Swelling-induced patterning in soft microchannels.

We study the effect of inflation on the swelling-induced wrinkling of thin elastic membranes in a set-up that is commonly used to create microchannels in lab-on-chip applications. Using a combination of experiments and associated numerical simulations, we demonstrate that the out-of-plane deformation of the inflated membrane and the resulting anisotropic stress lead to two distinct instabilities as the swelling progresses. The membrane first develops small-amplitude wrinkles that retain the cross-channel symmetry. Their wavelength depends on the pressure and is set in a process similar to the axisymmetric buckling of pressurised, uni-axially compressed cylindrical shells. As swelling increases, the membrane undergoes a secondary instability during which the wrinkles coarsen into large-amplitude folds whose morphology can be controlled by the degree of pre-inflation. We elucidate the fundamental mechanisms responsible for this behaviour and explain how inflation can be used as a control mechanism in the manufacture of microchannels.

Modeling Graphene-Polymer Heterostructure MEMS Membranes with the Föppl-von Kármán Equations.

Ultra-thin graphene-based membranes have shown significant promise for high-performance nano-electro-mechanical (NEMS) devices. The key challenge in the modeling of such membranes is that they often operate in deflection regimes where the assumptions or approximations of "pure bending" or "pure stretching" are not satisfied. We present a model of graphene-polymer heterostructure (GPH) NEMS membranes based on Föppl-von Kármán (FvK) equations which take into account both bending and stretching forces. The experimental GPH membrane shape obtained through atomic force microscopy topography mapping is compared to the inflation shapes predicted by FvK-based finite element method simulation, and they show excellent agreement with each other. When the GPH membranes are deflected under pressure in a capacitive pressure sensor configuration, the effectiveness of this model is further exemplified through accurately predicting the capacitance change of deflecting GPH membrane devices at varying pressures. This model serves as a powerful new tool in the design and development of graphene-based NEMS devices, being able to predict the performance of graphene NEMS devices or to aid in the design of device geometries to match required performances.

Resonances in Pulsatile Channel Flow with an Elastic Wall.

Interactions between fluids and elastic solids are ubiquitous in applications ranging from aeronautical and civil engineering to physiological flows. Here we study the pulsatile flow through a two-dimensional Starling resistor as a simple model for unsteady flow in elastic vessels. We numerically solve the equations governing the flow and the large-displacement elasticity and show that the system responds as a forced harmonic oscillator with nonconventional damping. We derive an analytical prediction for the amplitude of the oscillatory wall deformation, and thus the conditions under which resonances occur or vanish.

Modulating functional connectivity between medial frontopolar cortex and amygdala by inhibitory and excitatory transcranial magnetic stimulation.

The prefrontal-limbic network in the human brain plays a major role in social cognition, especially cognitive control of emotion. The medial frontopolar cortex (mFP; Brodmann Area 10) and the amygdala are part of this network and display correlated neuronal activity in time, as measured by functional magnetic resonance imaging (fMRI). This functional connectivity is dynamic, sensitive to training, and affected in mental disorders. However, the effects of neurostimulation on functional connectivity within this network have not yet been systematically investigated. Here, we investigate the effects of both low- and high-frequency repetitive transcranial magnetic stimulation (rTMS) to the right mFP on functional connectivity between mFP and amygdala, as measured with resting state fMRI (rsfMRI). Three groups of healthy participants received either low-frequency rTMS (1 Hz; N = 18), sham TMS (1 Hz, subthreshold; N = 18) or high-frequency rTMS (20 Hz; N = 19). rsfMRI was acquired before and after (separate days). We hypothesized a modulation of functional connectivity in opposite directions compared to sham TMS through adjustment of the stimulation frequency. Groups differed in functional connectivity between mFP and amygdala after stimulation compared to before stimulation (low-frequency: decrease, high-frequency: increase). Motion or induced changes in neuronal activity were excluded as confounders. Results show that rTMS is effective for increasing and decreasing functional coherence between prefrontal and limbic regions. This finding is relevant for social and affective neuroscience as well as novel treatment approaches in psychiatry.

Curvature-Sensitive Kinesin Binding Can Explain Microtubule Ring Formation and Reveals Chaotic Dynamics in a Mathematical Model.

