Highlights d Endothelial cells communicate mechanically in a basement membrane material d A defined stiffness range, plasticity, and the presence of laminin are mandatory d Cells remodel the matrix forming stiff bridges, which precede cell protrusions d These processes are an organizing principle for vascular network formation

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For biomedical applications of nanoconstructs, it is a general prerequisite to efficiently reach the desired target site. In this regard, it is crucial to determine the spatiotemporal distribution of nanomaterials at the microscopic tissue level. Therefore, the effect of different surface modifications on the distribution of microinjected quantum dots (QDs) in mouse skeletal muscle tissue has been investigated. In vivo real-time fluorescence microscopy and particle tracking reveal that carboxyl QDs preferentially attach to components of the extracellular matrix (ECM), whereas QDs coated with polyethylene glycol (PEG) show little interaction with tissue constituents. Transmission electron microscopy elucidates that carboxyl QDs adhere to collagen fibers as well as basement membranes, a type of ECM located on the basolateral side of blood vessel walls. Moreover, carboxyl QDs have been found in endothelial junctions as well as in caveolae of endothelial cells, enabling them to translocate into the vessel lumen. The in vivo QD distribution is confirmed by in vitro experiments. The data suggest that ECM components act as a selective barrier depending on QD surface modification. For future biomedical applications, such as targeting of blood vessel walls, the findings of this study offer design criteria for nanoconstructs that meet the requirements of the respective application.

A ngiogenesis, the formation of new blood vessels from pre-existing ones, is essential during development and wound healing and in diseases, such as cancer. Invasion into and migration through the extracellular matrix (ECM) are mandatory for endothelial cells (ECs) to assemble a vascular system. Thus, there is tremendous interest in understanding the properties and behavior of ECs in 3-dimensional (3D) environments.The basic concepts underlying EC migration have mostly been gleaned from observations in 2D cell culture systems. 1,2 However, the 3D environment encountered in vivo is far more complex. Cells have to integrate and coordinate their adhesion with the ECM and interpret attractive and repulsive cues to choose their pathway. 3,4 Many studies in recent years have revealed that different cell types use specific mechanisms to migrate into and navigate through the ECM. Studies in primary human fibroblasts showed that structurally distinct 3D environments support different modes of cell migration and that the polarization of phosphatidylinositol (3,4,5)-triphosphate and Rho family GTPase signaling differs between lobopodia-and lamellipodia-based 3D migration. 5 Studies on leukocyte motility and cancer cell migration in 3D environments revealed a switch between adhesion-dependent mesenchymal (elongated) and adhesion-independent amoeboid (rounded) cell motility in these cell types. Adhesion-independent motility is driven by actin polymerization and actomyosin contraction. 6,7 Plasma membrane blebbing was once primarily viewed as a by-product of apoptotic and necrotic processes. However, subsequent studies showed that cell blebbing is not limited to the execution of cell death programs 8,9 but is also implicated in cell movement. Plasma membrane blebs are now accepted as one of the types of cell protrusions mediating migration, similar to filopodia, lamellipodia, invadopodia, and podosomes. 10 However, which types of migration occur in ECs and whether ECs can switch between them remain largely unknown.ECMs are major components of the body's connective tissue and influence cellular functions, in addition to acting as a major reservoir of releasable growth factors and peptide mediators. These matrices vary in terms of fiber thickness, density, and stiffness as well as the pore size between fibers. The main ECM component of interstitial tissues is fibrillar collagen type © 2016 American Heart Association, Inc. Objective-Cell-matrix interactions are crucial for regulating cellular activities, such as migration. This is of special importance for morphogenic processes, such as angiogenesis (the development of new blood vessels). Most of our understanding of cell migration relies on 2-dimensional (2D) experiments. However, the awareness that 3D settings might elicit different results has increased. Knowledge about endothelial cell (EC) behavior in 3D environments and the influence of matrix composition on EC migration, in particular, is still limited. Approach and Results-We characterize the migration of single E...

Endothelial tube formation on a reconstituted extracellular matrix (Matrigel) is a wellestablished in vitro model for studying the processes of angiogenesis and vasculogenesis. However, to date, the organizing principles that underlie the morphogenesis of this network, and that shape the initial process of cell-cell finding remain elusive. Furthermore, it is unclear how in vitro results extrapolate to in vivo morphogenesis. Here, we identify a mechanism that allows cells to form networks by mechanically reorganizing and stiffening their extracellular matrix, independent of chemical guidance cues. Interestingly, we find that this cellular self-organization strongly depends on the connectivity and topology of the surrounding matrix, as well as on cell contractility and cell density. Cells rearrange the matrix, and form bridges of matrix material that are stiffer than their surroundings, thus creating a durotactic track for the initiation of cell-cell contacts. This contractility-based communication via strain stiffening and matrix rearrangement might be a general organizing principle during tissue development or regeneration.3 Significance StatementIn addition to chemotactic gradients, biomechanical cues are important for guiding biological pattern formation. Self-assembly of cells has often been ascribed to reorganization of collagen fibres in the extracellular matrix. However, the basement membrane surrounding vascular cells, is per se non-fibrous. Here, we find that this difference in matrix topology can crucially influence cell behaviour and pattern formation. In a homogeneously elastic environment like the basement membrane, endothelial cells rearrange extracellular matrix proteins by contractile force, forming stiff intercellular bridges as tracks for cell-cell contacts. Our findings shine some light why there is a lot of merit in having multiple approaches to matrix elasticity (like continuum theories or dilated network approaches). Our observations might help to understand why vascular nets look different in different tissues and after rearrangement of the extracellular matrix during disease.

On page 2641, K. Nekolla, M. Rehberg, and co‐workers investigate the microdistribution of surface modified quantum dots in vivo. As depicted, microinjected carboxyl quantum dots into tissue attach to muscle and collagen fibers, as well as endothelial basement membranes. In conclusion the study indicates that the choice of surface modification guides the distribution of quantum dots in the tissue.