Mast-cell-mediated angiogenesis: a novel experimental model using the rat mesentery

On Connective Tissue Mast Cells as Protectors of Life, Reproduction, and Progeny

The connective tissue mast cell (MC), a sentinel tissue-residing secretory immune cell, has been preserved in all vertebrate classes since approximately 500 million years. No physiological role of the MC has yet been established. Considering the power of natural selection of cells during evolution, it is likely that the MCs exert essential yet unidentified life-promoting actions. All vertebrates feature a circulatory system, and the MCs interact readily with the vasculature. It is notable that embryonic MC progenitors are generated from endothelial cells. The MC hosts many surface receptors, enabling its activation via a vast variety of potentially harmful exogenous and endogenous molecules and via reproductive hormones in the female sex organs. Activated MCs release a unique composition of preformed and newly synthesized bioactive molecules, like heparin, histamine, serotonin, proteolytic enzymes, cytokines, chemokines, and growth factors. MCs play important roles in immune responses, tissue remodeling, cell proliferation, angiogenesis, inflammation, wound healing, tissue homeostasis, health, and reproduction. As recently suggested, MCs enable perpetuation of the vertebrates because of key effects-spanning generations-in ovulation and pregnancy, as in life-preserving activities in inflammation and wound healing from birth till reproductive age, thus creating a permanent life-sustaining loop. Here, we present recent advances that further indicate that the MC is a specific life-supporting and progeny-safeguarding cell.

Estimation of shear stress heterogeneity along capillary segments in angiogenic rat mesenteric microvascular networks

OBJECTIVE

Fluid shear stress is thought to be a regulator of endothelial cell behavior during angiogenesis. The link, however, requires an understanding of stress values at the capillary level in angiogenic microvascular networks. Critical questions remain. What are the stresses? Do capillaries experience similar stress magnitudes? Can variations explain vessel-specific behavior? The objective of this study was to estimate segment-specific shear stresses in angiogenic networks.

METHODS

Images of angiogenic networks characterized by increased vascular density were obtained from rat mesenteric tissues stimulated by compound 48/80-induced mast cell degranulation. Vessels were identified by perfusion of a 40 kDa fixable dextran prior to harvesting and immunolabeling for PECAM. Using a network flow-based segment model with physiologically relevant parameters, stresses were computed per vessel for regions across multiple networks.

RESULTS

Stresses ranged from 0.003 to 2328.1 dyne/cm2 and varied dramatically at the capillary level. For all regions, the maximum segmental shear stresses were for capillary segments. Stresses along proximal capillaries branching from arteriole inlets were increased compared to stresses along capillaries in more distal regions.

CONCLUSIONS

The results highlight the variability of shear stresses along angiogenic capillaries and motivate new discussions on how endothelial cells may respond in vivo to segment-specific microenvironment during angiogenesis.

Angiogenic Microvascular Wall Shear Stress Patterns Revealed Through Three-dimensional Red Blood Cell Resolved Modeling

The wall shear stress (WSS) exerted by blood flowing through microvascular capillaries is an established driver of new blood vessel growth, or angiogenesis. Such adaptations are central to many physiological processes in both health and disease, yet three-dimensional (3D) WSS characteristics in real angiogenic microvascular networks are largely unknown. This marks a major knowledge gap because angiogenesis, naturally, is a 3D process. To advance current understanding, we model 3D red blood cells (RBCs) flowing through rat angiogenic microvascular networks using state-of-the-art simulation. The high-resolution fluid dynamics reveal 3D WSS patterns occurring at sub-endothelial cell (EC) scales that derive from distinct angiogenic morphologies, including microvascular loops and vessel tortuosity. We identify the existence of WSS hot and cold spots caused by angiogenic surface shapes and RBCs, and notably enhancement of low WSS regions by RBCs. Spatiotemporal characteristics further reveal how fluctuations follow timescales of RBC "footprints." Altogether, this work provides a new conceptual framework for understanding how shear stress might regulate EC dynamics in vivo.

Mast cell-mediated immune regulation in health and disease

Mast cells are important components of the immune system, and they perform pro-inflammatory as well as anti-inflammatory roles in the complex process of immune regulation in health and disease. Because of their strategic perivascular localization, sensitivity and adaptability to the microenvironment, and ability to release a variety of preformed and newly synthesized effector molecules, mast cells perform unique functions in almost all organs. Additionally, Mast cells express a wide range of surface and cytoplasmic receptors which enable them to respond to a variety of cytokines, chemicals, and pathogens. The mast cell's role as a cellular interface between external and internal environments as well as between vasculature and tissues is critical for protection and repair. Mast cell interactions with different immune and nonimmune cells through secreted inflammatory mediators may also turn in favor of disease promoting agents. First and forefront, mast cells are well recognized for their multifaceted functions in allergic diseases. Reciprocal communication between mast cells and endothelial cells in the presence of bacterial toxins in chronic/sub-clinical infections induce persistent vascular inflammation. We have shown that mast cell proteases and histamine induce endothelial inflammatory responses that are synergistically amplified by bacterial toxins. Mast cells have been shown to exacerbate vascular changes in normal states as well as in chronic or subclinical infections, particularly among cigarette smokers. Furthermore, a potential role of mast cells in SARS-CoV-2-induced dysfunction of the capillary-alveolar interface adds to the growing understanding of mast cells in viral infections. The interaction between mast cells and microglial cells in the brain further highlights their significance in neuroinflammation. This review highlights the significant role of mast cells as the interface that acts as sensor and early responder through interactions with cells in systemic organs and the nervous system.

Discovery and Development of Tumor Angiogenesis Assays

The angiogenesis process was described in its basic concepts in the works of the Scottish surgeon John Hunter and terminologically assessed in the early twentieth century. An aberrant angiogenesis is a prerequisite for cancer cells in solid tumors to grow and metastasize. The sprouting of new blood vessels is one of the major characteristics of cancer and represents a gateway for tumor cells to enter both the blood and lymphatic circulation systems. In vivo, ex vivo, and in vitro models of angiogenesis have provided essential tools for cancer research and antiangiogenic drug screening. Several in vivo studies have been performed to investigate the various steps of tumor angiogenesis and in vitro experiments contributed to dissecting the molecular bases of this phenomenon. Moreover, coculture of cancer and endothelial cells in 2D and 3D matrices have contributed to improve the recapitulation of the complex process of tumor angiogenesis, including the peculiar conditions of tumor microenvironment.

Do Mast Cells Contribute to the Continued Survival of Vertebrates?

