**6.2** *In vivo* **imaging of embryonic vascular development**

BAC transgenic mice expressing reporter constructs can also be used to monitor and measure the dynamic emergence and fate of distinct cell types. For example, in our studies,

we adopted an embryo culture system previously used for in vivo imaging of the cardiovascular system and hemodynamics (Garcia, Udan et al. 2011; Garcia, Udan et al. 2011). We used the SMA-mCherry reporter mice, crossed to an endothelial specific Flk1-YFP reporter mouse line, to monitor endothelial-mesenchymal interactions during vascular development (Fraser, Hadjantonakis et al. 2005). Still images of a time-lapse experiment are shown in Figure 4. At early time points in the experiment (~E8.5), only (YFP+) endothelial cells are present within the developing yolk sac (Fig. 4a,d). As vascular development progresses during embryo culture (~E9.0), SMA expressing cells appear (Fig. 4b,e, arrowhead); their migration within the tissue can be monitored over time (Fig. 4c,f, arrowhead). Heart development and function can also be monitored and measured using these mice, as previously reported (Armstrong et al. 2010).

Fig. 4. Still series from in vivo imaging. (a,d) YFP+ endothelial cells prior to the emergence of SMA+ cells. At later time points, SMA-expressing cells beginning to appear (b,e), and their movement within the tissue can be monitored over time (c,d).
