of PTHrP being to stimulate delamination of parietal endodermal cells from the basement membrane rather than promoting migration per se.

Ultrastructure and composition of basement membranes in the tooth

The tooth, the hardest organ in the body, is known to be formed through highly elaborate, unique processes of differentiation and development. Basement membranes play critical roles in fundamentally important biological processes such as growth and differentiation, and for better understanding of the mechanism of development and maintenance of the tooth, specializations of tooth basement membranes are reviewed in detail in relation to their roles. The basement membrane at such diverse locations in the tooth as the inner enamel epithelium, maturation-stage ameloblasts, and junctional epithelium at the dentogingival border are specialized in their own highly unique ways for anchoring, firm binding, or mediation in the transport of substances. Thus, the role of basement membranes in the developing and mature tooth is manifold and for these roles individual basement membranes are specialized in their own specific ways which are rare or not seen in nondental tissues, and these specializations are essential for successful development and maintenance of the tooth.

Regulation of programmed cell death by basement membranes in embryonic development

The formation of the proamniotic cavity in the mammalian embryo is the earliest of many instances throughout development in which programmed cell death and the formation of epithelia play fundamental roles (Coucouvanis, E., and G.R. Martin. 1995. Cell. 83:279-287). To determine the role of the basement membrane (BM) in cavitation, we use embryoid bodies derived from mouse embryonic stem cells in which the LAMC1 genes have been inactivated to prevent BM deposition (Smyth, N., H.S. Vatansever, P. Murray, M. Meyer, C. Frie, M. Paulsson, and D. Edgar. 1999. J. Cell Biol. 144:151-610). We demonstrate here that LAMC1-/- embryoid bodies are unable to cavitate, and do not form an epiblast epithelium in the absence of a BM, although both embryonic ectodermal cells and extraembryonic endodermal cells do differentiate, as evidenced by the expression of cell-specific markers. Acceleration or rescue of BM deposition by exogenous laminin in wild-type or LAMC1-/- embryoid bodies, respectively, results in cavitation that is temporally and spatially associated with restoration of epiblast epithelial development. We conclude that the BM not only directly regulates development of epiblast epithelial cells, but also indirectly regulates the programmed cell death necessary for cavity formation.

Basement membranes, microfibrils and beta amyloid fibrillogenesis in Alzheimer's disease: high resolution ultrastructural findings

It is known that beta amyloid fibrils are deposited at the basement membrane of the cerebromicrovasculature in the brains of patients with Alzheimer's disease, and the assembly of the fibrils may be in continuation with the core of senile plaques. The fibrils accumulate in a manner similar to that in which microfibrils accumulate in the glomerular basement membrane of the rat kidney during long-term experimental diabetes, and in the alveolar-capillary basement membrane of the normal lung. beta amyloid fibrils in-situ are known to be about 10 nm wide tubular structures and they closely resemble connective tissue microfibrils. Our recent high resolution ultrastructural studies combined with immunogold labeling demonstrated that beta amyloid fibrils in-situ are indeed microfibril-like structures, and the beta protein is associated with their surface in the form of loose assemblies of 1 nm wide flexible filaments. Thus, the result of this study indicates that in-situ a major component of the beta amyloid deposit is the microfibril-like structure. The elucidation of the mechanism of cerebral beta amyloid fibrillogenesis in Alzheimer's disease may therefore require understanding the mechanism of 'normal' microfibrils biogenesis.

Ultrastructural triple localization of laminin-1, nidogen-1, and collagen type IV helps elucidate basement membrane structure in vivo

The basement membrane models which have been proposed to date are generally based on biochemical data, mainly binding studies and artificially synthesized polymers in vitro. Basically these have led to models proposing two three-dimensional laminin-1 and collagen type IV networks interconnected by nidogen-1. Whether they reflect the in vivo basement membrane structure is still not clear. We localized laminin-1, nidogen-1, and collagen type IV ultrastructurally in adult and fetal mouse kidney basement membranes with the help of immunogold-histochemistry performing double and triple localization to try to elucidate the molecular organization of basement membranes in vivo. We found laminin-1, nidogen-1, and collagen type IV distributed over the entire basement membranes in adult and fetal kidneys. This contradicts earlier studies ascribing laminin-1 to the lamina lucida and collagen type IV to the lamina densa. In addition, various basement membrane segments exhibited an organized labeling pattern for the BM components. Double-labeling revealed co-localization of laminin-1 and nidogen-1. We conclude that the combination of laminin-1 with collagen type IV as double-network basement membrane partially interconnected by nidogen-1 is found already in the early fetal kidney in vivo. However, our data cannot exclude the possibility of other variants of basement membrane assemblages. This is also indicated by a changing structure even in individual segments of one basement membrane type which renders a more flexible basement membrane architecture plausible.