This study is an attempt to shed light on why the connective tissue mast cell (MC) is preserved in all species with a blood circulatory system, i.e., the vertebrates since >500 million years, which suggests that the MC performs as yet not understood indispensible life‐promoting actions. The literature survey focuses on data in published papers on MC functions in immunological and nonimmunological reactions, host protection, pregnancy, inflammation, and wound healing. All data are thus accessible to the reader. The MC is a secretory cell with a unique mediator profile. A distinctive role for MCs is defined not only by their extensive mediator composition but also by their prominent ability to affect the vasculature to expedite selective cell recruitment and permeability changes and to set the stage for an appropriate acquired response. MCs, harboring a wide range of surface membrane receptors, are activated by the major female sex hormones as well as by diverse potentially adverse stimuli. MC activation/degranulation creates a presumably unique triad tissue response in physiological and pathological situations alike: extracellular matrix degradation and tissue remodeling, de novo cell proliferation, and de novo angiogenesis. As shown in the literature, MC‐activation is crucial for successful female reproduction in the mouse, implying one of possibly several yet unidentified physiological roles of MCs. Moreover, the activated MC aids newborns to survive to reproductive age owing to its key beneficial actions in inflammation and wound healing. Thus, a not previously described life‐perpetuating loop spanning generations are apparently formed, which, hypothetically, could contribute to the continued survival of the vertebrates.

Microvascular Experimentation in the Chick Chorioallantoic Membrane as a Model for Screening Angiogenic Agents including from Gene-Modified Cells

The chick chorioallantoic membrane (CAM) assay model of angiogenesis has been highlighted as a relatively quick, low cost and effective model for the study of pro-angiogenic and anti-angiogenic factors. The chick CAM is a highly vascularised extraembryonic membrane which functions for gas exchange, nutrient exchange and waste removal for the growing chick embryo. It is beneficial as it can function as a treatment screening tool, which bridges the gap between cell based in vitro studies and in vivo animal experimentation. In this review, we explore the benefits and drawbacks of the CAM assay to study microcirculation, by the investigation of each distinct stage of the CAM assay procedure, including cultivation techniques, treatment applications and methods of determining an angiogenic response using this assay. We detail the angiogenic effect of treatments, including drugs, metabolites, genes and cells used in conjunction with the CAM assay, while also highlighting the testing of genetically modified cells. We also present a detailed exploration of the advantages and limitations of different CAM analysis techniques, including visual assessment, histological and molecular analysis along with vascular casting methods and live blood flow observations.

Adipocytes, mast cells and angiogenesis

Healthy adipose tissue contains a wide variety of innate and adaptive immune cells, including macrophages, dendritic cells, mast cells, eosinophils, neutrophils, and lymphocytes. Numerous signaling molecules in the adipose microenvironment can positively or negatively modulate angiogenic processes, regulate the interaction between the vascular system and adipocytes, and participate in tumor progression. Mast cells are involved in the new formation or metabolism of fat, are present in abundant quantities in fatty tissue, among fat cells, and a number of mediators released from mast cells play a role in adipogenesis. Moreover, mast cells produce several pro-angiogenic factors and are involved in tumor angiogenesis. In this context, the angiogenic effect might be amplified when the adipocytes and mast cells act in concert, and treatment of adipose tissue- and mast cell-associated cancers with anti-angiogenic drugs may represent an alternative or adjuvant strategy for the treatment of these tumors.

Mast Cells and Angiogenesis in Human Plasma Cell Malignancies

Bone marrow angiogenesis plays an important role in the pathogenesis and progression of hematological malignancies. It is well known that tumor microenvironment promotes tumor angiogenesis, proliferation, invasion, and metastasis, and also mediates mechanisms of therapeutic resistance. An increased number of mast cells has been demonstrated in angiogenesis associated with hematological tumors. In this review we focused on the role of mast cells in angiogenesis in human plasma cell malignancies. In this context, mast cells might act as a new target for the adjuvant treatment of these tumors through the selective inhibition of angiogenesis, tissue remodeling and tumor-promoting molecules, permitting the secretion of cytotoxic cytokines and preventing mast cell-mediated immune suppression.

Establishment of an in ovo chick embryo yolk sac membrane (YSM) assay for pilot screening of potential angiogenic and anti-angiogenic agents

Angiogenesis, the process of new blood vessel formation from pre-existing vessels, is essential for growth and development. Development of drugs that can accelerate or decelerate angiogenesis in the context of various diseases requires appropriate preclinical screening. As angiogenesis involves complex cellular and molecular processes, in vivo studies are superior to in vitro investigations. Conventional in vitro, in vivo, and ex ovo models of angiogenesis are time consuming and tedious, and require sophisticated infrastructure for embryo culture. In the present study, we established an in ovo chick embryo yolk sac membrane (YSM) assay for angiogenesis and tested the angiogenic potential of arginine, conditioned medium (CM) from human adipose tissue and placenta-derived mesenchymal stem cells (ADMSCs-CM and PDMSCs-CM), avastin and vitamin C. The obtained results were confirmed with the routinely employed chick embryo Chorioallantoic Membrane (CAM) assay. Both assays revealed the pro-angiogenic nature of arginine, ADMSCs-CM, and PDMSCs-CM, and the anti-angiogenic effect of avastin and vitamin C. This novel in ovo YSM model is simple, reproducible, and highly economic in terms of the time frame and cost incurred. The proposed model is thus a suitable substitute to the CAM model for pilot screening of potential angiogenic and anti-angiogenic agents.

Cross talk between natural killer cells and mast cells in tumor angiogenesis

Natural killer (NK) cells are large granular lymphocytes of the innate immune system, responsible for direct targeting and killing of both virally infected and transformed cells. Under pathological conditions and during inflammation, NK cells extravasate into the lymph nodes and accumulate at inflammatory or tumor sites. The activation of NK cells depends on an intricate balance between activating and inhibitory signals that determines if a target will be susceptible to NK-mediated lysis. Many experimental evidences indicate that NK cells are also involved in several immunoregulatory processes and have the ability to modulate the adaptive immune responses. Many other important aspects about NK cell biology are emerging in these last years. The aim of this review is to elucidate the role of NK cells in tumor angiogenesis and their interaction with mast cells. In fact, it has been observed that NK cells produce pro-angiogenic factors and participate alone or in cooperation with mast cells to the regulation of angiogenesis in both physiological and pathological conditions including tumors.

