SummaryA computer-controlled microscopy system was devised to allow the observation of avian embryo development over an extended time period. Parallel experiments, as well as extended specimen volumes, can be recorded at cellular resolution using a three-dimensional scanning procedure. The resulting large set of data is processed automatically into registered, focal-and positional-drift corrected mosaic images, assembled as montages of adjacent microscopic fields. The configuration of the incubator and a sterile embryo chamber prevents condensation of the humidified culturing atmosphere in the optical path and is compatible with both differential interference contrast and epifluorescence optics. As a demonstration, recordings are presented showing the large-scale remodelling of the embryonic primordial vascular structure.

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A major challenge confronting developmental cell biologists is to understand how individual cell behaviors lead to global tissue organization. Taking advantage of an endothelial cell-specific marker and scanning time-lapse microscopy, we have examined the formation of the primary vascular pattern during avian vasculogenesis. Five types of distinguishable endothelial cell motion are observed during formation of a vascular plexus: (1) global tissue deformations that passively convect endothelial cells; (2) vascular drift, a sheet-like medial translocation of the entire vascular plexus; (3)structural rearrangements, such as vascular fusion; (4) individual cell migration along existing endothelial structures; and (5) cell process extension into avascular areas, resulting in new links within the plexus. The last four types of motion are quantified and found to be reduced in the presence of an αvβ3 integrin inhibitor. These dynamic cell motility data result in new hypotheses regarding primordial endothelial cell behavior during embryonic vasculogenesis.

In recent years, there has been a sustained interest in vascularization processes. Much, if not all, of the work has included the concept of new vessel morphogenesis. Surprisingly, most of the work has not addressed developmental mechanisms directly, but rather as an offshoot of a disease process, wound healing process, or from the perspective of inducing vessels in an ischemic site. One theme has dominated the various studies on capillary or endothelial tube morphogenesis-integrin-mediated cell behavior. Integrin biology impacts virtually every known step of nascent vessel formation. In this review article, we attempted to summarize key findings from the viewpoint of developmental biologists/morphologists. We also attempted to summarize and contrast data obtained using integrin gene ablation approaches in mice with other experimental systems. It is hoped this review will provide a distinct cell biological perspective to vascular scientists from the clinical, molecular, and tissue engineering communities.

Cellular invasive behavior through three-dimensional collagen gels was analyzed using computational time-lapse imaging. A subpopulation of endocardial cells, derived from explanted quail cardiac cushions, undergoes an epithelialto-mesenchymal transition and invades the substance of the collagen gels when placed in culture. In contrast, other endocardial cells remain epithelial and move over the gel surface. Here, we show that integrin ␣v␤3 and matrix metalloproteinase (MMP)2 are present and active in cushion mesenchymal tissue. More importantly, functional assays show that mesenchymal invasive behavior is dependent on MMP2 activity and integrin ␣v␤3 binding. Inhibitors of MMP enzymatic activity and molecules that prevent integrin ␣v␤3 binding to MMP2, via its hemopexin domain, result in significantly reduced cellular protrusive activity and invasive behavior. Computational analyses show diminished intensity and persistence time of motility in treated invasive mesenchymal cells, but no reduction in motility of the epithelial-like cells moving over the gel surface. Thus, quantitative time-lapse data show that mesenchymal cell invasive behavior, but not epithelial cell locomotion over the gel surface, is partially regulated by the MMP2-integrin interactions. INTRODUCTIONCell motility within a three-dimensional (3D) tissue space, often termed invasion, is a dynamic process involved in a host of morphological and pathological events (Harris, 1987). Extracellular matrix (ECM) fibers provide mechanical support for tissue integrity/deformations, act as a scaffold for cell motility, and act as a repository of growth factors and latent enzymes (McCarthy and Turley, 1993;Damsky et al., 1997;Friedl and Brocker, 2000;Davis and Senger, 2005). Cell invasion is often coupled to local ECM degradation (Ellis and Murphy, 2001). In an extreme case, structures termed invadopodia have been observed on the surface of invasive tumor cells. These structures are cellular protrusions that contain surface-bound, externally facing proteases (Chen, 1989;Mueller and Chen, 1991;Monsky et al., 1993Monsky et al., , 1994Kelly et al., 1994).During normal embryonic development extracellular matrix protease activity is important for successful epithelialto-mesenchymal transformations (EMT) that accompany neural crest, sclerotome, and renal tubular cell delamination (Cheng and Lovett, 2003;Song et al., 2000;Duong and Erickson, 2004). During epithelial-to-mesenchymal transformations ECM proteolysis may not only remove a physical barrier, but also expose cell receptor sites and regulatory molecules sequestered in the ECM. Similar proteolytic events are recapitulated during angiogenesis and tubulogenesis in vitro (Bayless and Davis, 2003;Fisher et al., 2006). When deprived of specific proteolytic mechanisms, some cell types change their motility strategy and move through the ECM by means of amoeboid behavior (Wolf et al., 2003).Degradation of the ECM occurs by a number of secreted proteinases. One family, the matrix metalloproteinases (MMPs), is compose...