Morphogenesis of the glomerular filter: the synchronous assembly and maturation of two distinct extracellular matrices

The morphogenesis of the glomerular filtration apparatus during pre- and postnatal development in the rodent involves the coordinated assembly of two closely apposed but morphologically different extracellular matrices, the glomerular capillary basement membrane and the mesangial matrix. The cellular origin of these matrices is known to be distinct and complex; however, the mechanisms by which these matrices are assembled during morphogenesis are not entirely understood. It has been shown that in the earliest stages of glomerular morphogenesis the nascent glomerular basement membrane exists as a four-layered structure, the product of both the visceral epithelium and capillary endothelium. During the latter stages of glomerular development, the quadrilaminar structure becomes a trilaminar basement membrane, the event thought to occur by fusion of closely apposed basement membrane layers. In subsequent stages of maturation and throughout the life of the animal, the visceral epithelial cells, which line the periphery of the glomerular capillary, are the primary source of newly synthesized basement membrane material. The mesangial matrix, which lacks the specific organization of a basement membrane, first occurs in the developing glomerulus as a diffuse matrix central to the developing glomerular capillaries. During glomerular maturation the mesangial matrix undergoes a compaction/arborization coincident with the ramification of the vascular histoarchitecture of the glomerular tuft. Recent advances in the cell biology of basement membrane now demonstrate that there is a divergence in isoforms of the molecules that comprise the glomerular capillary basement membrane and mesangial matrices during development, possibly coincidental with functional specialization during the process of glomerular maturation.

Metalloproteinases regulate parietal endoderm differentiating and migrating in cultured mouse embryos

Extracellular matrix (ECM) adhesion and proteolysis play important roles in embryonic development. In previous work (Behrendtsen et al. [1992] Development 114:447-456) we showed that gelatinase B activity is rate-limiting for trophoblast-mediated invasion and degradation of ECM in culture. In the present study, we show that metalloproteinases (MMPs) have distinct roles in migration along ECM as opposed to invasion through ECM. We investigated the role of ECM proteolysis in the differentiation and migration of parietal endoderm (PE), the first embryonic migratory cell type, adhering to ECM surfaces. Gelatinase B was the major MMP of PE; mRNA and protein were detected in PE of 7.5- and 8.5-day embryos. Using cultures of inner cell masses (ICMs) isolated from mouse blastocysts, we found that inhibitors of metalloproteinases, specifically, tissue inhibitor of metalloproteinases (TIMP)-1 and a peptide hydroxamic acid stimulated outgrowth and differentiation of PE from ICMs cultured on fibronectin, but inhibitors of plasminogen activators did not. TIMP-1 increased the number of PE cells and mean distance migrated and increased expression of the PE differentiation marker vimentin; the increase in cell number was not at the expense of other cell types. The stimulatory effect of TIMP-1 was most marked on low concentrations of substrate fibronectin, decreasing as concentrations of fibronectin increased. TIMP-1 also stimulated the outgrowth of PE in blastocyst cultures and in ICM/trophectoderm co-cultures; in ICM/trophectoderm co-cultures TIMP-1 stimulated PE differentiation on higher concentrations of fibronectin than was permissive for ICMs cultured alone. These data indicate that metalloproteinase inhibitors preserved the migration-inducing status of the ECM. We conclude that metalloproteinases have distinct roles in invasive activity through ECM barriers and migratory activity along ECM surfaces.

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.

Ultrastructural characterization of internal basement membrane of junctional epithelium at dentogingival border

BACKGROUND

Although a number of electron microscopic observations of the dentogingival border have been reported, observation of the detailed ultrastructure of the internal basement membrane of the junctional epithelium at the border has not yet been done.

METHODS

The internal basement membrane of a rhesus monkey (Macaca mulatta) was ultrastructurally characterized with high-resolution electron microscopy. The head and neck region of a 5-year-old animal was perfused with a fixative containing 1.25% glutaraldehyde, 0.75% formaldehyde, and 0.04% CaCl2 in 0.1 M phosphate buffer, pH 7.4. Isolated mandibular segments were further fixed with 2.5% glutaraldehyde, 2% formaldehyde, and CaCl2 (300 mg/l) in 0.1 M sodium cacodylate buffer, pH 7.4, for 24 hr at 4 degrees C. The segments were then demineralized and cut into small pieces that were postfixed with 1% osmium tetroxide and further processed for electron microscopy.

RESULTS

The internal basement membrane takes the form of both a common basement membrane and, in places, a multilayered basement membrane. This basement membrane was found to have fundamental ultrastructural features common to other basement membranes. That is, the bulk of the basement membrane is composed of a network of irregular anastomosing strands referred to as "cords." Heparan sulfate proteoglycan-containing ribbonlike 4.5-5-nm-wide "double tracks" were also present in this basement membrane. However, the lamina densa of this basement membrane was unusually thick (160 nm) compared with the thickness (30-60 nm) in other basement membranes, and the cords were also much thicker (8.3 +/- 1.9 nm vs. the usual 3-5 nm).