A new role of mast cells in arteriogenesis

Arteriogenesis is defined as the growth of functional collateral arteries from pre-existing arterio-arteriolar anastomoses. The role of mast cells in arteriogenesis is largely unexplored. Recent evidences suggest that mast cells together with other inflammatory cells, including monocytes-macrophages, lymphocytes, NK cells and endothelial precursor cells (EPCs) may be involved in this process. This review article analyzes the literature concerning this new aspect of biological activity of mast cells.

Mouse Bone Marrow-Derived Mast Cells Induce Angiogenesis by Tissue Engineering in Rats: Histological Evidence

Objective

Therapeutic angiogenesis is employed to induce vascular network formation and improve functional recovery in ischemia. The aim of this study is to find an appropriate method to recover local ischemic conditions.

Materials and Methods

In this experimental survey, 20 male Wistar rats weighing approximately 200-250 g were randomly divided into four experimental groups respectively: ischemia group in which the femoral artery was transected; phosphate buffer solution group (PBS) in which the femoral artery transected location was immersed with PBS; chitosan (CHIT) group in which the transected location was immersed in a 50 µL CHIT solution; and mast cell transplanted group in which the transected location was immersed with a mixture of 50 µL CHIT and 50 µL PBS that contained 1×10⁶ mast cells.

Results

On day 14 after surgery, mean numbers of blood vessels of different sizes in the CHIT/mast cell group significantly increased compared to the other experimental groups (P<0.05).

Conclusion

Our data suggest that mast cell reconstitution could offer a new approach for therapeutic angiogenesis in cases of peripheral arterial diseases.

Aging is associated with impaired angiogenesis, but normal microvascular network structure, in the rat mesentery

A big problem associated with aging is thought to be impaired microvascular growth or angiogenesis. However, to link the evidence for impaired angiogenesis to microvascular dysfunction in aged tissues, we must compare adult vs. aged microvascular networks in unstimulated scenarios. The objective of this study was to test the hypothesis that aged microvascular networks are characterized by both fewer vessels and the impaired ability to undergo angiogenesis. Mesentery tissues from adult (9-mo) and aged (24-mo) male Fischer 344 rats were harvested and immunolabeled for platelet/endothelial cell adhesion molecule (an endothelial cell marker) according to two scenarios: unstimulated and stimulated. For unstimulated groups, tissues harvested from adult and aged rats were compared. For stimulated groups, tissues were harvested 3 or 10 days after compound 48/80-induced mast cell degranulation stimulation. Unstimulated aged microvascular networks displayed larger mean vascular area per tissue area compared with the unstimulated adult networks. The lack of a decrease in vessel density was supported at the gene expression level with RNA-Seq analysis and with comparison of vessel densities in soleus muscle. Following stimulation, capillary sprouting and vessel density were impaired in aged networks at 3 and 10 days, respectively. Our results suggest that aging associated with impaired angiogenesis mechanisms might not influence normal microvascular function, since unstimulated aged microvascular networks can display a "normal adult-like" vessel density and architecture.

NEW & NOTEWORTHY

Using a multidimensional approach, we present evidence supporting that aged microvascular networks display vessel density and patterning similar to adult networks despite also being characterized by a decreased capacity to undergo angiogenesis. Thus, vessel loss is not necessarily a characteristic of aging.

Tryptase, a novel angiogenic factor stored in mast cell granules

Human mast cells (MCs) are a rich reservoir of neutral proteases, packed in large amounts in their granules and comprising a high fraction of all cellular proteins. Among these proteases, tryptase is involved in angiogenesis after its release from activated MC granules, as it has been demonstrated in different in vitro and in vivo assays. Moreover, tryptase-positive MCs increase in number and vascularization increases in a linear fashion in different solid and hematological tumors. This complex interplay between MCs and tumor angiogenesis have led to consider the therapeutic use of angiogenesis inhibitors, which specifically target the angiogenic activity of tryptase, such as gabexate mesilate and nafamostat mesilate, two inhibitors of trypsin-like serine proteases.

Development of mast cells and importance of their tryptase and chymase serine proteases in inflammation and wound healing

Mast cells (MCs) are active participants in blood coagulation and innate and acquired immunity. This review focuses on the development of mouse and human MCs, as well as the involvement of their granule serine proteases in inflammation and the connective tissue remodeling that occurs during the different phases of the healing process of wounded skin and other organs. The accumulated data suggest that MCs, their tryptases, and their chymases play important roles in tissue repair. While MCs initially promote healing, they can be detrimental if they are chronically stimulated or if too many MCs become activated at the same time. The possibility that MCs and their granule serine proteases contribute to the formation of keloid and hypertrophic scars makes them potential targets for therapeutic intervention in the repair of damaged skin.

The role of inflammatory cells in angiogenesis in multiple myeloma

Both innate and adaptive immune cells are involved in the mechanisms of endothelial cell proliferation, migration and activation, via production and release of a large spectrum of pro-angiogenic mediators, thus creating the specific microenvironment that favors increased rate of tissue vascularization. In this article, we focus on the immune cell component of the angiogenic process occurring during multiple myeloma progression. We also provide information on some anti-angiogenic properties of immune cells that may be applied for a potential pharmacological use as anti-angiogenic agents in the disease treatment.

Metronomic chemotherapy and anti-angiogenesis: can upgraded pre-clinical assays improve clinical trials aimed at controlling tumor growth?

Metronomic chemotherapy, which is continuously administered systemically at close to non-toxic doses, targets the endothelial cells (ECs) that are proliferating during tumor angiogenesis. This leads to harmful effects of an even greatly increased number contiguous tumor cells. Although pre-clinical studies of angiogenesis-related EC features in vitro and of the anti-angiogenic and anti-tumor effects in vivo of metronomic chemotherapy have provided valuable insights, clinical trials with this type of therapy have been less successful in inhibiting tumor growth. One possible reason for the apparent disconnect between the pre-clinical and clinical outcomes is that most of the currently used experimental angiogenesis assays and tumor models are incapable of yielding data that can be translated readily into the clinical setting. Many of the assays used suffer from unintentional artifactual effects, e.g., oxidative stress in vitro, and inflammation in vivo, which reduces the sensitivity and discriminatory power of the assays. Co-treatment with an antioxidant or the inclusion of antioxidants in the vehicle often significantly affects the angiogenesis-modulating outcome of metronomic mono-chemotherapy in vivo. This ‘metronomic chemotherapy vehicle factor’ merits further study, as do the observations of antagonistic effects following metronomic treatment with a combination of standard chemotherapeutic drugs in vivo.

