Expression of fibronectin, laminin, and factor VIII-related antigen during development of the human cutaneous microvasculature

Development of Blood and Lymphatic Endothelial Cells in Embryonic and Fetal Human Skin

Blood and lymphatic vessels provide nutrients for the skin and fulfill important homeostatic functions, such as the regulation of immunologic processes. In this study, we investigated the development of blood and lymphatic endothelial cells in prenatal human skin in situ using multicolor immunofluorescence and analyzed angiogenic molecules by protein arrays of lysates and cell culture supernatants. We found that at 8 to 10 weeks of estimated gestational age, CD144(+) vessels predominantly express the venous endothelial cell marker PAL-E, whereas CD144(+)PAL-E(-) vessels compatible with arteries only appear at the end of the first trimester. Lymphatic progenitor cells at 8 weeks of estimated gestational age express CD31, CD144, Prox1, and temporary PAL-E. At that developmental stage not all lymphatic progenitor cells express podoplanin or Lyve-1, which are acquired with advancing gestational age in a stepwise fashion. Already in second-trimester human skin, the phenotype of blood and lymphatic vessels roughly resembles the one in adult skin. The expression pattern of angiogenic molecules in lysates and cell culture supernatants of prenatal skin did not reveal the expected bent to proangiogenic molecules, indicating a complex regulation of angiogenesis during ontogeny. In summary, this study provides enticing new insights into the development and phenotypic characteristics of the vascular system in human prenatal skin.

Inhibition of Angiogenesis In Vitro by Chebulagic Acid: A COX-LOX Dual Inhibitor

Angiogenesis is a crucial step in the growth of cancer and its metastasis. It is regulated by several endogenous factors which may stimulate or inhibit the new blood vessel growth. Besides these endogenous factors, several exogenous factors including some natural compounds are known to modulate angiogenesis. Angiogenesis being a potential target for drugs against a number of pathological conditions, search for compounds from natural sources that can affect angiogenesis is of great interest. The objective of our present study was to understand the effect of chebulagic acid, a COX-LOX dual inhibitor isolated from the fruits of Terminalia chebula Retz., on angiogenesis. The model systems used were rat aortic rings and human umbilical vein endothelial cells. The results showed that chebulagic acid exerts an antiangiogenic effect. This was evidenced from decreased sprouting in rat aortic rings and decrease in biochemical markers in endothelial cells treated with chebulagic acid. It downregulated the production of CD31, E-selectin, and vascular endothelial growth factor in human umbilical vein endothelial cells in culture (HUVEC). Further studies to understand the molecular mechanism of action of chebulagic acid revealed that CA exerts its anti angiogenic effect by modulating VE cadherin-β catenin signalling in human umbilical vein endothelial cells.

Extensive vascular remodeling in the spinal cord of pre-symptomatic experimental autoimmune encephalomyelitis mice; increased vessel expression of fibronectin and the α5β1 integrin

Alterations in vascular structure and function are a central component of demyelinating disease. In addition to blood-brain barrier (BBB) breakdown, which occurs early in the course of disease, recent studies have described angiogenic remodeling, both in multiple sclerosis tissue and in the mouse demyelinating model, experimental autoimmune encephalomyelitis (EAE). As the precise timing of vascular remodeling in demyelinating disease has yet to be fully defined, the purpose of the current study was to define the time-course of these events in the MOG35-55 EAE model. Quantification of endothelial cell proliferation and vessel density revealed that a large part of angiogenic remodeling in cervical spinal cord white matter occurs during the pre-symptomatic phase of EAE. At the height of vascular remodeling, blood vessels in the cervical spinal cord showed strong transient upregulation of fibronectin and the α5β1 integrin. In vitro experiments revealed that α5 integrin inhibition reduced brain endothelial cell proliferation under inflammatory conditions. Interestingly, loss of vascular integrity was evident in all vessels during the first 4-7days post-immunization, but after 14days, was localized predominantly to venules. Taken together, our data demonstrate that extensive vascular remodeling occurs during the pre-symptomatic phase of EAE and point to a potential role for the fibronectin-α5β1 integrin interaction in promoting vascular remodeling during demyelinating disease.

Fibrin and collagen differentially but synergistically regulate sprout angiogenesis of human dermal microvascular endothelial cells in 3-dimensional matrix

Angiogenesis is a highly regulated event involving complex, dynamic interactions between microvascular endothelial cells and extracellular matrix (ECM) proteins. Alteration of ECM composition and architecture is a hallmark feature of wound clot and tumor stroma. We previously reported that during angiogenesis, endothelial cell responses to growth factors are modulated by the compositional and mechanical properties of a surrounding three-dimensional (3D) extracellular matrix (ECM) that is dominated by either cross-linked fibrin or type I collagen. However, the role of 3D ECM in the regulation of angiogenesis associated with wound healing and tumor growth is not well defined. This study investigates the correlation of sprout angiogenesis and ECM microenvironment using in vivo and in vitro 3D angiogenesis models. It demonstrates that fibrin and type I collagen 3D matrices differentially but synergistically regulate sprout angiogenesis. Thus blocking both integrin alpha v beta 3 and integrin alpha 2 beta 1 might be a novel strategy to synergistically block sprout angiogenesis in solid tumors.