CONCLUSIONS

From these results, it is concluded that this particular basement membrane is specialized for mechanical strength. This specialization is likely to be suitable for the purpose of the tight sealing of the periodontal tissues from the oral environment.

High-resolution ultrastructural study of the rat glomerular basement membrane in aminonucleoside nephrosis

In the initial stages of aminonucleoside nephrosis, functional alterations in the glomerular basement membrane occur, as evidenced by the development of proteinuria. However, it has not been possible to observe important ultrastructural modifications at the level of the basement membrane, probably because the changes are taking place at the molecular level. In this study, by the use of high-resolution electron microscopy, an attempt was made to evaluate such changes in rat glomerular basement membrane during acute aminonucleoside nephrosis. As previously reported, in control animals the glomerular basement membrane is composed of a network of 4-nm-wide irregular anastomosing strands, referred to as "cords," which are known to contain a core filament of type IV collagen surrounded by a "sheath" of other components, such as laminin and heparan sulfate proteoglycan (HSPG). The most conspicuous ultrastructural alteration of the nephrotic glomerular basement membrane, recognizable only at high magnification, is that the cords were denuded leaving only the core filament through the loss of the sheath material. Thus, the cord network was transformed, with the progress of pathological conditions, into a network of fine filaments. On the other hand, abundance and distribution of HSPG molecules known to be present in the form of 4.5- to 5-nm-wide ribbon-like "double tracks," were found to be similar in control and nephrotic tissues. Since HSPG is one of the charge proteins of the basement membrane, the little changes observed for HSPG are difficult to interpret in view of reported decreases in basement membrane anionic sites in nephrosis. In conclusion, the glomerular basement membrane in aminonucleoside nephrosis loses its cord network components and replaces them with a more perforated network, which could be a cause for the increased permeability of this basement membrane.

Entactin modulates the attachment of rabbit corneal epithelial cells

PURPOSE

To understand the biological activity of entactin, a component of the basement membrane of the corneal epithelium, we investigated the ability of rabbit corneal epithelial cells to attach to an entactin matrix and the effect of entactin on the cells' attachment to other corneal basement proteins.

METHODS

Multiwell plastic plates were coated with bovine serum albumin (BSA), alone or with BSA and entactin, laminin, fibronectin or collagen type IV. Cultured rabbit corneal epithelial cells were seeded on the plates. After incubation (usually 90 min), the cells were fixed and stained with 1% crystal violet. The number of attached cells was counted under a light microscope.

RESULTS

The numbers of attached cells increased in proportion to both the incubation period and the concentration of entactin coated. Furthermore, the number of cells attached to the entactin-coated plate was greater than the number attached to the BSA-coated plate for each incubation period (30 to 120 min). Likewise, when laminin-coated plates were treated with entactin, the number of the attached cells increased in proportion to the concentration of entactin. However, entactin did not affect the cellular attachment of fibronectin or type IV collagen. Cellular attachment to entactin was partially inhibited by the cells' preincubation with the synthetic peptide (GRGDSP).

CONCLUSIONS

The present results showed that cultured corneal epithelial cells adhere to entactin and that entactin stimulated the attachment of these cells to the laminin matrix. These findings suggest that entactin plays a specific role in maintaining the normal integrity of the corneal epithelium.

Three-dimensional analysis of the substrate-dependent invasive behavior of a human lung tumor cell line with a confocal laser scanning microscope

Matrigel and collagen G gels were used as models for basement membrane and interstitial space-collagen, respectively, to study the invasive behavior of cells of the human lung tumor cell line EPLC 32M1, which was derived from a squamous cell carcinoma. For three dimensional analysis of the invasive process, cells were seeded onto the gels in a slide chamber and observed with a confocal laser scanning microscope. Optical sectioning in the xy and xz directions and image reconstruction with computer programs allowed us readily to obtain a three-dimensional overview of the invasive process in situ. Both types of gel showed a smooth surface. Matrigel had a granular structure whereas collagen G revealed a fiber-like morphology. The tumor cells showed a matrix-dependent behavior. On Matrigel, within 24 h of incubation, a network of cells appeared on the surface, which developed further within 72 h to interconnected multicellular cords also invading the gel. Tumor cells seeded on collagen G remained individual. They formed pseudopodia and achieved tight contact with the matrix, eventually also invading the gels in a time-dependent manner. Therefore, the composition of the substrate crucially influences the invasion path.