Relationships between lymphangiogenesis and angiogenesis during inflammation in rat mesentery microvascular networks

BACKGROUND

Lymphatic and blood microvascular systems play a coordinated role in the regulation of interstitial fluid balance and immune cell trafficking during inflammation. The objective of this study was to characterize the temporal and spatial relationships between lymphatic and blood vessel growth in the adult rat mesentery following an inflammatory stimulus.

METHODS AND RESULTS

Mesenteric tissues were harvested from unstimulated adult male Wistar rats and at 3, 10, and 30 days post compound 48/80 stimulation. Tissues were immunolabeled for PECAM, LYVE-1, Prox1, podoplanin, CD11b, and class III β-tubulin. Vascular area, capillary blind end density, and vascular length density were quantified for each vessel system per time point. Blood vascular area increased compared to unstimulated tissues by day 10 and remained increased at day 30. Following the peak in blood capillary sprouting at day 3, blood vascular area and density increased at day 10. The number of blind-ended lymphatic vessels and lymphatic density did not significantly increase until day 10, and lymphatic vascular area was not increased compared to the unstimulated level until day 30. Lymphangiogenesis correlated with the upregulation of class III β-tubulin expression by endothelial cells along lymphatic blind-ended vessels and increased lymphatic/blood endothelial cell connections. In local tissue regions containing both blood and lymphatic vessels, the presence of lymphatics attenuated blood capillary sprouting.

CONCLUSIONS

Our work suggests that lymphangiogenesis lags angiogenesis during inflammation and motivates the need for future investigations aimed at understanding lymphatic/blood endothelial cell interactions. The results also indicate that lymphatic endothelial cells undergo phenotypic changes during lymphangiogenesis.

Mast cells and macrophages exert beneficial and detrimental effects on tumor progression and angiogenesis

Mast cells and macrophages are critical regulators of inflammation and immunological response in the tumor microenvironment. Increased number of mast cells and macrophages have been reported to correlate with poor prognosis in numerous solid and hematological tumors. In contrast to their pro-tumorigenic role, mast cells and macrophages have shown also anti-tumorigenic effect in certain malignancies, for example by supporting cancer rejection. Thus, mast cells and macrophages can exert both detrimental and beneficial effects on tumor progression. Mast cell- and macrophages-derived growth factors able to promote tumor development and angiogenesis include TNF-α, TGF-β1, FGF-2, VEGF, PDGF, IL-8, osteopontin, and NGF. On the contrary, mast cell- and macrophages-produced cytokines that may participate in anti-tumor response include IL-1, IL-2, IL-4, IL-10, and IFN-γ. It is to note that mast cells and macrophages may also show beneficial and detrimental effects in the same cancer depending on the tumor stage.

Drug testing with angiogenesis models

BACKGROUND

The possibility to treat cancers and several angiogenesis- dependent diseases with non-toxic, antiangiogenic agents has revolutionized the therapeutic capabilities in the fields of oncology and ophthalmology, whereas therapeutic angiogenesis, governed by angiogenesis stimulators, is about to enter clinical medicine.

OBJECTIVE

To describe and critically evaluate the advantages and limitations of the most important and most frequently used preclinical in vivo angiogenesis assays as well as to appraise the preclinical models that are most widely used for studying antiangiogenic effects in tumors.

METHODS

Up-to-date literature survey.

RESULTS/CONCLUSION

Only few angiogenesis and tumor models appear to meet realistic standards fully in terms of biological relevance. Improvement of the biological pertinence and sensitivity of such models would apparently facilitate the translatability of preclinical data into clinical practice.

Spatiotemporal distribution of neurovascular alignment in remodeling adult rat mesentery microvascular networks

An emerging area of microvascular research focuses on the links between neural and vascular patterning. However, the functional dependence between vascular and neural growth in adult tissues remains underinvestigated. The objective of this study was to determine the spatial and temporal coordination between vascular and neural networks over a time course of adult microvascular growth. Mesentery tissues from adult male Wistar rats were harvested prior to stimulation, and 2, 10 and 30 days after angiogenesis stimulated by mast cell degranulation. Tissues were immunolabeled for PECAM (endothelial cell marker) and class III β-tubulin (peripheral nerve marker). Neurovascular alignment was quantified per vessel category: arterioles (>20 µm), pre-capillary arterioles (10-20 µm), post-capillary venules (10-20 µm), venules (>20 µm), capillaries (<10 µm) and capillary sprouts. Neurovascular alignment along pre-capillary arterioles, capillaries, post-capillary venules and venules was decreased compared to unstimulated levels on days 2 and 10. These decreases inversely correlated with increases in vessel density per vessel category. By day 30, alignment either returned to unstimulated levels or was increased compared to day 10. These results suggest that neurovascular alignment arises after microvascular network growth and is present along arterioles, venules and even capillaries.

Identification of class III β-tubulin as a marker of angiogenic perivascular cells

A full understanding of the functional role for pericytes in microvascular network growth requires identifying the specific cell phenotypes involved in angiogenesis. The objective of this study was to evaluate class III β-tubulin expression along remodeling adult rat mesenteric microvascular networks. Mesenteric tissues were harvested from unstimulated adult male Wistar rats and at 2, 10 and 30 days post-compound 48/80 stimulation (n=4 per experimental group). Tissues were immunohistochemically labeled with antibodies for class III β-tubulin, NG2 and PECAM. In unstimulated microvascular networks, class III β-tubulin was nerve specific, and did not identify vascular cells along PECAM positive arterioles, venules, and capillaries. Two days post 48/80 stimuli, class III β-tubulin labeling of perivascular cells, including pericytes and smooth muscle cells, was observed along capillary sprouts, capillaries, venules, and arterioles in network regions characterized by increased vessel density and tortuosity. Pericyte identity along capillaries and capillary sprouts was confirmed by cell morphology and co-labeling with NG2. The percentage of vessels with class III β-tubulin positive labeling decreased at subsequent time points and temporally correlated with the time course of capillary sprouting. The results identify class III β-tubulin as a marker of angiogenic perivascular cells and suggest that specific pericyte phenotypes are associated with capillary sprouting.

Rat mesentery angiogenesis assay

The adult rat mesentery window angiogenesis assay is biologically appropriate and is exceptionally well suited to the study of sprouting angiogenesis in vivo [see review papers], which is the dominating form of angiogenesis in human tumors and non-tumor tissues, as discussed in invited review papers^1,2. Angiogenesis induced in the membranous mesenteric parts by intraperitoneal (i.p.) injection of a pro-angiogenic factor can be modulated by subcutaneous (s.c.), intravenous (i.v.) or oral (p.o.) treatment with modifying agents of choice. Each membranous part of the mesentery is translucent and framed by fatty tissue, giving it a window-like appearance.

