Body Cavity Development Is Guided by Morphogen Transfer between Germ Layers

The body cavity is a space where internal organs develop and are placed. Despite the importance of this internal space, how the body cavity forms specifically within the mesoderm remains largely unknown. Here, we report that upon the onset of dorsal mesodermal cell polarization and initial lumen formation, mesodermal cells form filamentous projections that are directed toward the ectoderm. This cell behavior enables the dorsal population of mesodermal cells to receive BMP7 that is expressed by the ectoderm. Suppression of ectodermal BMP7 diminishes mesodermal cell projection and results in the loss of body cavity development. The data reveal that body cavity induction depends on signaling factor transfer from ectoderm to mesoderm. [Display omitted] •Live visualization of early body cavity formation and mesodermal cell behavior•Mesodermal cells form long projections and reach out toward surface ectoderm•Ectodermal BMP7 is delivered to mesodermal cells via cellular projections•BMP7 is crucial for body cavity formation by promoting mesodermal epithelization Schlueter and Mikawa show that body cavity formation is initiated by mesodermal cell projections that are directed toward the ectoderm. These projections are promoted by ectodermal BMP7, which plays a crucial role in the establishment of two distinct mesodermal layers at the onset of body cavity development.