Possible continuity of subplasmalemmal cytoplasmic network with basement membrane cord network: ultrastructural study

The ultrastructure of the subplasmalemmal cytoplasm of the cell and the associated basement membrane as well as the area of the cell-basement membrane border were observed with high resolution electron microscopy after preparation of the tissues with cryofixation or glutaraldehyde fixation followed by freeze substitution. The subplasmalemmal cytoplasm of the smooth muscle cells of rat epididymal tubules and the podocyte processes of the mouse glomerular visceral epithelium were found to be composed of a fine network of irregular anastomosing strands. This network closely resembled the previously characterized cord network of the basement membrane. The cords are known to be composed of a 1.5 to 3 nm thick core filament made up of type IV collagen which is surrounded by an irregular ‘sheath’ of other components. The strands in the subplasmalemmal network showed ultrastructural features similar to those of the cord network. Ribbon-like, 4.5 nm wide heparan sulfate proteoglycan ‘double tracks’ were previously reported to be associated with the cord network. Structures similar in size and appearance to the double tracks were also found in the subplasmalemmal network. At the cell-basement membrane border, the lamina densa of the basement membrane was in contact with the cell without the intervening space of a lamina lucida which was recently found to be an artefact caused by conventional tissue processing. Furthermore, the subplasmalemmal network appeared to be continuous through the plasma membrane, with the cord network of the basement membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

High-resolution ultrastructural study of the rat glomerular basement membrane in long-term experimental diabetes

The ultrastructure of the glomerular basement membrane of the long-term diabetic and age-matched control rats was studied with the application of advanced high-resolution microscopy. By using the freeze substitution method for the preparation of the renal tissue, it was possible to observe that the glomerular basement membrane in control and diabetic animals is composed on only a single lamina densa without the presence of a lamina lucida interna or externa. High-resolution electron microscopy of the diabetic glomerular basement membrane showed significant alterations in its morphology and ultrastructure. First, the basement membrane in diabetic condition appeared to be split into two halves, endothelial and epithelial. In the epithelial half of the membrane, the network of distinct strands referred to as cords, which were clearly present in the glomerular basement membrane of age-matched control animals, became less distinct and showed a diffused appearance being evenly replaced by thin filaments. The openings of the network were filled with a granular material. In the endothelial half of the membrane, on the other hand, the cord network was variably lost in diabetic condition and, within the resulting vacant spaces, bundles of fibrils 12 nm in width, identified as basotubules, were deposited. Immunolabeling for type IV collagen was found to be enriched in the endothelial half of the basement membrane being associated with the bundles of basotubules. The ultrastructural changes reported by high-resolution microscopy could be related to the molecular alterations of the basement membrane components and to the loss in permselectivity occurring during diabetes.

Absence of the blood-clotting regulator thrombomodulin causes embryonic lethality in mice before development of a functional cardiovascular system

We have targeted the murine thrombomodulin (TM) gene in embryonic stem cells and generated embryos as well as mice with TM deficiency. The heterozygous TM-deficient (TM+/-) mice as compared to wild-type (TM+/+) littermates exhibit 50% reductions in the levels of TM mRNA and TM protein. However, TM+/- mice appear normal and are free of thrombotic complications. The homozygous TM-deficient (TM-/-) embryos die before embryonic day 9.5. An overall retardation in growth and development of TM-/- embryos is first evident on embryonic day 8.5 (8-12 somite pairs). However, no specific pathologic abnormalities are observed. These initial changes take place at a time when TM is normally expressed in the parietal yolk sac. The removal of embryonic day 7.5 TM-/- embryos from maternal decidua and their subsequent culture in vitro allow development to proceed to stages not observed in vivo (13-20 somite pairs) with the appearance of normal blood vessels in the visceral yolk sac and embryo. The results of our studies suggest that the failure of TM-/- embryos to survive at mid-gestation is a consequence of dysfunctional maternal-embryonic interactions caused by the absence of TM in the parietal yolk sac and demonstrate that the receptor is necessary for normal embryonic development in utero.

Expression of basement membrane components in the dental papilla mesenchyme of monkey tooth germs--an immunohistochemical study

The present work, which employs indirect immunoperoxidase methods, demonstrates electron microscopic localization of three major basement membrane (BM) components--type IV collagen, laminin, and heparan sulfate proteoglycan--at the early stages of odontogenesis in tooth germs of the Japanese macaque (Macaca fuscata). Intense immunostaining for each examined component occurred at the interface between the inner enamel epithelium and the dental papilla mesenchyme. At higher magnification, immunoreaction products were observed both in the lamina densa and lamina fibroreticularis. Fuzzy substances occurring very close to the lamina fibroreticularis manifested moderate immunoreactivity. In addition, immunostaining took place in the dental papilla mesenchyme. The dental papilla cells located close to the BM demonstrated immunoreactive material mainly on plasma membranes facing the BM. Reaction products were also observed in large concavities formed in some areas of the cell surfaces; and small, immunopositive vesicles occurred close to the plasma membrane. Immunoreaction products could be found in the cisternae of the rough endoplasmic reticulum of some mesenchymal cells. These findings suggest that dental papilla mesenchymal cells may produce the three major BM components and those of the components that are incorporated into the dental BM--particularly into the lamina fibroreticularis--during tooth development.