The assay has the following advantageous features: (i) the test tissue is natively vascularized, albeit sparsely, and since it is extremely thin, the microvessel network is virtually two-dimensional, which allows the entire network to be assessed microscopically in situ; (ii) in adult rats the test tissue lacks significant physiologic angiogenesis, which characterizes most normal adult mammalian tissues; the degree of native vascularization is, however, correlated with age, as discussed in¹; (iii) the negligible level of trauma-induced angiogenesis ensures high sensitivity; (iv) the assay replicates the clinical situation, as the angiogenesis-modulating test drugs are administered systemically and the responses observed reflect the net effect of all the metabolic, cellular, and molecular alterations induced by the treatment; (v) the assay allows assessments of objective, quantitative, unbiased variables of microvascular spatial extension, density, and network pattern formation, as well as of capillary sprouting, thereby enabling robust statistical analyses of the dose-effect and molecular structure-activity relationships; and (vi) the assay reveals with high sensitivity the toxic or harmful effects of treatments in terms of decreased rate of physiologic body-weight gain, as adult rats grow robustly.

Mast-cell-mediated angiogenesis was first demonstrated using this assay^3,4. The model demonstrates a high level of discrimination regarding dosage-effect relationships and the measured effects of systemically administered chemically or functionally closely related drugs and proteins, including: (i) low-dosage, metronomically administered standard chemotherapeutics that yield diverse, drug-specific effects (i.e., angiogenesis-suppressive, neutral or angiogenesis-stimulating activities⁵); (ii) natural iron-unsaturated human lactoferrin, which stimulates VEGF-A-mediated angiogenesis⁶, and natural iron-unsaturated bovine lactoferrin, which inhibits VEGF-A-mediated angiogenesis⁷; and (iii) low-molecular-weight heparin fractions produced by various means^8,9. Moreover, the assay is highly suited to studies of the combined effects on angiogenesis of agents that are administered systemically in a concurrent or sequential fashion.

The idea of making this video originated from the late Dr. Judah Folkman when he visited our laboratory and witnessed the methodology being demonstrated.

Review papers (invited) discussing and appraising the assay

Norrby, K. In vivo models of angiogenesis. J. Cell. Mol. Med. 10, 588-612 (2006).

Norrby, K. Drug testing with angiogenesis models. Expert Opin. Drug. Discov. 3, 533-549 (2008).

Regulation of mast cell responses in health and disease

Mast cells are multifunctional cells that initiate not only IgE-dependent allergic diseases but also play a fundamental role in innate and adaptive immune responses to microbial infection. They are also thought to play a role in angiogenesis, tissue remodeling, wound healing, and tumor repression or growth. The broad scope of these physiologic and pathologic roles illustrates the flexible nature of mast cells, which is enabled in part by their phenotypic adaptability to different tissue microenvironments and their ability to generate and release a diverse array of bioactive mediators in response to multiple types of cell-surface and cytosolic receptors. There is increasing evidence from studies in cell cultures that release of these mediators can be selectively modulated depending on the types or groups of receptors activated. The intent of this review is to foster interest in the interplay among mast cell receptors to help understand the underlying mechanisms for each of the immunological and non-immunological functions attributed to mast cells. The second intent of this review is to assess the pathophysiologic roles of mast cells and their products in health and disease. Although mast cells have a sufficient repertoire of bioactive mediators to mount effective innate and adaptive defense mechanisms against invading microorganisms, these same mediators can adversely affect surrounding tissues in the host, resulting in autoimmune disease as well as allergic disorders.

Mast cells, angiogenesis, and tumour growth

Accumulation of mast cells (MCs) in tumours was described by Ehrlich in his doctoral thesis. Since this early account, ample evidence has been provided highlighting participation of MCs to the inflammatory reaction that occurs in many clinical and experimental tumour settings. MCs are bone marrow-derived tissue-homing leukocytes that are endowed with a panoply of releasable mediators and surface receptors. These cells actively take part to innate and acquired immune reactions as well as to a series of fundamental functions such as angiogenesis, tissue repair, and tissue remodelling. The involvement of MCs in tumour development is debated. Although some evidence suggests that MCs can promote tumourigenesis and tumour progression, there are some clinical sets as well as experimental tumour models in which MCs seem to have functions that favour the host. One of the major issues linking MCs to cancer is the ability of these cells to release potent pro-angiogenic factors. This review will focus on the most recent acquisitions about this intriguing field of research. This article is part of a Special Issue entitled: Mast cells in inflammation.

Vasculogenesis and angiogenesis in the endometrium during menstrual cycle and implantation

Blood vessels develop via two subsequent processes, vasculogenesis and angiogenesis, both being of crucial importance during menstrual cycle and implantation. These processes are also involved in the development of the fetal and placental vasculatures. During vasculogenesis, formation of the earliest primitive capillaries is achieved by in situ differentiation of hemangiogenic stem cells that are derived from pluripotent mesenchymal cells. The subsequent process, angiogenesis, is characterized by development of new vessels from already existing vessels, and is a well coordinated process initiated by stimulation of various growth factors. Vasculogenesis and angiogenesis are important and complex processes involving extensive interplay between cells and growth factors. The development, maturation and maintenance of the vascular network are necessary for successful hemochorial placentation as well as normal embryonic development and growth. In this review, we outline the basic mechanisms of vasculogenesis and angiogenesis in the endometrium during the menstrual cycle and different stages of implantation, and consider how this data can be applied to human pregnancy. Recent studies have shown that during the initiation steps of implantation, angiogenic factors trigger vasculogenesis and angiogenesis. Different inducers and stimulators affect angiogenesis and vasculogenesis by directly or indirectly stimulating proliferation, differentiation and migration of endothelial or respective precursor cells. As a conclusion, understanding the mechanisms of angiogenesis and the roles of angiogenic factors during the menstrual cycle and implantation may provide new insights and possible approaches for embryo implantation and healthy pregnancy.

Immune cells and angiogenesis

Both innate and adaptive immune cells are involved in the mechanisms of endothelial cell proliferation, migration and activation, through the production and release of a large spectrum of pro-angiogenic mediators. These may create the specific microenvironment that favours an increased rate of tissue vascularization. In this review, we will focus on the immune cell component of the angiogenic process in inflammation and tumour growth. As angiogenesis is the result of a net balance between the activities exerted by positive and negative regulators, we will also provide information on some antiangiogenic properties of immune cells that may be utilized for a potential pharmacological use as antiangiogenic agents in inflammation as well as in cancer.

