Transitioning lung for postnatal life The lung is a complex organ composed of multiple cell types, and its alveolus serves as the functional unit of gas exchange. The alveolar type 1 cell (AT1) serves as an active signaling hub in the developing and postnatal mouse and human lung. Zepp et al. generated a comprehensive single-cell atlas of the developing murine lung and identified cell differentiation and cell-to-cell communication as the lung transitions to air breathing. The AT1 cells spatially aligned with stromal progenitors and formed a signaling hub that preferentially communicated with a transient, force-exerting, myofibroblast through signaling factors including Shh and Wnts to actively remodel the alveolus after the transition to air breathing. Science , this issue p. eabc3172