Characterization of the fibrillar layer at the epithelial-mesenchymal junction in tooth germs

A characteristic layer containing numerous fibrils is associated with the basement membrane of the inner enamel epithelium during the early stages of odontogenesis. However, its nature is not well understood. In this study, the layer was examined with high-resolution electron microscopy and immuno-histochemical staining. Tooth germs of monkeys (Macaca fuscata) were studied and each fibril in the layer was found to be a tubular structure, 8-9 nm in width, resembling a "basotubule", the tubular structure previously observed in various basement membranes. The space between the fibrils was filled with a network formed by irregular anastomosing strands with an average thickness of 4 nm; these strands resembled the "cords" forming the network in the lamina densa of basement membranes. After immunoperoxidase staining, fine threads immunoreactive for laminin staining were seen winding along the strands of the network, and 1.5-nm wide filaments, immunoreactive for type IV collagen, took the form of a network arrangement. The 5-nm-wide ribbon-like structures associated with the strands were identified as heparan sulfate proteoglycan by immunostaining. These results are similar to those obtained for the cord network of the lamina densa. The "fibrillar layer" therefore represents a highly specialized lamina fibroreticularis of the basement membrane of the inner enamel epithelium, and rich in basotubules.

Vascular basement membrane components and the lesions of Alzheimer's disease: light and electron microscopic analyses

Alzheimer's disease (AD) is one of several systemic and cerebral diseases that involve the abnormal deposition of fibrillar proteins called amyloids. All amyloids share conformational and staining characteristics, as well as an association with resident tissue macrophages and two extracellular matrix components [heparan sulfate proteoglycan (HSPG) and amyloid P component]. Vascular, glomerular, and Schwann cell basement membrane pathologies have been documented in many forms of amyloidosis, and often amyloid fibrils fuse to and project from the basement membrane in these diseases. The present report demonstrates the vascular basement membrane (VBM) alterations in AD autopsy samples, and details the methodologies used. Electron microscopy reveals the fusion of amyloid fibrils with the VBM and the alteration of the VBM in the absence of amyloid accumulation. Double-labelling and pre-embed immuno-electron microscopy techniques demonstrate the colocalization of amyloid P component and VBM components with amyloid, and also reveal that amyloid P component is not localized to the cerebral VBM. Finally, a novel correlative light/electron microscopy technique demonstrates the association between amyloid P component and cerebral resident tissue macrophages, the microglia. Taken together, these data suggest that the physicochemical processes of amyloid formation, rather than amyloid deposition, may be responsible for VBM pathology.

Morphology of the basement membrane

The aim of this contribution is to summarize our knowledge of the morphology of the basement membrane (BM). The first step in this direction is the attempt to define this term. The BM is composed of the Lamina lucida, densa, and fibroreticularis. Subsequently, the historical development of this term is discussed. Our main interest is, of course, focused on the description of the BM-structure up to the macromolecular level and the special forms of this structure. This is supplemented by discussing its chemical composition and establishing a relationship between morphology and biochemistry. The obtained findings yielded some indications as to the molecular composition of the BM which may serve for the construction of "models." The composition of the Lamina lucida (L.l.) and the Lamina or Pars fibroreticularis (L.f.) must be discussed separately, since, if present, they show a different and strongly varying structure (L.f.). An important aspect is the function of this extracellular layer which comprises mechanical tasks up to inductive effects. Finally, the concepts of the formation of the BM, especially of the Lamina densa (L.d.), are summarized. It obviously consists of a sequence of individual steps which starts with expression and secretion of the L.d.-components and is followed by an induction of integrin expression.

Quick-freeze, deep-etch studies of renal basement membranes

The fine structure of the renal (i.e., glomerular, tubular, and capillary) basement membranes was re-evaluated with the aid of a deep-etch replica method. The structure of the laminae rarae interna and externa of the rat glomerular basement membrane (GBM) and laminae lucida of other basement membranes were basically identical in that 6 to 8 nm fibrils were interconnected to form a three-dimensional, polygonal network. By contrast, all of the laminae densa examined were composed of closely packed granules, and a filamentous substructure was identified only in a limited area. These granular components were demonstrated to be an integral component of the lamina densa. From additional observations on the trypsinized bovine GBM, it appeared that the basic structure of renal basement membranes was almost identical, namely, that a three-dimensional fibrillar meshwork existed throughout the individual layers to form a structural framework upon which fine particles were variably attached. In addition, we observed some of the fine structure of the pars fibroreticularis; the laminae densa of the tubular and capillary basement membranes continued to the fibrillar meshwork resembling the structural backbone of the glomerular basement membrane. The network was sometimes directly connected to the extracellular matrix, but more often changed into a rough fibrillar framework and connected to the extracellular matrix.