Mast cells and angiogenesis in haematological malignancies

Tumor cells are surrounded by an infiltrate of inflammatory cells, namely lymphocytes, neutrophils, macrophages and mast cells (MCs). Increasing evidence indicates that MCs play a role in tumor growth and tumor-related angiogenesis in both solid and haematological tumors. In this review article, we discuss the involvement of MCs in angiogenesis in haematological malignancies and suggest that MCs might act as a new target for the adjuvant treatment of these tumors through the selective inhibition of angiogenesis.

Mast cells in tumor growth: angiogenesis, tissue remodelling and immune-modulation

There is a growing acceptance that tumor-infiltrating myeloid cells play an active role in tumor growth and mast cells are one of the earliest cell types to infiltrate developing tumors. Mast cells accumulate at the boundary between healthy tissues and malignancies and are often found in close association with blood vessels within the tumor microenvironment. They express many pro-angiogenic compounds, and may play an early role in angiogenesis within developing tumors. Mast cells also remodel extracellular matrix during wound healing, and this function is subverted in tumor growth, promoting tumor spread and metastasis. In addition, mast cells modulate immune responses by dampening immune rejection or directing immune cell recruitment, depending on local stimuli. In this review, we focus on key roles for mast cells in angiogenesis, tissue remodelling and immune modulation and highlight recent findings on the integral role that mast cells play in tumor growth. New findings suggest that mast cells may serve as a novel therapeutic target for cancer treatment and that inhibiting mast cell function may lead to tumor regression.

Capillary sprout endothelial cells exhibit a CD36 low phenotype: regulation by shear stress and vascular endothelial growth factor-induced mechanism for attenuating anti-proliferative thrombospondin-1 signaling

Endothelial cells acquire distinctive molecular signatures in their transformation to an angiogenic phenotype that are indicative of changes in cell behavior and function. Using a rat mesentery model of inflammation-induced angiogenesis and a panel of known endothelial markers (CD31, VE-cadherin, BS-I lectin), we identified a capillary sprout-specific endothelial phenotype that is characterized by the marked down-regulation of CD36, a receptor for the anti-angiogenic molecule thrombospondin-1 (TSP-1). TSP-1/CD36 interactions were shown to regulate angiogenesis in this model as application of TSP-1 inhibited angiogenesis and blockade of both TSP-1 and CD36 accelerated angiogenesis. Vascular endothelial growth factor, which was up-regulated in the in vivo model, elicited a dose- and time-dependent down-regulation of CD36 (ie, to a CD36 low phenotype) in cultured human umbilical vein endothelial cells. Human umbilical vein endothelial cells that had been conditioned to a CD36 low phenotype with VEGF were found to be refractory to anti-proliferative TSP-1 signaling via a CD36-dependent mechanism. The loss of exposure to wall shear stress, which occurs in vivo when previously quiescent cells begin to sprout, also generated a CD36 low phenotype. Ultimately, our results identified the regulation of endothelial cell CD36 expression as a novel mechanism through which VEGF stimulates and sustains capillary sprouting in the presence of TSP-1. Additionally, CD36 was shown to function as a potential molecular linkage through which wall shear stress may regulate both microvessel sprouting and quiescence.

HIF-1alpha is up-regulated in activated mast cells by a process that involves calcineurin and NFAT

Mast cells play important roles in many pathological conditions where local hypoxia is observed, including asthma, rheumatic diseases, and certain types of cancer. Here, we investigated how expression of the hypoxia-inducible factor 1, alpha subunit gene (HIF1A), is regulated in mast cells. The product of HIF1A is hypoxia-inducible factor 1alpha (HIF-1alpha), is a major nuclear transcription factor modulating gene expression in response to hypoxic conditions. We observed that under hypoxic conditions, exposure of mast cells to ionomycin and substance P resulted in significant up-regulation of HIF1A expression as compared with resting mast cells incubated under identical conditions. The ionomycin-mediated increase in HIF-1alpha protein levels was sensitive to the transcription inhibitor actinomycin D and to inhibitors of calcineurin, cyclosporin A (CsA), and FK506. The increased HIF-1alpha protein level was paralleled by a severalfold increase in HIF-1alpha mRNA that could be also inhibited with actinomycin D and CsA. The HIF1A promoter activity was significantly increased in ionomycin-activated mast cells, and the promoter activity could be inhibited by CsA and FK506. Furthermore, in situ mutagenesis experiments showed that the ionomycin-mediated HIF1A promoter activity depends on a conservative NFAT-binding site. Thus, accumulation of HIF-1alpha in activated mast cells requires up-regulation of HIF1A gene transcription and depends on the calcineurin-NFAT signaling pathway.

IFATS collection: The role of human adipose-derived stromal cells in inflammatory microvascular remodeling and evidence of a perivascular phenotype

A growing body of literature suggests that human adipose-derived stromal cells (hASCs) possess developmental plasticity both in vitro and in vivo, and might represent a viable cell source for therapeutic angiogenesis and tissue engineering. We investigate their phenotypic similarity to perivascular cell types, ability to contribute to in vivo microvascular remodeling, and ability to modulate vascular stability. We evaluated hASC surface expression of vascular and stem/progenitor cell markers in vitro, as well as any effects of platelet-derived growth factor B chain (PDGF-BB) and vascular endothelial growth factor 165 on in vitro hASC migration. To ascertain in vivo behavior of hASCs in an angiogenic environment, hASCs were isolated, expanded in culture, labeled with a fluorescent marker, and injected into adult nude rat mesenteries that were stimulated to undergo microvascular remodeling. Ten, 30, and 60 days after injection, tissues from anesthetized animals were harvested and processed with immunohistochemical techniques to determine hASC quantity, positional fate in relation to microvessels, and expression of endothelial and perivascular cell markers. After 60 days, 29% of hASCs exhibited perivascular morphologies compared with 11% of injected human lung fibroblasts. hASCs exhibiting perivascular morphologies also expressed markers characteristic of vascular pericytes: smooth muscle alpha-actin (10%) and neuron-glia antigen 2 (8%). In tissues treated with hASCs, vascular density was significantly increased over age-matched controls lacking hASCs. This study demonstrates that hASCs express pericyte lineage markers in vivo and in vitro, exhibit increased migration in response to PDGF-BB in vitro, exhibit perivascular morphology when injected in vivo, and contribute to increases in microvascular density during angiogenesis by migrating toward vessels. Disclosure of potential conflicts of interest is found at the end of this article.

