CXCL12 and pulmonary emphysema: Identification of factors modulating stem cell functions of AECII and epithelial homeostasis such as BMP/SMAD signaling and SDF-1 represent the most promising approach to understand lung regrowth after PNX and further to develop strategies to induce lung regeneration during pulmonary diseases such as emphysema/COPD and fibrosis (Fig. 4).