A New Approach to Model Confined Suspensions Flows in Complex Networks: Application to Blood Flow

The modeling of blood flows confined in micro-channels or micro-capillary beds depends on the interactions between the cell-phase, plasma and the complex geometry of the network. In the case of capillaries or channels having a high aspect ratio (their longitudinal size is much larger than their transverse one), this modeling is much simplified from the use of a continuous description of fluid viscosity as previously proposed in the literature. Phase separation or plasma skimming effect is a supplementary mechanism responsible for the relative distribution of the red blood cell’s volume density in each branch of a given bifur- cation. Different models have already been proposed to connect this effect to the various hydrodynamics and geometrical parameters at each bifurcation. We discuss the advantages and drawbacks of these models and compare them to an alternative approach for modeling phase distribution in complex channels networks. The main novelty of this new formulation is to show that albeit all the previous approaches seek for a local origin of the phase segre- gation phenomenon, it can arise from a global non-local and nonlinear structuration of the flow inside the network. This new approach describes how elementary conservation laws are sufficient principles (rather than the complex arametric models previously proposed) to provide non local phase separation. Spatial variations of the hematocrit field thus result from the topological complexity of the network as well as nonlinearities arising from solving a new free boundary problem associated with the flux and mass conservation. This network model approach could apply to model blood flow distribution either on artificial micro-models, micro-fluidic networks, or realistic reconstruction of biological micro-vascular networks

Coupling and robustness of intra-cortical vascular territories

Vascular domains have been described as being coupled to neuronal functional units enabling dynamic blood supply to the cerebral cyto-architecture. Recent experiments have shown that penetrating arterioles of the grey matter are the building blocks for such units. Nevertheless, vascular territories are still poorly known, as the collection and analysis of large three-dimensional micro-vascular networks are difficult. By using an exhaustive reconstruction of the micro-vascular network in an 18 mm 3 volume of marmoset cerebral cortex, we numerically computed the blood flow in each blood vessel. We thus defined arterial and venular territories and examined their overlap. A large part of the intracortical vascular network was found to be supplied by several arteries and drained by several venules. We quantified this multiple potential to compensate for deficiencies by introducing a new robustness parameter. Robustness proved to be positively correlated with cortical depth and a systematic investigation of coupling maps indicated local patterns of overlap between neighbouring arteries and neighbouring venules. However, arterio-venular coupling did not have a spatial pattern of overlap but showed locally preferential functional coupling, especially of one artery with two venules, supporting the notion of vascular units. We concluded that intra-cortical perfusion in the primate was characterised by both very narrow functional beds and a large capacity for compensatory redistribution, far beyond the nearest neighbour collaterals

Le réseau micro-vasculaire structure la distribution de la pression sanguine

Cerebral micro-vascular networks control the blood pressure distribution when considering in vitro blood rheology models. Blood rheology is complex and non-linear. In small vessels, the effective viscosity variations are important due to red blood cells packing in capillaries, the so-called Fåhræus-Lindquist effect, whilst concomitantly phase segregation appears in bifurcations. Direct numerical simulations of different non-linear rheological models of the blood are performed on realistic three-dimensional micro-vascular networks. These simulations exhibit two significant results. First, various rheological models lead to very similar pressure distribution over the whole range of physiologically relevant hematocrits. Secondly, different models for phase segregation lead to very distinct hematocrit distributions in the micro-vacular network. Nevertheless, the hematocrit distribution very weakly affects the pressure distribution. Hence, our results suggest that the micro-vacular network structure mainly controls the pressure distribution in micro-circulation, whilst the effect of hematocrit distribution is weak

From cerebral blood flow modeling to vascular units map in primate cortex

The amazing topological and geometrical complexity of micro-vascular networks in the brain, and in other organs, has challenged many researchers for decades. Since the brain's vascular system is structured by a highly reticulated pial surface network which plunges down into a set of penetrating vessels, it is tempting to attribute a vascular unit to each penetrating arteriole. Recent experimental analysis have led to a breakthrough on the properties of the blood supply in the brain. Penetrating arterioles have been identified as the bottleneck of brain perfusion. Furthermore, it has also been realized that targeted clots of penetrating arterioles are not compensated by active changes in the diameter of their neighbor arteries. This observation suggests passive compensatory mechanisms resulting from the couplings between arteriolar territories consistent with other recent observations of active blood flow reorganization via collateral vessels (inter-arterial connections). A systematic investigation of the three-dimensional extent of compensation is not possible with experimental measurements but in silico simulations permit a systematic investigation of the spatial distribution of the brain perfusion