Microtubules are filamentous tubular protein polymers which are essential for a range of cellular behaviour, and are generally straight over micron length scales. However, in some gliding assays, where microtubules move over a carpet of molecular motors, individual microtubules can also form tight arcs or rings, even in the absence of crosslinking proteins. Understanding this phenomenon may provide important explanations for similar highly curved microtubules which can be found in nerve cells undergoing neurodegeneration. We propose a model for gliding assays where the kinesins moving the microtubules over the surface induce ring formation through differential binding, substantiated by recent findings that a mutant version of the motor protein kinesin applied in solution is able to lock-in microtubule curvature. For certain parameter regimes, our model predicts that both straight and curved microtubules can exist simultaneously as stable steady states, as has been seen experimentally. Additionally, unsteady solutions are found, where a wave of differential binding propagates down the microtubule as it glides across the surface, which can lead to chaotic motion. Whilst this model explains two-dimensional microtubule behaviour in an experimental gliding assay, it has the potential to be adapted to explain pathological curling in nerve cells.

Lack of ecto-5'-nucleotidase (CD73) promotes arteriogenesis.

AIMS: Adenosine can stimulate angiogenesis, but its role in the distinct process of arteriogenesis is unknown. We have previously reported that mice lacking ecto-5'-nucleotidase (CD73-/-) show enhanced monocyte adhesion to the endothelium after ischaemia, which is considered to be an important trigger for arteriogenesis. METHODS AND RESULTS: Hindlimb ischaemia was induced in wild-type (WT) and CD73-/- mice to study the role of extracellularly formed adenosine in arteriogenesis. Magnetic resonance angiography (MRA) was performed for serial visualization of newly developed vessels at a spatial resolution of 1 nL, and high-energy phosphates (HEP) were quantified by (31)P MR spectroscopy (MRS). MRA of CD73-/- mice revealed substantially enhanced collateral artery conductance at day 7 [CD73-/-: 0.73 ± 0.11 a.u. (arbitrary units); WT: 0.44 ± 0.13 a.u.; P < 0.01, n = 6], and MRS of the affected hindlimb showed a faster restoration of HEP in correlation with enhanced functional recovery in the mutant. Additionally, histology showed no differences in capillary density between the groups but showed an increased monocyte infiltration in hindlimbs of CD73-/- mice. CONCLUSION: Serial assessment of dynamic changes of vessel growth and metabolism in the process of arteriogenesis demonstrate that the lack of CD73-derived adenosine importantly promotes arteriogenesis but does not alter angiogenesis in our model of hindlimb ischaemia.

Role of early growth response 1 in arteriogenesis: impact on vascular cell proliferation and leukocyte recruitment in vivo.

Based on previous findings that early growth response 1 (Egr-1) participates in leukocyte recruitment and cell proliferation in vitro, this study was designed to investigate its mode of action during arteriogenesis in vivo. In a model of peripheral arteriogenesis, Egr-1 was significantly upregulated in growing collaterals of wild-type (WT) mice, both on mRNA and protein level. Egr-1(-/-) mice demonstrated delayed arteriogenesis after femoral artery ligation. They further showed increased levels of monocytes and granulocytes in the circulation, but reduced levels in adductor muscles under baseline conditions. After femoral artery ligation, elevated numbers of macrophages were detected in the perivascular zone of collaterals in Egr-1(-/-) mice and mRNA of leukocyte recruitment mediators was upregulated. Other Egr family members (Egr-2 to -4) were significantly upregulated only in Egr-1(-/-) mice, suggesting a mechanism of counterbalancing Egr-1 deficiency. Moreover, splicing factor-1, downregulated in WT mice after femoral artery ligation in the process of increased vascular cell proliferation, was upregulated in Egr-1(-/-) mice. αSM-actin on the other hand, significantly downregulated in WT mice, showed no differential expression in Egr-1(-/-) mice. While cell cycle regulator cyclin E and cdc20 were upregulated in Egr-1(-/-) mice, cyclin D1 expression decreased below the detection limit in collaterals, and the proliferation marker ki67 was not differentially expressed. In conclusion, compensation for deficiency in Egr-1 function in leukocyte recruitment can presumably be mediated by other transcription factors; however, Egr-1 is indispensable for effective vascular cell cycle progression in arteriogenesis.

The energetics of flow through a rapidly oscillating tube with slowly varying amplitude.

