Osteoblasts act in a group to make bone over the course of several months. However, how osteoblasts cooperate with each other in vivo is still unknown. Now, researchers at Osaka University established an advanced high-resolution microscopy system to visualize extracellular vesicles secreted and captured by mature osteoblasts in vivo and identified a subset of osteoblast-derived vesicles limiting bone formation and stimulating bone-resorbing osteoclast differentiation through a microRNA (miRNA)-mediated mechanism. The osteoblast communication via extracellular vesicles controls the dynamic transition from bone-forming to bone-resorbing phases in vivo and could be a target for new treatments for bone diseases.