Lamina lucida of basement membrane: an artefact

In tissues prepared with chemical fixation followed by conventional dehydration, basement membranes have been observed to be laminated structures composed of a lamina lucida and lamina densa as well as a poorly limited transitional zone referred to as the pars fibroreticularis. Scattered attempts in the application of new techniques of tissue preparation such as cryofixation or freeze substitution for the study of the basement membrane structure have been made in recent years. From these studies, the possibility has arisen in which basement membranes are composed of only the lamina densa without a lamina lucida. In recent studies in this laboratory, the attempt was made to determine whether or not this lamina lucida is an artefact, and if so, which step in the conventional method of tissue preparation is responsible for its formation. Basement membranes from diverse sources in the mouse and rat including the testis, ductus epididymis, eye, thyroid, kidney, and skin, were observed after either cryofixation by slam freezing followed by freeze substitution, or aldehyde fixation followed by freeze substitution. The basement membranes after preservation with either of these two methods were composed of only the lamina densa with no lamina lucida. It indicates that an artefactual formation of the lamina lucida occurs during dehydration in conventional tissue preparation rather than during chemical fixation. In view of the well known superiority of freeze substitution over conventional dehydration, the lamina lucida of the basement membrane is likely to be an artefact. Therefore, it is concluded that the lamina lucida is an artefact formed during conventional tissue preparation, and in its original condition in the living state, the basement membrane is composed of a single layer made up of lamina densa material.

Basic structure of basement membranes is a fine network of "cords," irregular anastomosing strands

A three-dimensional network of irregular anastomosing strands, referred to as "cords," was found to be the main component of the lamina densa of a) common, "thin" basement membranes in tissues from diverse origins including foot pad epidermis, trachea, jejunum, seminiferous tubule and vas deferens of the rat, monkey seminiferous tubule, and mouse ciliary process, b) a "double" basement membrane, the rat glomerular basement membrane, and c) "thick" basement membranes including rat Reichert's membrane, mouse lens capsule and the Engelbreth-Holm-Swarm (EHS) tumor matrix. The average thickness of the cords was 3.2-4.8 nm, 4 nm, and 4.7-5 nm, respectively, in these three types of basement membranes. The mean diameter of the intercordal spaces, or openings of the network, averaged 14 nm with a range from 8 nm in the glomerular basement membrane to 21.9 nm in the lens capsule. After cryofixation followed by freeze substitution or freeze drying, similar cord networks were observed in all basement membranes examined which included two thin basement membranes, that of the rat epididymis and seminiferous tubules, and three thick basement membranes, that is, the lens capsule and the EHS tumor matrix of the mouse, and rat Reichert's membrane. In addition, following the co-incubation of laminin, type IV collagen and heparan sulfate proteoglycan at 35 degrees C, a precipitate was formed which was found to contain lamina densa-like sheets and large semisolid masses. Both types of structures were found to be made up of a network of 3 nm wide cords, which resembled that of natural basement membranes. With the immunoperoxidase technique, these cords were stained for major basement membrane components including laminin, type IV collagen, heparan sulfate proteoglycan, entactin, and fibronectin. Ribbon-like "double tracks" 4.5 nm in width and being distributed along cords have been identified as the form taken by heparan sulfate proteoglycan in basement membranes. Following mild plasmin treatment, most of the cord components were digested away leaving behind a network of fine filaments found to contain type IV collagen. Each cord, therefore, is organized by a type IV collagen core filament which is surrounded by a plasmin-sensitive sheath containing other basement membrane components. Two types of minor structural components, that is, 7-10 nm wide straight "basotubules" and 3.5 nm wide particulate structures referred to as "pentosomes" were associated with cord network in some basement membranes.(ABSTRACT TRUNCATED AT 400 WORDS)

A line of rat ovarian surface epithelium provides a continuous source of complex extracellular matrix

A spontaneously immortalized, yet non-tumorigenic rat ovarian surface epithelial (ROSE 199) cell line, deposits large amounts of extracellular matrix (ECM) in response to crowding. The characteristics and components of ROSE 199-derived cell-free ECM were compared after three different preparative techniques: treatment with 20 mM ammonium hydroxide, with 1% sodium deoxycholate, or by repeated freeze-thaws. The ECMs were analyzed by histochemistry, immunofluorescence, electron microscopy, and Western immunoblotting. Components of ROSE 199 ECM included laminin, fibronectin, and collagen types I and III. Even though ROSE 199 is an epithelial cell line, striated collagen fibers formed a major part of its matrix. Thus, ROSE 199 matrix consists of both basement membrane and stromal matrix components. This matrix supported the adhesion, spreading, and growth of several cell types without altering their morphology or growth pattern, and enhanced the attachment of some cell types that spread on plastic only with difficulty. Immunofluorescence, electron microscopy, and dry weight determinations indicated that a greater proportion of matrix was retained in preparations obtained by ammonium hydroxide or freeze thaw techniques than after sodium deoxycholate treatment. Ammonium hydroxide and freeze-thaw treated matrices were also superior to sodium deoxycholate preparations as evidenced by enhanced initial cellular adhesion and spreading compared to cells plated on plastic. Residual nuclear material did not seem to affect the biological activity of this matrix. ROSE 199 extracellular matrix provides a novel, complex substratum for cell culture and for studies of matrix functions and synthesis.

