Physical mechanisms of tissue compartmentalization andinternalizationin the Drosophila embryo

Compartment boundaries prevent cell mixing during embryonic development. Cables formed by actin and the molecular motor myosin II are often found at compartment boundaries. How boundaries are established and maintained remains unclear. In the Drosophila embryo, the mesectoderm separates ectoderm and mesoderm, forming the ventral midline. Eventually, mesectoderm cells are internalized becoming part of the central nervous system. We found that ectoderm and mesectoderm remained separated as the mesectoderm was internalized, suggesting the presence of a boundary between the tissues. Using live microscopy, we found an enrichment of myosin at the mesectoderm-ectoderm boundary (MEB), forming a supracellular cable. Myosin levels at the MEB decreased as the mesectoderm was internalized. To study the role of myosin cables at the MEB, we simulated the internalization using a vertex model. Our model predicted that tension at the MEB maintains the interface linearity, prevents cell mixing, and controls the timing of internalization. Consistent with this, pharmacological inhibition of myosin disrupted the MEB, leading to mesectoderm-ectoderm cell mixing and premature internalization. Our model also predicted cell divisions in the ectoderm play a role in maintaining the linearity of the MEB.