During its development, the heart tube undergoes rapid elongation, fuelled by the addition of cardiac progenitors from the second heart field (SHF). The gene regulatory networks governing SHF formation have been studied extensively, but little is known about the basic cellular features of SHF cells. Now, Robert Kelly and co-workers show that the transcription factor TBX1, which is implicated in both normal SHF development and congenital heart defects, regulates the epithelial properties of mouse SHF cells (p. ). Using immunofluorescence microscopy, they first show that SHF cells in the dorsal pericardial wall constitute an apicobasally polarised epithelium. Transmission and scanning electron microscopy reveal the presence of monocilia on the apical surface of SHF cells and of actin-rich filopodia on their basal surface. Using live-imaging of thick-slice cultures, the researchers demonstrate that these filopodia are dynamic, extending towards and making contact with surrounding tissues. Importantly, they report that TBX1 plays a crucial role in regulating these epithelial cell features; cell shape, cell polarity and filopodia dynamics are perturbed in Tbx1-/- mutants. These exciting findings suggest that TBX1-mediated control of epithelial state is crucial for heart development.