Motivated by the problem of self-excited oscillations in fluid-filled collapsible tubes, we examine the flow structure and energy budget of flow through an elastic-walled tube. Specifically, we consider the case in which a background axial flow is perturbed by prescribed small-amplitude high-frequency long-wavelength oscillations of the tube wall, with a slowly growing or decaying amplitude. We use a multiple-scale analysis to show that, at leading order, we recover the constant-amplitude equations derived by Whittaker et al. (Whittaker et al. 2010 J. Fluid Mech. 648, 83-121. (doi:10.1017/S0022112009992904)) with the effects of growth or decay entering only at first order. We also quantify the effects on the flow structure and energy budget. Finally, we discuss how our results are needed to understand and predict an instability that can lead to self-excited oscillations in collapsible-tube systems.

Adiponectin-mediated changes in effector cells involved in the pathophysiology of rheumatoid arthritis.

OBJECTIVE: Rheumatoid arthritis (RA) is associated with increased production of adipokines, which are cytokine-like mediators that are produced mainly in adipose tissue but also in synovial cells. Since RA synovial fibroblasts (RASFs), lymphocytes, endothelial cells, and chondrocytes are key players in the pathophysiology of RA, this study was undertaken to analyze the effects of the key adipokine adiponectin on proinflammatory and prodestructive synovial effector cells. METHODS: Lymphocytes were activated in part prior to stimulation. All cells were stimulated with adiponectin, and changes in gene and protein expression were determined by Affymetrix and protein arrays. Messenger RNA and protein levels were confirmed using semiquantitative reverse transcription-polymerase chain reaction (PCR), real-time PCR, and immunoassays. Intracellular signal transduction was evaluated using chemical signaling inhibitors. RESULTS: Adiponectin stimulation of human RASFs predominantly induced the secretion of chemokines, as well as proinflammatory cytokines, prostaglandin synthases, growth factors, and factors of bone metabolism and matrix remodeling. Lymphocytes, endothelial cells, and chondrocytes responded to adiponectin stimulation with enhanced synthesis of cytokines and various chemokines. Additionally, chondrocytes released increased amounts of matrix metalloproteinases. In RASFs, adiponectin-mediated effects were p38 MAPK and protein kinase C dependent. CONCLUSION: Our previous findings indicated that adiponectin was present in inflamed synovium, at sites of cartilage invasion, in lymphocyte infiltrates, and in perivascular areas. The findings of the present study indicate that adiponectin induces gene expression and protein synthesis in human RASFs, lymphocytes, endothelial cells, and chondrocytes, supporting the concept of adiponectin being involved in the pathophysiologic modulation of RA effector cells. Adiponectin promotes inflammation through cytokine synthesis, attraction of inflammatory cells to the synovium, and recruitment of prodestructive cells via chemokines, thus promoting matrix destruction at sites of cartilage invasion.

VEGFR-1 signaling regulates the homing of bone marrow-derived cells in a mouse stroke model.

Vascular endothelial growth factor receptor 1 (VEGFR-1) is highly expressed in endothelial cells and regulates developmental angiogenesis by acting as a decoy receptor and trapping VEGF-A. Vascular endothelial growth factor receptor 1 is also expressed in monocytes and macrophages; mice lacking the VEGFR-1 tyrosine kinase (TK) domain (VEGFR-1 TK mice) display impaired macrophage function. Because macrophages are recruited to sites of cerebral ischemic infarcts, we hypothesized that lack of VEGFR-1 TK in bone marrow(BM) cells would affect the outcome in an experimental stroke model. We performed BM transplantation experiments in C57BL/6J mice using VEGFR-1 TK and VEGFR-1 TK mice as BM donors and analyzed cell infiltration after cerebral ischemia. There was reduced initial recruitment of VEGFR-1 TK myeloid cells into the infarcted tissue and reduced postischemic angiogenesis at 3days postischemia. By 10 days, the numbers of infiltrating cells and the densities of vessels in the infarct peri-infarct zone were similar for both groups. Neither infarct size at 3 and 10 days postischemia nor neurological performance at 24 hours was different between the experimental groups. These results support a role of VEGFR-1 signaling in the early regulation of BM infiltration and angiogenesis after brain ischemia.

Flt-1 signaling in macrophages promotes glioma growth in vivo.