Nonenzymatic glycosylation-induced modifications of intact bovine kidney tubular basement membrane

We examined structural changes in bovine kidney tubular basement membrane (TBM) following in vitro nonenzymatic glycosylation (NEG). Isolated TBM was incubated for 2 wk at 37 degrees C in the absence of sugar or in the presence of either glucose or ribitol under conditions that minimized degradation and oxidative damage. NEG and crosslink formation in glycated TBM were confirmed by decreased solubility, increased amounts of low mobility material by SDS-PAGE, and increased specific fluorescence compared to controls. Morphological analysis using high resolution, low voltage scanning electron microscopy (LV-SEM) revealed a complex three-dimensional meshwork of interconnecting strands with intervening openings. Glycated TBM underwent distinct morphological changes, including a 58% increase in the amount of image surface area occupied by openings. This was due to an apparent increase in the number of large openings (diameters > 12.5 nm), whereas the number of small openings (diameters < 12.5 nm) remained unchanged. These findings corroborate earlier physiological studies, which established that the loss of glomerular permselectivity seen in patients with diabetic nephropathy is due to the formation of large pores in the kidney filtration barrier of which the BM is a major component. We conclude that NEG and crosslink formation among BM components lead to modifications of BM ultrastructure, which could play a role in loss of barrier function in diabetic microangiopathy and nephropathy.

Ultrastructural localization of carbohydrates in Reichert's membrane of the mouse

In the present investigation, we examined the role of trophoblast and parietal endoderm cells in the synthesis of carbohydrate-containing components of Reichert's membrane. To eliminate the function of Reichert's membrane as a filter between maternal and embryonal tissues we carried out our examination under in vitro conditions. Parietal yolk sac from mouse embryos on day 9 post coitum (p.c.) were cultivated for 0 to 5 days. Because tannic acid enables a complex formation between carbohydrates and osmium we chose the fixation with this acid for the ultrastructural study. Electron microscopy showed that for assembly of Reichert's membrane, trophoblast cells produce and then release components that were detected as tannic acid-positive granules both in the Reichert's membrane and in the vacuoles of the trophoblast cells. To localize specific carbohydrates we used postembedding-gold-lectin histochemistry on LR-GoldR-embedded tissues. Strong binding sites for the lectins WGA (Triticum vulgare), RCA I (Ricinus communis) and Con A (Canavalia ensiformis) were observed in Reichert's membrane and trophoblast cells but not in the parietal endoderm cells. The LTA (Lotus tetragonolobus)-binding pattern was positive in the membrane and its adjacent cells but that of the LFA (Limax flavus) was negative in the parietal endoderm cells and very weak in Reichert's membrane and trophoblast cells. Our results demonstrate that trophoblast cells are involved in the construction of Reichert's membrane through the production and release of specific glycoconjugates.

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.

Renal basement membranes by ultrahigh resolution scanning electron microscopy

Three-dimensional ultrastructures of basement membranes of the rat kidney were investigated with an ultrahigh resolution scanning electron microscope (HSEM) equipped with a resolving power of 0.5 nm. All cellular components were extracted from renal cortical tissues by sequential-detergent treatment. Four types of acellular basement membranes were observed after tannin-osmium conductive staining: the glomerular basement membrane (GBM) associated with the mesangial matrix, the tubular basement membrane (TBM), the Bowman's capsule basement membrane (BCBM), and the peritubular capillary basement membrane (PTCBM). We could demonstrate the polygonal meshwork structures composed of strands in the respective basement membranes. The strands averaged 6 to 7 nm wide, whereas the pore sizes within the meshworks were variable and differed according to the basement membrane type. Moreover, we confirmed the presence of the heterogeneity of the GBM suggested by several approaches. Present data support the proposition that a polygonal meshwork structure may represent the basic structure of basement membrane. Some of the observed architectural dissimilarities in basement membrane types may reflect their different functional properties, which in turn may reflect the heterogeneous distribution of major basement membrane components as demonstrated by immunohistochemical and biochemical studies.

Localization of laminin in nephritic glomeruli as revealed by a quick-freezing and deep-etching method with immunohistochemistry

The three-dimensional localization of laminin in rat glomeruli at the chronic phase of Masugi nephritis was investigated by a quick-freezing and deep-etching method combined with immunohistochemistry. Light-microscopically, laminin was localized in increased mesangial matrix and thickened glomerular basement membrane. The quick-freezing and deep-etching method revealed that the increased mesangial matrix, which was newly formed in axial portions and areas of mesangial interposition, was composed of fine fibrillar networks. They were revealed with the 3,3'-diaminobenzidine tetrahydrochloride (DAB) reaction products of peroxidase-labelled secondary antibody following anti-laminin antibody. However, these reaction products were not uniformly distributed in the newly formed matrix. Although the fibrils organizing lamina densa were also immunostained with anti-laminin antibody, the fibrils connected to mesangial cells, podocytes and endothelial cells had smaller amounts of DAB reaction products for laminin. These results indicate that one of the components of fibrils in the mesangial matrix and lamina densa is laminin, which is heterogeneously distributed in the newly formed matrix.