Endoglin mediates fibronectin/α5β1 integrin and TGF-β pathway crosstalk in endothelial cells

Both the transforming growth factor β (TGF-β) and integrin signalling pathways have well-established roles in angiogenesis. However, how these pathways integrate to regulate angiogenesis is unknown. Here, we show that the extracellular matrix component, fibronectin, and its cellular receptor, α5β1 integrin, specifically increase TGF-β1- and BMP-9-induced Smad1/5/8 phosphorylation via the TGF-β superfamily receptors endoglin and activin-like kinase-1 (ALK1). Fibronectin and α5β1 integrin increase Smad1/5/8 signalling by promoting endoglin/ALK1 cell surface complex formation. In a reciprocal manner, TGF-β1 activates α5β1 integrin and downstream signalling to focal adhesion kinase (FAK) in an endoglin-dependent manner. α5β1 integrin and endoglin form a complex on the cell surface and co-internalize, with their internalization regulating α5β1 integrin activation and signalling. Functionally, endoglin-mediated fibronectin/α5β1 integrin and TGF-β pathway crosstalk alter the responses of endothelial cells to TGF-β1, switching TGF-β1 from a promoter to a suppressor of migration, inhibiting TGF-β1-mediated apoptosis to promote capillary stability, and partially mediating developmental angiogenesis in vivo. These studies provide a novel mechanism for the regulation of TGF-β superfamily signalling and endothelial function through crosstalk with integrin signalling pathways.

Upregulation of fibronectin and the α5β1 and αvβ3 integrins on blood vessels within the cerebral ischemic penumbra

Following focal cerebral ischemia, blood vessels in the ischemic border, or penumbra, launch an angiogenic response. In light of the critical role for fibronectin in angiogenesis, and the observation that fibronectin and its integrin receptors are strongly upregulated on angiogenic vessels in the hypoxic CNS, the aim of this study was to establish whether angiogenic vessels in the ischemic CNS also show this response. Focal cerebral ischemia was established in C57/Bl6 mice by middle cerebral artery occlusion (MCA:O), and brain tissue analyzed 7 days following re-perfusion, a time at which angiogenesis is ongoing. Within the ischemic core, immunofluorescent (IF) studies demonstrated vascular expression of MECA-32, a marker of leaky cerebral vessels, and vascular breakdown, defined by loss of staining for the endothelial marker, CD31, and the vascular adhesion molecules, laminin, dystroglycan and α6 integrin. Within the ischemic penumbra, dual-IF with CD31 and Ki67 revealed the presence of proliferating endothelial cells, indicating ongoing angiogenesis. Significantly, vessels in the ischemic penumbra showed strong upregulation of fibronectin and the fibronectin receptors, α5β1 and αvβ3 integrins. Taken together with our recent finding that the α5β1 integrin plays an important role in promoting cerebral angiogenesis in response to hypoxia, these results suggest that stimulation of the fibronectin-α5β1 integrin signaling pathway may provide a novel approach to amplifying the intrinsic angiogenic response to cerebral ischemia.

Increased expression of fibronectin and the alpha 5 beta 1 integrin in angiogenic cerebral blood vessels of mice subject to hypobaric hypoxia

The extracellular matrix (ECM) is an important regulator of angiogenesis and vascular remodeling. We showed previously that angiogenic capillaries in the developing CNS express high levels of fibronectin and its receptor alpha5beta1 integrin, and that this expression is developmentally downregulated. As cerebral hypoxia leads to an angiogenic response, we sought to determine whether angiogenic vessels in the adult CNS re-express fibronectin and the alpha5beta1 integrin. Ten-week old mice were subject to hypobaric hypoxia for 0, 4, 7 and 14 days, and fibronectin/integrin expression examined. Fibronectin and the alpha5 integrin subunit were strongly upregulated on capillaries in the hypoxic CNS, with the effect maximal at the earliest time point examined (4 days). Immunofluorescent studies demonstrated that the alpha5 integrin was expressed by angiogenic endothelial cells. In light of the defined angiogenic role for fibronectin in other systems, this work suggests that induction of fibronectin-alpha5beta1 integrin expression may be an important molecular switch driving angiogenesis in the hypoxic CNS.

In silico analysis of angiogenesis associated gene expression identifies angiogenic stage related profiles

In vitro models have been extensively used to map gene expression in ECs but few studies have used cells from in vivo sources directly. Here, we compare different gene expression surveys on both cultured and fresh tissue derived ECs, and it emerges that gene expression profiles can be paralleled with the angiogenic stage of the cells. ECs stimulated with different growth factors in monolayer cultures exhibit gene expression profiles indicative of an active proliferative state, whereas gene expression in tube forming cells in vitro involves genes implicated in cell adhesion processes. Genes overexpressed in tumor ECs are biased towards extracellular matrix remodeling, a late event in angiogenesis. The elucidation of gene expression profiles under these different conditions will contribute to a better understanding of the molecular mechanisms during angiogenesis in both pathological and physiological circumstances and will have implications for the development of angiogenesis interfering treatment strategies.

