Medial epithelial seam cell migration during palatal fusion

The mammalian secondary palate forms from two shelves of mesenchyme sheathed in a single‐layered epithelium. These shelves meet during embryogenesis to form the midline epithelial seam (MES). Failure of MES degradation prevents mesenchymal confluence and results in a cleft palate. Previous studies indicated that MES cells undergo features of epithelial‐to‐mesenchymal transition (EMT) and may become migratory as part of the fusion mechanism. To detect MES cell movement over the course of fusion, we imaged the midline of fusing embryonic ephrin‐B2/GFP mouse palates in real time using two‐photon microscopy. These mice express an ephrin‐B2‐driven green fluorescent protein (GFP) that labels the palatal epithelium nuclei and persists in those cells through the time window necessary for fusion. We observed collective migration of MES cells toward the oral surface of the palatal shelf over 48 hr of imaging, and we confirmed histologically that the imaged palates had fused by the end of the imaged period. We previously reported that ephrin reverse signaling in the MES is required for palatal fusion. We therefore added recombinant EphA4/Fc protein to block this signaling in imaged palates. The blockage inhibited fusion, as expected, but did not change the observed migration of GFP‐labeled cells. Thus, we uncoupled migration and fusion. Our data reveal that palatal MES cells undergo a collective, unidirectional movement during palatal fusion and that ephrin reverse signaling, though required for fusion, controls aspects of the fusion mechanism independent of migration. Dissolution of the midline epithelial seam during development is required for fusion of the secondary palate from its two lateral shelves. We used live‐cell imaging in embryonic mouse palates to reveal for the first time a concerted, directional migration of these cells during fusion. Fusion could be blocked without inhibiting migration, indicating that elements of signaling in the epithelial seam other than simple removal by translational motion are crucial for fusion.