Evaluating integrin function in models of angiogenesis and vascular permeability

All vascular biological processes are influenced to some degree by integrins expressed on endothelial cells, vascular smooth muscle cells, fibroblasts, platelets, or other circulating cells. In particular, angiogenesis requires cells to process signals from their microenvironment and respond by altering their cell-cell and cell-matrix adhesion, events which allow migration and vascular remodeling over the period of days to weeks. On the other hand, endothelial cells can respond to a permeability stimulus and alter their junctional adhesion molecules or vesicular transport machinery within seconds or minutes. This chapter will discuss the current understanding of how integrins participate in these processes, and explore the in vitro and in vivo models available to study the role of integrin function during angiogenesis and vascular leak.

Mast cells are required for normal healing of skin wounds in mice

Mast cells (MCs) have recently been reported to play a pivotal role in the elicitation of inflammatory reactions that are beneficial to the host, e.g., during innate immune responses to bacteria. To explore whether MCs also contribute to wound repair, we studied experimentally induced skin wounds in MC-deficient Kit(W)/Kit(W-v) mice, normal Kit+/+ mice, and MC-reconstituted Kit(W)/Kit(W-v) mice. Wound closure was significantly impaired in the absence of MCs during the first 6 days of wound healing and histomorphometric analyses of MC degranulation at the wound edges revealed distance-dependent MC activation, i.e., MC degranulation was most prominent directly adjacent to the wound. In addition, Kit(W)/Kit(W-v) mice showed impaired extravasation and recruitment of neutrophils to the wounded areas. Notably, wound closure, extravasation, and neutrophil recruitment were found to be normal in MC-reconstituted Kit(W)/Kit(W-v) mice. Therefore, we examined whether MCs promote wound healing by releasing histamine or TNF-alpha. Interestingly, wound closure was reduced in mice treated with an H1-receptor antagonist but not after treatment with an H2-receptor antagonist or in the absence of TNF-alpha. Taken together, our findings indicate that MC activation and histamine release are required for normal cutaneous wound healing.

In vivo models of angiogenesis

The process of building new blood vessels (angiogenesis) and controlling the propagation of blood vessels (anti-angiogenesis) are fundamental to human health, as they play key roles in wound healing and tissue growth. More than 500 million people may stand to benefit from anti- or pro-angiogenic treatments in the coming decades [National Cancer Institute (USA), Cancer Bulletin, volume 3, no. 9, 2006]. The use of animal models to assay angiogenesis is crucial to the search for therapeutic agents that inhibit angiogenesis in the clinical setting. Examples of persons that would benefit from these therapies are cancer patients, as cancer growth and spread is angiogenesis-dependent, and patients with aberrant angiogenesis in the eye, which may lead to blindness or defective sight. Recently, anti-angiogenesis therapies have been introduced successfully in the clinic, representing a turning point in tumor therapy and the treatment of macular degeneration and heralding a new era for the treatment of several commonly occurring angiogenesis-related diseases. On the other hand, pro-angiogenic therapies that promote compensatory angiogenesis in hypoxic tissues, such as those subjected to ischemia in myocardial or cerebral hypoxia due to occluding lesions in the coronary or cerebral arteries, respectively, and in cases of poor wound healing, are also being developed. In this review, the current major and newly introduced preclinical angiogenesis assays are described and discussed in terms of their specific advantages and disadvantages from the biological, technical, economical and ethical perspectives. These assays include the corneal micropocket, chick chorioallantoic membrane, rodent mesentery, subcutaneous (s.c.) sponge/matrix/alginate microbead, s.c. Matrigel plug, s.c. disc, and s.c. directed in vivo angiogenesis assays, as well as, the zebrafish system and several additional assays. A note on quantitative techniques for assessing angiogenesis in patients is also included. The currently utilized preclinical assays are not equivalent in terms of efficacy or relevance to human disease. Some of these assays have significance for screening, while others are used primarily in studies of dosage-effects, molecular structure activities, and the combined effects of two or more agents on angiogenesis. When invited to write this review, I was asked to describe in some detail the rodent mesenteric-window angiogenesis assay, which has not received extensive coverage in previous reviews.

Peritoneal Dialysis Fluid–Induced Angiogenesis in Rat Mesentery Is Increased by Lactate in the Presence or Absence of Glucose

Angiogenesis may be an important mechanism behind the functional deterioration of the peritoneum leading to ultrafiltration failure in peritoneal dialysis. The present study was designed to compare the angiogenic properties of lactate-, bicarbonate-, and pyruvate-buffered fluids, evaluated separately with and without glucose. Five different fluids (lactate and bicarbonate with and without 2.5% glucose and pyruvate without glucose) were studied for 5 weeks of twice-daily injections in rats. The respective buffers (40 mmol/l) were adjusted to pH 7.2, and sodium, chloride, calcium, and magnesium were present at standard concentrations. The mesenteric window model, based on observation of the translucent peritoneal sections of the small intestine mesentery, was used for immunohistochemical imaging of microvessels (RECA-1 antigen) and macrophages (ED1 and ED2 antigens). All fluids induced angiogenesis as compared with untreated controls. The lactate-buffered fluids induced larger vascularized zones than did their bicarbonate- and pyruvate-buffered counterparts. Angiogenesis was accompanied by a local recruitment of ED1 macrophages from blood. Addition of glucose to the lactate- and bicarbonate-buffered fluids did not seem to alter their pro-angiogenic properties. In conclusion, intraperitoneal exposure to lactate buffer, compared with bicarbonate, stimulates angiogenesis in the presence or absence of glucose.

