Description:
In the developing heart, the epicardium is derived from the pro‐epicardial organ (PEO), which spreads over the cardiac tube (purportedly as an epithelium), and then undergoes what is believed to be an epithelial‐to‐mesenchymal transition (EMT) to generate the subepicardial mesenchyme. We are interested in the process of epicardial cell spreading and the mechanisms that regulate the epicardial EMT. To study these processes, we have developed fluorescent labeling techniques to visualize the PEO and the spreading epicardium using time‐lapse confocal imaging in intact hearts and on a planar substratum. When the PEO is labeled and then explanted before it begins to spread in vivo, we saw that each cell has numerous filopodia and lamellipodia around its borders. Fixed images of spreading PEO cells labeled in ovo confirm the presence of numerous filopodia and lamellipodia in the spreading epicardium in vivo. Although the PEO has been assumed to spread as an epithelium with the force being generated at the free edge, our live‐cell imaging suggests that each cell in the epicardium is generating tractional force. These results show that real‐time behavior studies are critical since morphology alone does not reveal behavior. Our observations challenge the notion of how the PEO‐derived cells spread over the cardiac tube during cardiac morphogenesis.Grant Funding Source: Supported by NIHT32OD011147 (JV‐M), UCD GSSP Fellowship (JV‐M), UCD GGCP Fellowship (JV‐M), IOS1146480 (CAE)