Several lines of evidence indicate that Flt-1, a fms-like tyrosine kinase receptor, which binds to vascular endothelial growth factor (VEGF)-A, VEGF-B, and PlGF, is a positive regulator of angiogenesis in the context of tumor growth and metastasis. However, the molecular basis of its action is still not clear. Besides endothelial cells, Flt-1 is also expressed by other different cell types, including myeloid hematopoeitic cells (monocytes and macrophages). To examine the functions of Flt-1 expressed by bone marrow-derived myeloid cells in supporting tumor growth and angiogenesis, Flt-1 tyrosine kinase-deficient (Flt-1 TK-/-) bone marrow cells were transplanted into lethally irradiated syngeneic recipients. After hematopoietic reconstitution, we orthotopically implanted syngeneic wild-type glioma cells or glioma cells overexpressing either VEGF(164) or PlGF-2. Loss of Flt-1 signaling in bone marrow-derived myeloid cells led to a significant decrease in tumor volume and vascularization in gliomas. VEGF but not PlGF overexpressed by glioma cells restored the tumor growth rate in Flt-1 TK-/- bone marrow chimera. VEGF and PlGF overexpression by tumor cells induced an accumulation of bone marrow-derived myeloid cells into tumor tissue. This infiltration was decreased in tumors grown in Flt-1 TK-/- bone marrow chimeras. When investigating chemokines and growth factors involved in myeloid cell recruitment, we determined elevated SDF-1/CXCL12 levels in VEGF- and PlGF-overexpressing tumors. Collectively, these results suggest that Flt-1 signaling in myeloid cells is essential to amplify the angiogenic response and to promote glioma growth.

The mechanics of airway closure.

We describe how surface-tension-driven instabilities of the lung's liquid lining may lead to pulmonary airway closure via the formation of liquid bridges that occlude the airway lumen. Using simple theoretical models, we demonstrate that this process may occur via a purely fluid-mechanical "film collapse" or through a coupled, fluid-elastic "compliant collapse" mechanism. Both mechanisms can lead to airway closure in times comparable with the breathing cycle, suggesting that surface tension is the primary mechanical effect responsible for the closure observed in peripheral regions of the human lungs. We conclude by discussing the influence of additional effects not included in the simple models, such as gravity, the presence of pulmonary surfactant, respiratory flow and wall motion, the airways' geometry, and the mechanical structure of the airway walls.

Angiopoietin-2 impairs revascularization after limb ischemia.

Angiopoietins play important roles in the formation of neovessels and complex vascular networks. Angiopoietin (Ang)-1 and Ang-2 belong to a family of growth factors that display opposing effects on the activation of Tie2 (tyrosine kinase with immunoglobulin and epidermal growth factor homology domain 2). Endothelial Ang-2 expression is associated with vessel destabilization and regulates a balance between vascular regression and growth. To elucidate, in particular, the role of Ang-2 after arterial artery occlusion in the mouse limb, we applied a transgenic animal model with targeted Ang-2 expression in endothelial cells. We show here that restoration of blood flow in Ang-2:Tie1 transgenic mice is dramatically impaired when Ang-2 expression is induced in the vasculature. The defective restoration of perfusion in Ang-2 transgenic mice is evidenced by reduced collateral artery growth, which typically occurs to compensate for flow deficits after occlusion of the large conductance artery. Furthermore, reduced movement capacities and higher incidents of necrosis are consequently observed in the transgenic limbs as compared with controls. Mechanistically, the observed effects are attributed to defective smooth muscle cell recruitment in Ang-2 transgenic mice. Moreover, distinct Ang-2 levels in the genetically modified animals clearly correlated with the magnitude of reduced perfusion. In conclusion, our studies define Ang-2 as an important molecule for the progression of collateral artery growth and angiogenesis during ischemia and suggest precise Ang-2 dosage activities to accomplish blood vessel growth.

Endothelial nitric oxide synthase activity is essential for vasodilation during blood flow recovery but not for arteriogenesis.

OBJECTIVE: Arteriogenesis is the major mechanism of vascular growth, which is able to compensate for blood flow deficiency after arterial occlusion. Endothelial nitric oxide synthase (eNOS) activity is essential for neovascularization, however its specific role in arteriogenesis remains unclear. We studied the role of eNOS in arteriogenesis using 3 mouse strains with different eNOS expression. METHODS AND RESULTS: Distal femoral artery ligation was performed in eNOS-overexpressing mice (eNOStg), eNOS-deficient (eNOS-/-) mice, and wild type (WT) controls. Tissue perfusion and collateral-dependent blood flow were significantly increased in eNOStg mice compared with WT only immediately after ligation. In eNOS-/- mice, although tissue perfusion remained significantly decreased, collateral-dependent blood flow was only decreased until day 7, suggesting normal, perhaps delayed collateral growth. Histology confirmed no differences in collateral arteries of eNOStg, eNOS-/-, and WT mice at 1 and 3 weeks. Administration of an NO donor induced vasodilation in collateral arteries of eNOS-/- mice, but not in WT, identifying the inability to vasodilate collateral arteries as main cause of impaired blood flow recovery in eNOS-/- mice. CONCLUSIONS: This study demonstrates that eNOS activity is crucial for NO-mediated vasodilation of peripheral collateral vessels after arterial occlusion but not for collateral artery growth.

