Remodeling of the Microvasculature: May the Blood Flow Be With You

Abstract

The vasculature ensures optimal delivery of nutrients and oxygen throughout the body, and to achieve this function it must continually adapt to varying tissue demands. Newly formed vascular plexuses during development are immature and require dynamic remodeling to generate well-patterned functional networks. This is achieved by remodeling of the capillaries preserving those which are functional and eliminating other ones. A balanced and dynamically regulated capillary remodeling will therefore ensure optimal distribution of blood and nutrients to the tissues. This is particularly important in pathological contexts in which deficient or excessive vascular remodeling may worsen tissue perfusion and hamper tissue repair. Blood flow is a major determinant of microvascular reshaping since capillaries are pruned when relatively less perfused and they split when exposed to high flow in order to shape the microvascular network for optimal tissue perfusion and oxygenation. The molecular machinery underlying blood flow sensing by endothelial cells is being deciphered, but much less is known about how this translates into endothelial cell responses as alignment, polarization and directed migration to drive capillary remodeling, particularly in vivo. Part of this knowledge is theoretical from computational models since blood flow hemodynamics are not easily recapitulated by in vitro or ex vivo approaches. Moreover, these events are difficult to visualize in vivo due to their infrequency and briefness. Studies had been limited to postnatal mouse retina and vascular beds in zebrafish but new tools as advanced microscopy and image analysis are strengthening our understanding of capillary remodeling. In this review we introduce the concept of remodeling of the microvasculature and its relevance in physiology and pathology. We summarize the current knowledge on the mechanisms contributing to capillary regression and to capillary splitting highlighting the key role of blood flow to orchestrate these processes. Finally, we comment the potential and possibilities that microfluidics offers to this field. Since capillary remodeling mechanisms are often reactivated in prevalent pathologies as cancer and cardiovascular disease, all this knowledge could be eventually used to improve the functionality of capillary networks in diseased tissues and promote their repair.

Keywords: 3D-confocal microscopy; blood flow; capillary pruning; capillary splitting; endothelial cells; microfluidics; microvascular remodeling; shear stress.

FIGURE 1

Capillary remodeling by pruning/regression and splitting/duplication. (A) In capillary pruning the poorly perfused vessel is selected to regress, its lumen collapses and the endothelial cells inside (dark orange) polarized against flow and migrate toward the higher flow adjacent vessel. (B) In capillary splitting, highly perfused vessels vasodilate and endothelial cells nearby the intersections (Y bifurcation; dark orange) reorganize their cytoskeleton, migrate toward the lumen and form an intraluminal pillar that will eventually split the vessel forming two daughter vessels. Arrows indicate the direction and intensity of the blood flow (arterial flow in red and venous flow in blue). Endothelial cell nuclei and Golgi are colored in yellow and red, respectively, to show endothelial cell polarization preferentially against the flow.

FIGURE 2

Blood flow-driven capillary pruning and splitting: two sides of the same coin. (A) Heterogeneous flow distribution in dynamic microvascular networks results in shear stress gradients (left). As a consequence, lower perfused segments will be selected to regress (*) and highly perfused segments to split (#) in order to redistribute flow in a more efficient manner in the remodeled network (right). (B) Endothelial cells sense the blood flow and shear stress gradients and respond by elongating and aligning in the direction of flow. In the case of pruning, endothelial cells in the regressing segment (left panel) detach from their neighbors (red) during lumen collapse, polarize (Golgi in brown) and migrate toward the region of higher flow in the adjacent vessel (curved arrows). In capillary splitting (right panel), endothelial cells close to shear stress gradient at the bifurcation, reorganize their cytoskeleton (pink), protrude luminal filopodia (red), and polarize (Golgi in brown) and migrate toward the lumen to form the intraluminal pillar; note the area drawn with low blood flow/shear stress at the nascent pillar which could be permissive for filopodia growth and fusion. Heat map colors indicate the predicted/simulated shear stress values in the modeled microvascular network. Straight arrows indicate the flow direction. Adapted from Chen et al. (2012).

FIGURE 3

Image-based approaches to capillary remodeling in vivo and proposed device-based simulation in vitro. (A, B) Examples of whole-mount staining and confocal microscopy for the visualization and analysis of capillary pruning in the mouse postnatal retina (A) and of capillary splitting in the inflamed mouse intestine by means of 3D reconstruction with Imaris software. Scale bar, 20 μm. (C) Top view of proposed microfluidic chips for the study of capillary regression in an H-type geometry (left panel) and of capillary splitting in Y-type geometry (right panel), mimicking the geometries found in microvascular networks (blue boxes in panels A and B). Inlets and outlets for flow are indicated and are exchangeable depending on the preferred direction of flow in the different segments. Vessel channels are covered with a basement membrane and endothelial cells. In purple auxiliary channels for creating biochemical gradients along the extracellular matrix (ECM)-like hydrogel region (light blue). This hydrogel could also incorporate cells for co-culture analysis. Heat map color represent theoretical shear stress values (as in Figure 2) for established flow gradient profiles and shading represent biochemical gradients along the ECM. The rest of the microfluidic chip would be made of PDMS.