Low-molecular-weight heparins and angiogenesis

The involvement of the vascular system in malignancy encompasses not only angiogenesis, but also systemic hypercoagulability and a pro-thrombotic state, and there is increasing evidence that pathways of blood coagulation and angiogenesis are reciprocally linked. In fact, cancer atients often display hypercoagulability resulting in markedly increased thromboembolism, which requires anti-coagulant treatment using heparins, for example. Clinical trials reveal that treatment with various low-molecular-weight heparins (LMWHs) improves the survival time in cancer patients receiving chemotherapy compared with those receiving unfractionated standard heparin (UFH) or no heparin treatment, as well as in cancer patients receiving LMWH as thrombosis prophylaxis during primary surgery. This anti-tumor effect of the heparins appears to be unrelated to their anti-coagulant activity, but the mechanisms involved are not fully understood. Tumor growth and spread are dependent on angiogenesis and it is noteworthy that the most potent endogenous pro- and anti-angiogenic factors are heparin-binding proteins that may be affected by systemic treatment with heparins. Heparin and other glycosaminoglycans play a role in vascular endothelial cell function, as they are able to modulate the activities of angiogenic growth factors by facilitating the interaction with their receptor and promoting receptor activation. To date, preclinical studies have demonstrated that only LMWH fragments produced by the heparinase digestion of UFH, i.e. tinzaparin, exert anti-angiogenic effects in any type of tissue in vivo. These effects are fragment-mass-specific and angiogenesis-type-specific. Data on the effect of various LMWHs and UFH on endothelial cell capillary tube formation and proliferation in vitro are also presented. We hope that this paper will stimulate and facilitate future research designed to elucidate whether the anti-angiogenic or anti-tumor effects of commercial LMWHs in their own right are agent specific and whether anti-angiogenic properties increase the anti-tumor properties of the LMWHs in the clinic.

Perivascular Cells Along Venules Upregulate NG2 Expression During Microvascular Remodeling

OBJECTIVE

Recently the authors have shown that neuron-glial antigen 2 (NG2) is expressed by perivascular cells along arterioles and capillaries, but not along venules in quiescent rat mesenteric microvascular networks. To investigate how the spatial distribution of this proteoglycan changes during microvascular remodeling, the objective of this study was to characterize the expression of NG2 in adult rat mesenteric microvascular networks undergoing active remodeling.

METHODS

The distribution of NG2 expression was evaluated in adult rat mesenteric microvascular networks. Tissues were harvested from 250 g, female, Sprague-Dawley rats at 1, 3, and 5 days poststimulation and double immunolabeled for NG2 and CD31 (endothelial cell marker).

RESULTS

After 1 day, NG2 expression was observed along 27 +/- 11% of network draining venules (14-55 microm) and after 3 days, 59 +/- 10% of draining venules (13-59 microm) stained positive for the proteoglycan. By 5 days poststimulation, the percentage of network draining venules (18-59 microm) staining positive for NG2 returned to 18 +/- 7%, indicating a downregulation of the proteoglycan toward quiescent levels along larger-sized venules.

CONCLUSIONS

The results suggest that NG2 proteoglycan expression is transiently upregulated along venules during microvascular remodeling, implicating NG2 as a marker of activated venules.

VEGF165b, an inhibitory vascular endothelial growth factor splice variant: mechanism of action, in vivo effect on angiogenesis and endogenous protein expression

Growth of new blood vessels (angiogenesis), required for all tumor growth, is stimulated by the expression of vascular endothelial growth factor (VEGF). VEGF is up-regulated in all known solid tumors but also in atherosclerosis, diabetic retinopathy, arthritis, and many other conditions. Conventional VEGF isoforms have been universally described as proangiogenic cytokines. Here, we show that an endogenous splice variant, VEGF(165)b, is expressed as protein in normal cells and tissues and is circulating in human plasma. We also present evidence for a sister family of presumably inhibitory splice variants. Moreover, these isoforms are down-regulated in prostate cancer. We also show that VEGF(165)b binds VEGF receptor 2 with the same affinity as VEGF(165) but does not activate it or stimulate downstream signaling pathways. Moreover, it prevents VEGF(165)-mediated VEGF receptor 2 phosphorylation and signaling in cultured cells. Furthermore, we show, with two different in vivo angiogenesis models, that VEGF(165)b is not angiogenic and that it inhibits VEGF(165)-mediated angiogenesis in rabbit cornea and rat mesentery. Finally, we show that VEGF(165)b expressing tumors grow significantly more slowly than VEGF(165)-expressing tumors, indicating that a switch in splicing from VEGF(165) to VEGF(165)b can inhibit tumor growth. These results suggest that regulation of VEGF splicing may be a critical switch from an antiangiogenic to a proangiogenic phenotype.

Immunohistochemical identification of an extracellular matrix scaffold that microguides capillary sprouting in vivo

To gain insight into how a naturally occurring scaffold composed of extracellular matrix (ECM) proteins provides directional guidance for capillary sprouting, we examined angiogenesis in whole-mount specimens of rat mesentery. Angiogenesis was studied in response to normal maturation, the injection of a mast cell degranulating substance (compound 48/80), and mild wounding. Confocal microscopy of specimens immunolabeled for elastin revealed a network of crosslinked elastic fibers with a density of 140.8 +/- 37 mm of fiber/mm(2) tissue. Fiber diameters ranged from 180 to 1400 nm, with a mean value of 710 +/- 330 nm. Capillary sprouts contained CD31- and OX-43-positive endothelial cells as well as desmin-positive pericytes. During normal maturation, leading endothelial cells and pericytes were in contact and aligned with an elastic fiber in approximately 80-90% of all sprouts. In wounding and compound 48/80-treated specimens, in which angiogenesis was markedly increased, leading endothelial cells remained in contact and aligned with elastic fibers in approximately 60-80% of all sprouts. These observations indicate that elastic fibers are used for endothelial and pericyte migration during capillary sprouting in rat mesentery. The composition of this elastic fiber matrix may provide important clues for the development of tissue-engineered scaffolds that support and directionally guide angiogenesis.

Absence of OX-43 antigen expression in invasive capillary sprouts: identification of a capillary sprout-specific endothelial phenotype

Endothelial cells exhibit a number of unique phenotypes, some of which are angiogenesis dependent. To identify a capillary sprout-specific endothelial phenotype, we labeled angiogenic rat mesentery tissue using a microvessel and capillary sprout marker (laminin), selected endothelial cell markers (CD31, tie-2, and BS-I lectin), and the OX-43 monoclonal antibody, which recognizes a 90-kDa membrane glycoprotein of unknown function. In tissues that were stimulated through wound healing and compound 48/80 application, double-immunolabeling experiments with an anti-laminin antibody revealed that the OX-43 antigen was expressed strongly in all microvessels. However, the OX-43 antigen was completely absent from a large percentage (>85%) of the capillary sprouts that were invading the avascular tissue space. In contrast, sprouts that were introverting back into the previously vascularized tissue retained high levels of OX-43 antigen expression. Double-labeling experiments with endothelial markers indicated that the OX-43 antigen was expressed by microvessel endothelium but was absent from virtually all invasive capillary sprout endothelial cells. We conclude that the absence of OX-43 antigen expression marks a novel, capillary sprout-specific, endothelial cell phenotype. Endothelial cells of this phenotype are particularly abundant in capillary sprouts that invade avascular tissue during angiogenesis.