Insights into pathways of arteriogenesis.

The compensatory growth of blood vessels after major arterial occlusions has been termed arteriogenesis. Although having some characteristics in common with angiogenesis, marked differences between both forms of vascular growth exist relating to triggers, underlying mechanisms and physiologic effects. Arteriogenesis describes the remodelling of small interconnecting arterial anastomoses with almost no net blood flow to large functional arteries. It has been shown that growth of these collateral arteries is triggered by physical forces, but does not require hypoxia as a stimulus. In this review we describe an animal model which we used to characterize the role of fluid shear stress for arteriogenesis. Fluid shear stress initiates the activation of endothelial cells and modulates processes which control attraction of circulating cells to the collateral wall. Monocytes were shown to have a pivotal role during arteriogenesis. After entering the vascular wall they function as micro-bioreactors producing cytokines and thereby controlling cell proliferation and remodelling. Furthermore, cell proliferation coincides with the transient dismantling of extracellular structures such as the elastic lamina which is required to provide space for the increasing number of wall cells. After the re-arrangement of wall structures collaterals with large calibres represent functional arteries with the ability to compensate blood flow deficits caused by arterial occlusions. It is therefore questionable, whether there is also a form of de novo collateral artery growth with physiologic relevance.

Expression of PSACH-associated mutant COMP in tendon fibroblasts leads to increased apoptotic cell death irrespective of the secretory characteristics of mutant COMP.

OBJECTIVE: Pseudoachondroplasia (PSACH) is a dominantly inherited chondrodysplasia associated with mutations of cartilage oligomeric matrix protein (COMP), characterized clinically by disproportionate dwarfism and laxity of joints and ligaments. Studies in chondrocytes and cartilage biopsies suggest that the cartilage disease is caused by retention of mutant COMP in the endoplasmic reticulum of chondrocytes and by disruption of the collagen network of the extracellular matrix. The pathogenesis of the tendon disease remains unclear in the absence of a cell culture model, with available tendon biopsies leading to conflicting results with respect to the intracellular retention of mutant COMP. METHODS: We established a cell culture model using adenoviral gene transfer in tendon fibroblast cultures. We compared the effect of expression of three PSACH-associated COMP mutants and the wildtype protein on COMP secretion, matrix composition and cellular viability. RESULTS: Our results show that mutants D475N and D469Delta are retained within the endoplasmic reticulum of tendon cells similar to what is known from chondrocytes, whereas mutant H587R is secreted like wildtype COMP. In spite of this difference, the collagen I matrix formed in culture appears disturbed for all three mutants. All COMP-mutants induce apoptotic cell death irrespective of their differing secretion patterns. CONCLUSION: Pathogenic pathways leading to tendon disease in humans appear to be heterogeneous between different COMP mutants.

Critical function of Bmx/Etk in ischemia-mediated arteriogenesis and angiogenesis.

Bmx/Etk non-receptor tyrosine protein kinase has been implicated in endothelial cell migration and tube formation in vitro. However, the role of Bmx in vivo is not known. Bmx is highly induced in the vasculature of ischemic hind limbs. We used both mice with a genetic deletion of Bmx (Bmx-KO mice) and transgenic mice expressing a constitutively active form of Bmx under the endothelial Tie-2 enhancer/promoter (Bmx-SK-Tg mice) to study the role of Bmx in ischemia-mediated arteriogenesis/angiogenesis. In response to ischemia, Bmx-KO mice had markedly reduced, whereas Bmx-SK-Tg mice had enhanced, clinical recovery, limb perfusion, and ischemic reserve capacity when compared with nontransgenic control mice. The functional outcomes in these mice were correlated with ischemia-initiated arteriogenesis, capillary formation, and vessel maturation as well as Bmx-dependent expression/activation of TNF receptor 2- and VEGFR2-mediated (TNFR2/VEGFR2-mediated) angiogenic signaling in both hind limb and bone marrow. More importantly, results of bone marrow transplantation studies showed that Bmx in bone marrow-derived cells plays a critical role in the early phase of ischemic tissue remodeling. Our study provides the first demonstration to our knowledge that Bmx in endothelium and bone marrow plays a critical role in arteriogenesis/angiogenesis in vivo and suggests that Bmx may be a novel target for the treatment of vascular diseases such as coronary artery disease and peripheral arterial disease.