Microvascular patterns in human medullary tegmentum at the level of the area postrema

Abstract The aim of the present study was to evaluate the regional differences in microvessel density (MVD) of the human medullary tegmentum in adults and newborns/infants. Transverse serial sections of formalin-fixed, paraffin-embedded brainstems, taken from 16 adult and eight newborn/infant subjects, were stained with anti-von Willebrand factor (vWF) polyclonal antibodies. The boundaries of the area postrema (AP), dorsal motor vagal nucleus (DMVN), solitary tract nucleus (STN), solitary tract (ST) and hypoglossal nucleus (XII) were defined, all vessels were counted and the values were divided by the areas. In adult cases, statistically significant heterogeneity in MVD was found among the nuclei studied (P < 0.001). DMVN and AP showed higher MVD with respect to XII and ST (P < 0.001). The MVD of STN was lower with respect to DMVN (P < 0.001) and higher with respect to XII and ST (P < 0.05). The MVD and capillary density of the AP of newborns/infants were not significantly different with respect to adults. In sections of the medulla of adult subjects stained with anti-vWF, all vessels showed an intense reaction of endothelial cells whereas in the DMVN, XII, STN and ST of newborns/infants, only rare, isolated vessels showed anti-vWF reactivity and in the AP, 41 +/- 21% of vessels expressed vWF. Differences in MVD among the nuclei may be related to their different functions and metabolic demands. Light and heterogeneous expression of vWF in endothelial cells of newborns/infants indicates that differentiation of microvasculature in the human medullary tegmentum extends beyond fetal stages.

Interaction between morphology and angiogenesis in human early colorectal cancers

Morphologically, early colorectal cancers are divided into two types: polypoid cancers and non-polypoid cancers. They vary in growth pattern, progression, and genetic alteration. Angiogenesis between polypoid and non-polypoid cancers may also be different. Therefore, the present study aims to evaluate angiogenesis in the early stages of colorectal malignancy, with particular attention to the morphological differences. The serial slides of all materials (48 polypoid cancers, 10 non-polypoid cancers, 20 adenomas and 10 normal tissues) were immunohistochemically stained for three endothelial cell markers (CD31, von Willebrand factor and CD105), counted for the number of microvessels in the same hot spots, and the angiogenic status was estimated. Polypoid cancers had higher microvessel counts and were more predominantly supplied by activated (CD105-positive, newly forming) microvessels than non-polypoid cancers. The present study indicated the possibility that the difference in growth pattern might be explained by the difference in angiogenesis between polypoid and non-polypoid cancers.

Culture of large vessel endothelial cells on floating collagen gels promotes a phenotype characteristic of endothelium in vivo

The vascular endothelium in vivo is a remarkably quiescent cell layer that displays a highly differentiated and tissue-specific phenotype. Once established in culture, endothelial cells (EC) are phenotypically different from their in situ counterparts, displaying altered gene expression, increased mitotic index, and decreased cell density. To determine whether manipulating the microenvironment of cells in vitro would lead to a more differentiated phenotype, we cultured bovine aortic EC on floating collagen gels. EC cultured to confluence on floating gels for 24 or 48 hr display mitotic indices nearly identical to those of quiescent endothelium in vivo, nearly two log orders lower than that of EC cultured to confluence on plastic, and cell density on floating gels also resembles that observed for endothelium in vivo. Culture of EC on floating gels leads to decreased expression of platelet-derived growth factor-B, fibronectin, and fibronectin isoform ED-B, and increased levels of connexin40, relative to cells cultured on plastic. We conclude that culture of bovine aortic EC under standard culture conditions results in a phenotype reminiscent of development and/or wound healing, and that culturing them on a floating collagen gel leads to a more differentiated phenotype, reminiscent of that observed for large vessel EC in vivo.

Angiogenesis in wound healing

During wound healing, angiogenic capillary sprouts invade the fibrin/fibronectin-rich wound clot and within a few days organize into a microvascular network throughout the granulation tissue. As collagen accumulates in the granulation tissue to produce scar, the density of blood vessels diminishes. A dynamic interaction occurs among endothelial cells, angiogenic cytokines, such as FGF, VEGF, TGF-beta, angiopoietin, and mast cell tryptase, and the extracellular matrix (ECM) environment. Specific endothelial cell ECM receptors are critical for these morphogenetic changes in blood vessels during wound repair. In particular, alpha(v)beta3, the integrin receptor for fibrin and fibronectin, appears to be required for wound angiogenesis: alpha(v)beta3 is expressed on the tips of angiogenic capillary sprouts invading the wound clot, and functional inhibitors of alpha(v)beta3 transiently inhibit granulation tissue formation. Recent investigations have shown that the wound ECM can regulate angiogenesis in part by modulating integrin receptor expression. mRNA levels of alpha(v)beta3 in human dermal microvascular endothelial cells either plated on fibronectin or overlaid by fibrin gel were higher than in cells plated on collagen or overlaid by collagen gel. Wound angiogenesis also appears to be regulated by endothelial cell interaction with the specific three-dimensional ECM environment in the wound space. In an in vitro model of human sprout angiogenesis, three-dimensional fibrin gel, simulating early wound clot, but not collagen gel, simulating late granulation tissue, supported capillary sprout formation. Understanding the molecular mechanisms that regulate wound angiogenesis, particularly how ECM modulates ECM receptor and angiogenic factor requirements, may provide new approaches for treating chronic wounds.