A murine model for evaluating metastatic potential: characterization of a 90-110-kDa metastasis-binding protein

Reliable discriminatory tests to predict metastatic disease would clearly facilitate the management of cancer in the elderly. We have recently identified a 90-110-kilodalton (kDa) cell surface glycoprotein that is differentially expressed in benign and malignant murine adrenal carcinoma cells. In view of the proteins highly glycosylated nature, we have tested its ability to bind to a panel of agarose-bound lectins. Wheat germ agglutinin (WGA), a lectin specific for terminal sialic acid and N-acetylglucosamine (G1cNAc), had a strong affinity for the metastasis-related protein but failed to detect such a glycoprotein in nonmetastatic cells. Treatment of cells with sialidase to remove terminal sialic acids did not affect the affinity of the protein for the lectin, indicating the presence of terminal G1cNAc. We show by in situ that this metastatic binding protein (MBP) is regionally concentrated on the surface of invasive cells but absent in cells unable to invade. We postulate that MBP plays an active role in cell migration through interactions with beta-1,4 galactosytransferase and basement membrane glycoproteines.

Ultrastructure of the basement membrane and its precursor in developing rat submandibular gland as shown by alcian blue staining

The ultrastructure of the epithelial basement membrane and membrane precursor was studied in rat submandibular rudiment and a model system of the reconstructed basement membrane, by transmission electron microscopy following alcian blue staining. Directly beneath the epithelial plasma membrane, a meshwork layer was found to consist of anastomosing thin fibers arranged as a three-dimensional meshwork (100-400 nm in thickness). Straight strands (5-10 nm in diameter) could sometimes be seen to pass through the meshwork. Adjacent to this layer, a coarse network composed of threads (20-40 nm in diameter) connected the meshwork layer with collagen fibers of the underlying connective tissue. The earliest precursors recognized in the reconstruction-model system were part of the fine-meshwork structure, and showed this structure to be a fundamental component of the basement membrane.

Reconstruction of the basement membrane in a cultured submandibular gland

In a rat submandibular rudiment on day 16, both laminin (LM) and type IV collagen (Col-IV) were found in all cases to colocalize not only in the basement membrane, but also in the rough endoplasmic reticulum of the epithelial cells, indicating that the synthesis of the components of basement membrane is greatly enhanced at this particular stage of extensive branch formation. Using the submandibular gland from a 16-day embryo, the model system was developed to determine the structural organization of the basement membrane. The pre-existing basement membrane was digested with collagenase and dispase, causing its complete disappearance. The subsequent gradual reconstruction of an authentic basement membrane was confirmed by electron microscopy and immunohistochemistry of LM and Col-IV. In the model system, this recovery started at 4 h of culture, and formation was complete by 8 h. During the recovery, thick bundles of actin filaments appeared transitionally in the basal cytoplasm. Electron microscopic analysis indicated two precursor structures, aggregated fuzzy fibers (type 1 extracellular matrix (ECM)) and 10-nm-thick strand piles (type 2 ECM), and an authentic basement membrane structure appeared during the course of membrane reconstruction. LM and Col-IV were always located together in these three structures. These observations clearly indicate that the precursors, containing LM, Col-IV and most likely heparan sulfate proteoglycan, appeared to form immediately following their secretion into the extracellular space, and assembled into the rigid structure of basement membrane within 8 h. The ultrastructural and immunohistochemical process of basement membrane reconstruction appeared to coincide closely with that of the glomerular basement membrane in developing kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of kidney tubular basement membranes

Although some progress has been made in recent years, there are truly large gaps in our basic knowledge on how the TBM is assembled during development. Some of the new evidence presented here indicates that both the tubular epithelium and interstitial fibroblasts participate in TBM protein biosynthesis during nephrogenesis. In addition, newly assembled segments of TBM are spliced or inserted into existing TBM during tubule expansion and elongation. A similar splicing mechanism has been described previously in the GBM, endocrine organs, and intestinal villi, and this mechanism therefore probably represents a fundamental process of basement membrane formation. A major unresolved question at present, however, is how this mechanism operates at the molecular level. Does the newly formed basement membrane contain identical components as that already present? Since an enzymatic process is likely occurring in the insertion of new matrix into old, which enzymes are involved? What is the cellular origin of these enzymes and which matrix component(s) is their substrate? Even more fundamental yet unanswered questions have to do with the mechanisms of epithelial induction, basement membrane gene activation, and tubular morphogenesis. Once the basement membrane is fully formed at the completion of nephrogenesis, what controls basement membrane turnover and how does this operate? Clearly, much additional research is necessary to address these questions. This work is needed, however, before we can fully understand the important roles basement membranes play in normal development as well as in disease.