The range of adaptation by collateral vessels after femoral artery occlusion.

Natural adaptation to femoral artery occlusion in animals by collateral artery growth restores only approximately 35% of adenosine-recruitable maximal conductance (C(max)) probably because initially elevated fluid shear stress (FSS) quickly normalizes. We tested the hypothesis whether this deficit can be mended by artificially increasing FSS or whether anatomical restraints prevent complete restitution. We chronically increased FSS by draining the collateral flow directly into the venous system by a side-to-side anastomosis between the distal stump of the occluded femoral artery and the accompanying vein. After reclosure of the shunt collateral flow was measured at maximal vasodilatation. C(max) reached 100% already at day 7 and had, after 4 weeks, surpassed (2-fold) the C(max) of the normal vasculature before occlusion. Expression profiling showed upregulation of members of the Rho-pathway (RhoA, cofilin, focal adhesion kinase, vimentin) and the Rho-antagonist Fasudil markedly inhibited arteriogenesis. The activities of Ras and ERK-1,-2 were markedly increased in collateral vessels of the shunt experiment, and infusions of L-NAME and L-NNA strongly inhibited MAPK activity as well as shunt-induced arteriogenesis. Infusions of the peroxinitrite donor Sin-1 inhibited arteriogenesis. The radical scavengers urate, ebselen, SOD, and catalase had no effect. We conclude that increased FSS can overcome the anatomical restrictions of collateral arteries and is potentially able to completely restore maximal collateral conductance. Increased FSS activates the Ras-ERK-, the Rho-, and the NO- (but not the Akt-) pathway enabling collateral artery growth.

Finite-Reynolds-number effects in steady, three-dimensional airway reopening.

Motivated by the physiological problem of pulmonary airway reopening, we study the steady propagation of an air finger into a buckled elastic tube, initially filled with viscous fluid. The system is modeled using geometrically non-linear, Kirchhoff-Love shell theory, coupled to the free-surface Navier-Stokes equations. The resulting three-dimensional, fluid-structure-interaction problem is solved numerically by a fully coupled finite element method. Our study focuses on the effects of fluid inertia, which has been neglected in most previous studies. The importance of inertial forces is characterized by the ratio of the Reynolds and capillary numbers, ReCa, a material parameter. Fluid inertia has a significant effect on the system's behavior, even at relatively small values of ReCa. In particular, compared to the case of zero Reynolds number, fluid inertia causes a significant increase in the pressure required to drive the air finger at a given speed.

Impact of mouse strain differences in innate hindlimb collateral vasculature.

OBJECTIVE: To assess the importance of genetic background for collateral artery development. METHODS AND RESULTS: C57BL/6, BALB/c and 129S2/Sv mice were studied after femoral artery ligation by laser Doppler imaging, visible light oximetry, time-of-flight-magnetic resonance imaging, and treadmill testing; C57BL/6 and BALB/c also underwent electron paramagnetic resonance (EPR) oximetry, x-ray angiography, and histology. C57BL/6 had the least initial distal ischemia and most complete recovery. BALB/c had the most severe initial ischemia and poorest recovery. BALB/c had some vasodilatory reserve in their ligated limbs not seen in the other strains at 3 weeks. By in vivo TOF-magnetic resonance angiography, C57BL/6 had larger preexistent and developed collaterals. By x-ray angiography, C57BL/6 had a higher collateral-dependent filling score and number of visible collaterals immediately after femoral ligation and a higher number of visible collaterals at 1 week but not at 4 weeks. EPR oximetry and histology revealed hypoxia and tissue damage in regions of collateral growth of BALB/c but not C57BL/6 mice. In C57BL/6 BrdUrd uptake in the thigh was limited to larger vessels and isolated perivascular cells. Proliferative activity in collateral arterioles was similar in both strains. CONCLUSIONS: Genetic differences in preexistent collateral vasculature can profoundly affect outcome and milieu for compensatory collateral artery growth after femoral artery occlusion.