Selenium-induced inhibition of angiogenesis in mammary cancer at chemopreventive levels of intake

The trace element nutrient selenium (Se) has been shown to possess cancer-preventive activity in both animal models and humans, but the mechanisms by which this occurs remain to be elucidated. Because angiogenesis is obligatory for the genesis and growth of solid cancers, we investigated, in the study presented here, the hypothesis that Se may exert its cancer-preventive activity, at least in part, by inhibiting cancer-associated angiogenesis. The effects of chemopreventive levels of Se on the intra-tumoral microvessel density and the expression of vascular endothelial growth factor in 1-methyl-1-nitrosourea-induced rat mammary carcinomas and on the proliferation and survival and matrix metalloproteinase activity of human umbilical vein endothelial cells in vitro were examined. Increased Se intake as Se-enriched garlic, sodium selenite, or Se-methylselenocysteine led to a significant reduction of intra-tumoral microvessel density in mammary carcinomas, irrespective of the manner by which Se was provided: continuous exposure (7-wk feeding) with a chemoprevention protocol or acute bolus exposure (3 d) after carcinomas had established. Compared with the untreated controls, significantly lower levels of vascular endothelial growth factor expression were observed in a sizeable proportion of the Se-treated carcinomas. In contrast to the mammary carcinomas, the microvessel density of the uninvolved mammary glands was not altered by Se treatment. In cell culture, direct exposure of human umbilical vein endothelial cells to Se induced cell death predominantly through apoptosis, decreased the gelatinolytic activities of matrix metalloproteinase-2, or both. These results indicate a potential for Se metabolites to inhibit key attributes (proliferation, survival, and matrix degradation) of endothelial cells critical for angiogenic sprouting. Therefore, inhibition of angiogenesis associated with cancer may be a novel mechanism for the anticancer activity of Se in vivo, and multiple mechanisms are probably involved in mediating the anti-angiogenic activity.

EMAP II: a modulator of neovascularization in the developing lung

Neovascularization is a key regulatory process in fetal growth and development. Although factors promoting growth and development of the pulmonary vasculature have been investigated, nothing is known regarding the molecular mechanisms that may counteract these stimuli. Endothelial monocyte-activating polypeptide (EMAP) II has recently been identified as an antiangiogenic factor in tumor vascular development. We postulated that EMAP II is a putative negative modulator of lung vascular growth. EMAP II mRNA and protein decrease fivefold (P < 0.01) as the developing lungs in the fetal mouse progress from having poor vascularization (day 14) to having complete vascular development at term (day 18.5). EMAP II protein expression continues to remain low throughout postnatal life and into adulthood, with the exception of a surge that correlates with microvascular maturation. Furthermore, through the use of in situ hybridization and immunohistochemistry, EMAP II is localized throughout the lung, with significant expression in the submyoepithelial area during the early stages of lung development when there is minimal vascular development. In contrast, EMAP II is distributed around the large vessels during the end of vascular development, suggesting that EMAP II modulates the neovascularization process. We speculate that EMAP II is a director of neovascularization in the developing lung.

Autoregulation of angiogenesis by cells of the vessel wall

The cells of the vessel wall can regulate angiogenesis by producing growth factors, proteolytic enzymes, extracellular matrix components, cell adhesion molecules, and vasoactive factors. This property enables preexisting blood vessels to generate new vessels in the absence of exogenous angiogenic stimuli. Vascular autoregulation of angiogenesis can be studied by culturing rat aortic or venous explants in collagen gels under serum-free conditions. In this system, the combined effect of injury and exposure of explants to collagen triggers a self-limited angiogenic response. Interactions among endothelial cells, smooth muscle cells, and fibroblasts play a critical role in the regulation of this process. This chapter reviews the literature on angiogenesis, focusing on the vessel wall as a highly specialized and plastic tissue capable of regenerating itself through autocrine, paracrine, and juxtacrine mechanisms.

Monoclonal antibody against adult marrow-derived mesenchymal stem cells recognizes developing vasculature in embryonic human skin