From homogeneous to fractal normal and tumorous microvascular networks in the brain

We studied normal and tumorous three-dimensional (3D) microvascular networks in primate and rat brain. Tissues were prepared following a new preparation technique intended for high-resolution synchrotron tomography of microvascular networks. The resulting 3D images with a spatial resolution of less than the minimum capillary diameter permit a complete description of the entire vascular network for volumes as large as tens of cubic millimeters. The structural properties of the vascular networks were investigated by several multiscale methods such as fractal and power- spectrum analysis. These investigations gave a new coherent picture of normal and pathological complex vascular structures. They showed that normal cortical vascular networks have scale- invariant fractal properties on a small scale from 1.4 lm up to 40 to 65 lm. Above this threshold, vascular networks can be considered as homogeneous. Tumor vascular networks show similar characteristics, but the validity range of the fractal regime extend to much larger spatial dimensions. These 3D results shed new light on previous two dimensional analyses giving for the first time a direct measurement of vascular modules associated with vessel-tissue surface exchange

Brain Tumor Vascular Network Segmentation from Micro-Tomography

Micro-tomography produces high resolution images of bio- logical structures such as vascular networks. In this paper, we present a new approach for segmenting vascular network into pathological and normal regions from considering their micro-vessel 3D structure only. We define and use a condi- tional random field for segmenting the output of a watershed algorithm. The tumoral and normal classes are thus character- ized by their respective distribution of watershed region size interpreted as local vascular territories

Vascular network segmentation: an unsupervised approach

Micro-tomography produces high resolution images of biological structures such as vascular networks. In this paper, we present a new approach for segmenting vascular network into pathological and normal regions from considering their micro-vessel 3D structure only. We consider a partition of the volume obtained by a watershed algorithm based on the distance from the nearest vessel. Each territory is characterized by its volume and the local vascular density. The volume and density maps are first regularized by minimizing the total variation. Then, a new approach is proposed to segment the volume from the two previous restored images based on hypothesis testing. Results are presented on 3D micro-tomographic images of the brain micro-vascular network

Automatic reconstruction of brain’s micro-vascular network based onX-ray synchrotron tomography

X-ray synchrotron tomographic microscopy enables the acquisition of large amount of images with a geometric and radiometric resolution sufficient for the morphometric and topologic analysis of vascular and even of microvascular network of different organs. In this study cylindrical shaped, NiDAB labelled brain samples of a diameter of half a millimeter were imaged and analyzed with an effective pixel size of 0.38 µm. Several thousands of tomographic slices build up each reconstructed volume meaning that an automatized analysis tool is indispensable. These big data sets are processed by the developed algorithms on commercially available PCs with an automated image analysis technology in acceptable processing time. The obtained vessel segments are stored for further topological and morphometric analyses or surface/volumetric visualization purposes. These analyses contain vessel feature distribution analysis followed by 3D reconstruction. The obtained results are in accordance to the literature data.DOI: 10.17489/biohun/2015/2/0

La communication chimique chez les bourdons (Bombus sp.) : une approche neurobiologique pluridisciplinaire

* Inra Poitou-Charentes, ERIST, Route de Saintes BP 6, 86600 Lusignan (FRA) Diffusion du document : Inra Poitou-Charentes, ERIST, Route de Saintes BP 6, 86600 Lusignan (FRA) Diplôme : Dr. d'Universit

La préparation à la mise en hivernage de l'abeille domestique (Apis mellifica L.)

* Inra Poitou-Charentes, ERIST, Route de Saintes BP 6, 86600 Lusignan (FRA) Diffusion du document : Inra Poitou-Charentes, ERIST, Route de Saintes BP 6, 86600 Lusignan (FRA