We have described previously a monoclonal antibody (SH2) that specifically recognizes undifferentiated mesenchymal progenitor cells isolated from adult human bone marrow. These cells, which we operationally refer to as mesenchymal stem cells, have the capacity to differentiate and form distinct mesenchymal tissues such as bone and cartilage when the isolated cells are placed in the appropriate in vivo or in vitro environment. We report here the partial biochemical characterization of the antigen recognized by the SH2 antibody. Metabolically radiolabelled adult marrow-derived mesenchymal stem cells in culture were extracted and immunoprecipitated with the SH2 antibody. The purified antigen migrated as a single band of 90 kDa after sodium dodecyl sulfate polyacrylamide gel electrophoresis was performed under reducing conditions. The SH2-immunoprecipitated protein exhibited a molecular weight band shift after removal of N-linked oligosaccharides. We investigated the expression of the SH2 antigen, along with the endothelial markers factor VIII-related antigen and Ulex europaeus I (UEA-I) lectin during specific developmental periods in human dermal embryogenesis and in the postnatal period through aged adults. Frozen sections of human embryonic, fetal, or postnatal skin ranging from 8 weeks estimated gestational age (EGA) through 84 years of age were immunostained or double immunolabelled with antibodies SH2, UEA-I, or factor VIII-related antigen followed by second antibodies with fluorescent markers. Positive cell surface reactivity with the SH2 antibody was seen in cells in the vascular plane in the earliest specimens (day 55 EGA) corresponding to the late cellular dermis period. During the period of the cellular to fibrous transition, in which the initiation of appendage development occurs, most SH2-reactive cells colocalized with vasculature markers UEA-I and factor VIII-related antigen, although there was a subset of cells recognized by SH2 antibody that did not colocalize with the endothelial markers. In contrast to the endothelial markers UEA-I and factor VIII-related antigen, in which the number of immunopositive cells became more prominent with age and maturation of the dermis, the frequency of cells that contained the SH2-reactive antigen diminished with age. The SH2 reactivity evident in embryonic, fetal, and early postnatal periods was not observed in human skin specimens taken from adults greater than 30 years old. These observations support the hypothesis that the SH2 antigen is a cell surface marker of developing microvasculature and may play a role in dermal embryogenesis and angiogenesis.

Confocal microscopic analysis of integrin expression on the microvasculature and its sprouts in the neonatal foreskin

Members of the integrin family of adhesion receptors are essential participants in blood vessel growth and remodeling. It is not known which integrins are involved in the initial stages of angiogenesis in vivo. In this study we determined the location of integrins on the blood vessels of a growing tissue, the neonatal foreskin, in which neovascularization is likely to occur. We used the confocal microscope to visually reconstruct vessels from the papillary dermis of the foreskin and to identify potential sprouts as narrow, tapering extensions from these vessels. Blood vessels were initially identified by their positive reaction with antibodies to von Willebrand factor or human platelet endothelial cell adhesion molecule and their negative response to anti-neurofilament antibodies. Later, vessels were identified by their shape and location. We screened vessels with anti bodies to integrin subunits alpha 1, alpha 2, alpha 3, alpha 5, alpha 6, alpha v, beta 1, beta 3 and beta 4. We found that integrin subunits alpha 6 and beta 4 were consistently found along the whole length of capillary loops and extended to the distal ends of presumed sprouts. The alpha 2 and alpha v integrin concentrations, which are normally low in the microvasculature, were increased on the sprouts. alpha 5 was either absent from vessels entirely or more concentrated on the body than on the sprout. alpha 1 was more commonly present on nerves than blood vessels. These studies suggest an important role for the alpha 6 beta 4 integrin in the initial stages of endothelial outmigration during new vessel growth.

The expression of transforming growth factor type beta in fetal and adult rabbit skin wounds

Transforming growth factor, subtype beta (TGF-beta) exists in several isoforms and is known to have important roles in adult wound healing by promoting collagen and extracellular matrix component deposition. It is also believed that TGF-beta influences normal developmental processes during embryo-genesis. Immunolocalization of two isoforms, TGF-beta 1 and TGF-beta 2, in healing fetal and adult rabbit skin wounds shows distinctly different forms of expression of these molecules. TGF-beta 1 and TGF-beta 2 are both expressed within the developing fetal dermis, but no differential upregulation in the area of the healing wound is noted. In contrast, the expression of TGF-beta 1 and TGF-beta 2 is increased in adult wounds by day 7 after wounding, within macrophages that are abundant by this time. High levels of TGF-beta 1 and TGF-beta 2 within adult wounds might indicate that the relative paucity and differential distribution of these factors in fetal wounds are important in the production of scar in adults and the absence of scar in the fetus. Further, these patterns of expression suggest fundamental differences between fetal and adult tissues in accomplishing wound repair.

Expression of type XIV collagen during the differentiation of fetal bovine skin: immunolabeling with monoclonal antibody is prominent in morphogenetic areas

Type XIV collagen belongs to the subclass of fibril-associated collagens with interrupted triple helices, which are composed of alternative triple helical and non-collagenous domains. Structural data show that these molecules interact with collagen fibrils and suggest that they might interact with cells. We have investigated the expression of type XIV collagen in bovine skin during development. Fetuses from 9 to 37 weeks were examined. Anti-type XIV collagen monoclonal antibody was produced, characterized, and used for immunofluorescence detection of the molecule. The localization of immunolabeling was analyzed by comparison with light and electron microscopic observations. In 9-week-old fetus, no type XIV collagen was found in the skin. From 19 weeks to birth, extensive immunofluorescence was observed on bundles of collagen fibrils in deep dermis. As shown by electron microscopy, this area exhibited bundles of collagen fibrils and cells with an abundant rough endoplasmic reticulum. In the upper dermis, a delicate fibrillar network of type XIV collagen was revealed by immunofluorescence around growing hair follicles at 19 and 24 weeks. Double labeling for type XIV collagen and fibronectin shows a more restricted pattern of expression of type XIV collagen in this area. The electron microscopic examination of skin of fetuses at these stages shows that the whole upper dermis is composed by a loose connective tissue containing scattered small bundles of collagen fibrils. Type XIV collagen was synthesized in the upper dermis between 24 weeks and birth. From this study, it appears that type XIV collagen expression is distinct from that of fibrillar collagens, at least during some developmental events. The prominent localization of type XIV collagen around growing hair follicles suggests a role for this molecule in epithelial-mesenchymal interactions.

Fibronectin promotes the elongation of microvessels during angiogenesis in vitro

Fibronectin is a component of the extracellular matrix of developing microvessels whose role in angiogenesis is poorly understood. This study evaluated the effect of plasma fibronectin on angiogenesis in serum-free collagen gel culture of rat aorta. Aortic explants embedded in collagen gels generated microvascular outgrowths. Fibronectin incorporated in the collagen gel promoted a selective dose-dependent elongation of the newly formed microvessels without stimulating vascular proliferation. The fibronectin-treated microvessels were longer due to a proportional increase in the number of microvascular cells. However, fibronectin had no effect on microvascular DNA synthesis and mitotic activity. Fibronectin stimulated microvascular length also in cultures in which mitotic activity was suppressed and angiogenesis was markedly reduced by pretreating the aortic explants with mitomycin C. The synthetic peptide Gly-Arg-Gly-Asp-Ser (GRGDS), which competes for the binding of fibronectin to its cell receptors and inhibits the adhesion of endothelial cells to substrates, arrested the elongation of developing microvessels causing regression and inhibition of angiogenesis. Conversely, Gly-Arg-Gly-Glu-Ser (GRGES), which lacks the RGD sequence, had no inhibitory effect. These data support the hypothesis that fibronectin promotes angiogenesis and suggest that developing microvessels elongate in response to fibronectin as a result of an adhesion-dependent migratory recruitment of endothelial cells that does not require increased cell proliferation.

T cell adhesion to extracellular matrix molecules secreted by endothelial cells cultured on a substrate of type IV collagen

T cell emigrating from the bloodstream into lymphoid organs or sites of inflammation in the connective tissue must adhere to, and traverse, the subendothelial basement membrane (BM). The goal of the current investigation was to develop a method to study the adhesion of T cells to endothelial cell (EC)-derived extracellular matrix (ECM) as a model for the interaction of T cells with the subendothelial BM in vivo. To be certain that we were truly measuring T cell adhesion to ECM molecules secreted by the EC, it was necessary to culture the EC on a substrate to which T cells could not attach. Non-tissue culture-treated microtiter plate wells which had been coated with type IV collagen (tIVC), a major constituent of BM in vivo, were found to be suitable for this purpose since EC, but very few T cells, adhered to such wells. After incubating the EC on a substrate of tIVC in non-treated wells for a period of 48 h, the EC were gently removed from their underlying ECM and T cell adhesion to that ECM was examined. Using this system, it was observed that approximately 15-40% of human peripheral blood T cells specifically adhered to ECM molecules produced by the EC. This method should be useful as a model for the interactions of T cells and other leukocytes with the vascular BM in vivo.

Animal models for the study of fetal tissue repair

Recent experimental and clinical evidence suggests that the fetus responds to injury in a fashion fundamentally different from that of the adult. Acute inflammation is almost always absent, hyaluronic acid is a prominent component of the wound matrix, and collagen is deposited in a scarless manner. Using a variety of animal models and techniques, numerous investigators have begun to analyze the constituents of the fetal wound healing process in an attempt to understand the control mechanisms that endow the fetus with unique healing abilities. Since scarring and fibrosis dominate some diseases in almost every medical specialty, the ultimate clinical aim is to delineate the biological principles of fetal wound healing and then apply them to modulate adult wound healing problems.

Brain angiogenesis: variations in vascular basement membrane glycoprotein immunoreactivity

Changes in the distribution and quantity of laminin and fibronectin within the basement membranes of developing or regenerating CNS blood vessels were investigated using two immunocytochemical techniques. Three models of angiogenesis were studied: normal pre- and postnatal development, wound healing, and vascularization of fetal neocortical transplants placed in the adult rat brain. Although all brain vessels were stained in enzymatically pretreated immunoreacted paraffin sections, those associated with wound and transplant sites were the most intensely reactive with both antisera during the first postoperative week. When 40-microns vibratome sections of normal adult brains were immunoprocessed, only the meninges and vessels of the circumventricular organs were stained. The remainder of the brain vasculature was immunoreactive only if sections were enzymatically treated prior to immunoprocessing. In contrast, the nascent vasculature in developing brain and the regenerating vessels at wound and transplant sites were reactive to both antisera without enzymatic pretreatment of the sections. This immunoreactivity decreased by 11 days postnatal in normal animals and 4 weeks postoperative in experimental animals, coinciding with the period of astrocytic contact and complete vascular basement membrane formation in both cases. The variations in staining pattern and intensity may be reflections of differences in the quantity of laminin and fibronectin within the basement membranes of proliferating and/or non-blood-brain barrier vasculature. However, the results of the different experimental protocols suggest that immature vascular basement membranes may have a molecular configuration that does not require an enzymatic unmasking step to react with the antisera. Alternatively, the looseness of the surrounding neuropil inherent in developing and injured CNS could allow the antisera greater access to basement membrane antigens.

An immunoradiometric assay for von Willebrand factor antigen to assess angiogenesis

A method of determining the degree of vascularization of tissues would be of great value in dermatological research such as the investigation of neoangiogenesis in wounds and in the study of the vascular abnormality in psoriasis. We describe an adaptation of an immunoradiometric assay (IRMA) for von Willebrand factor antigen for this purpose. The results show that in endothelial cells in culture there was a good relationship between the number of cells and von Willebrand factor antigen measured by the IRMA. The relationships between the numbers of endothelial cells in biopsies of normal and healing wound skin as estimated by the histometric technique and the IRMA are less good (r = 0.45 and 0.32, respectively) but suggest that there is a positive and proportionate variation. There is a much stronger relationship (r = 0.79) between the biochemical and histometric methods in psoriatic skin.

Microvascular changes in early and advanced dermatomyositis: a quantitative study

In adult dermatomyositis 10 muscle specimens with no or minimal histological alterations were compared with 7 that showed typical alterations. Five specimens from patients with inclusion body myositis, 5 from patients with polymyositis, and 8 from normal subjects served as controls. Vascular endothelium, visualized with the lectin Ulex europaeus agglutinin I, and complement membrane attack complex were demonstrated in the same cryostat sections by paired immunofluorescence. Large randomly selected fields were analyzed to determine the number of capillaries per square millimeter of fiber area (capillary density), per 1,000-microns 2 area of each muscle fiber (capillary index), and in 100 x 100-microns grid squares. In dermatomyositis specimens with minimal structural alterations there was focal capillary depletion, the capillary density was significantly reduced, and the frequency distributions of the capillary index and grid count were shifted to the left. In advanced dermatomyositis specimens, the findings were similar but more severe. In both kinds of specimens, clusters of capillaries reacted for complement membrane attack complex. The 2 patients with the highest proportion of vessels positive for membrane attack complex had a fulminant and fatal course. In polymyositis and inclusion body myositis specimens, the capillaries had a normal overall density and none reacted for membrane attack complex. The findings imply that the capillaries are an early and specific target of the disease process in dermatomyositis.

Modulation of microvascular growth and morphogenesis by reconstituted basement membrane gel in three-dimensional cultures of rat aorta: a comparative study of angiogenesis in matrigel, collagen, fibrin, and plasma clot

Rings of rat aorta cultured in Matrigel, a reconstituted gel composed of basement membrane molecules, gave rise to three-dimensional networks composed of solid cellular cords and occasional microvessels with slitlike lumina. Immunohistochemical and ultrastructural studies showed that the solid cords were composed of endothelial sprouts surrounded by nonendothelial mesenchymal cells. The angiogenic response of the aortic rings in Matrigel was compared to that obtained in interstitial collagen, fibrin, or plasma clot. Morphometric analysis demonstrated that the mean luminal area of the microvascular sprouts and channels was significantly smaller in Matrigel than in collagen, fibrin, or plasma clot. The percentage of patent microvessels in Matrigel was also markedly reduced. Autoradiographic studies of 3H-thymidine-labeled cultures showed reduced DNA synthesis by developing microvessels in Matrigel. The overall number of solid endothelial cords and microvessels was lower in Matrigel than in fibrin or plasma clot. A mixed cell population isolated from Matrigel cultures formed a monolayer in collagen or fibrin-coated dishes but rapidly reorganized into a polygonal network when plated on Matrigel. The observation that gels composed of basement membrane molecules modulate the canalization, proliferation, and organization into networks of vasoformative endothelial cells in three-dimensional cultures supports the hypothesis that the basement membrane is a potent regulator of microvascular growth and morphogenesis.

Studies in fetal wound healing: III. Early deposition of fibronectin distinguishes fetal from adult wound healing

Wound healing in the fetus proceeds through a series of steps that differ in the fetus and the adult. At each phase of this complex process, there is signaling between the tissue cells and the wound microenvironment, signals that are mediated by and through the extracellular matrix. We postulate that these signals occur earlier in fetal wounds, resulting in more rapid repair. To investigate this, we compared the first 24 hours of wound healing in the rabbit fetus and adult, using antibodies against key extracellular matrix macromolecular components: laminin, fibronectin, and type-specific collagens I, III, IV, and V. Fibronectin was the first matrix component to be deposited, and was visualized as early as four hours after fetal wounding and 12 hours after adult wounding. There was no evidence of new laminin or collagen deposition in either the fetal or adult wounds at any time point examined. The early deposition of fibronectin, a matrix adhesion molecule that provides a scaffolding for epithelial migration, may underlie the rapid reepithelialization observed in fetal wounds.

Development of human embryonic and fetal dermal vasculature

This report summarizes recent advances in the understanding of the structure and organization of the microvasculature in developing human skin. Previous observations suggested that the skin contains no blood vessels as late as eight weeks estimated gestational age (EGA). Computer reconstructions, in conjunction with light and transmission electron microscopy (TEM), however, demonstrated that specimens as young as 35-45 d show a level of vascular complexity previously unknown. The computer reconstructions showed that the vasculature was organized in one or two planes parallel to the epidermis. A simple, single plane was evident in specimens 40-50 d EGA, whereas specimens 50-75 d EGA showed two planes. Fewer vessels were continuous throughout the tissue sample in the younger specimens compared with the older specimens. Superior views highlighted the continuities and connections of vessels. The younger specimens showed more discontinuous segments of vessels when compared with the network established in the older specimens. In the earliest specimens examined morphologically (35-40 d), simple, capillarylike vessels were morphologically identifiable in presumptive dermis. The samples studied by TEM revealed detailed structure of the vessel wall including extreme attenuations and projections, plasmalemmal vesicles, and junctions similar to adult endothelial cells. Little or no basal lamina surrounded the vessel. The basal lamina first appeared in the form of amorphous deposits and eventually thickened and became continuous. By the end of the first trimester, the basal lamina still lacked the organization of adult cutaneous arterial and venous segments. These findings suggest that the major vascular organization of the dermis is defined in the first trimester of development.

Laminin: molecular organization and biological function

Laminin, the most abundant glycoprotein molecule found in basement membrane, has multiple functions in eukaryotic tissues. It serves to attach epithelial cells to basement membrane, aids development and migration of specific cell types in growth and maturation, and has been implicated in tumor metastasis and some types of infection. Current concepts of the molecular organization and myriad functions of the laminin molecule are reviewed.

The relationship between von Willebrand factor antigen and fibronectin in human plasma, endothelial cells and fibroblasts in culture

The distribution of von Willebrand factor antigen (vWFAg) and fibronectin (Fn) in endothelial cells and in fibroblasts in culture was examined using immunohistological methods. Extracellular matrices were studied after removal of cells by mild hypotonic lysis. Co-distribution of these antigens was also examined after culture of fibroblasts in the presence of endothelial cell conditioned medium or of exogenous vWFAg. The presence of intracellular vWFAg and its association with extracellular Fn in confluent endothelial cells was confirmed. Furthermore, both antigens were detected in matrices from fibroblasts grown in the presence of human plasma and endothelial cell medium. vWFAg was not found, however, in the absence of endothelial cell conditioned medium or in experiments using human serum in place of plasma. Cross-linking of vWFAg was examined using quantitative and qualitative electrophoretic methods. Levels of vWFAg in serum from patients with severe Haemophilia A were approximately 50% of those found in the corresponding plasma. Furthermore, vWFAg was reduced to similar levels in serum from normal blood to which heparin was added. The lowest levels of vWFAg were found in the presence of sufficient heparin to totally inhibit the formation of fibrin. In contrast, levels of Fn remained unchanged in these circumstances. The results did not support the view that vWFAg was cross-linked to Fn in plasma by thrombin-activated factor XIII. In addition, immunoelectrophoresis of cultured endothelial cell products did not demonstrate cross-linking of vWFAg to Fn. The data are consistent with the concept that deposition of vWFAg on the subendothelium is dependent on viable endothelial cells or on another product of these cells.

Basal cell carcinomas grown in nude mice produce and deposit fibronectin in the extracellular matrix

Epidermal cells in vitro produce and deposit fibronectin (FN) in the pericellular matrix. Such FN production by epidermal cells may be involved in vivo in wound reepithelialization, tissue morphogenesis, and growth of epithelial tumors. The purpose of this study was to examine whether the FN, previously shown to be within and surrounding human basal cell carcinoma (BCC) lobules, was in part the product of epidermal-derived tumor cells. To examine this question we took advantage of our ability to grow human BCC in nude mice. Since we could demonstrate that all stromal cells surrounding the BCC were of mouse origin, antibodies specific for human FN would distinguish epithelial-derived FN from mesenchymal-derived FN. Five solid BCCs were implanted subcutaneously in nude mice. Growing tumors were removed after 60 days, snap-frozen, sectioned on a cryostat, and verified microscopically as BCC. The Hoescht DNA stain, which can distinguish mouse and human nuclei, demonstrated that mouse, not human, fibroblasts occupied the stroma surrounding each tumor lobule. Sections of all 5 BCCs were stained by immunofluorescence and immunoperoxidase techniques with antibodies to bullous pemphigoid (BP) antigen, laminin (LM), and FN. BP antigen and LM were present at the basement membrane zone (BMZ) of all tumor lobules as previously described for in situ BCC. FN staining was present along the BMZ, within the tumor lobules, and in the surrounding stroma. Antibodies to human FN were passed over a mouse FN affinity column to absorb antibodies which cross-reacted with mouse FN. The resultant antibody preparation, which was specific for human FN in this system, continued to demonstrate FN along the BMZ and within the tumors, but failed to stain FN in more distant stroma. Epidermal-derived cells, therefore, can synthesize and deposit FN in vivo in adjacent extracellular matrix. We speculate that this FN matrix may facilitate growth of BCC in this model.