Signals and mechanisms of compensatory lung growth.

Growth of the lung involves unique structure-function interactions not seen in solid organs. Mechanical feedback between the lung and thorax constitutes a major signal that sustains developmental as well as compensatory lung growth. After the loss of lung units as by pneumonectomy (PNX), increased mechanical stress and strain on the remaining units induce adaptive responses to augment oxygen transport, including 1) recruitment of alveolar-capillary reserves, 2) remodeling of existing tissue, and 3) regenerative growth of acinar tissue when strain exceeds a critical threshold. Alveolar hypoxia, hormones, and growth factors may feed into the mechanical feedback system to modify an existing growth response but are unlikely to initiate compensatory growth in the absence of sufficient mechanical signals. Whereas endogenous post-PNX alveolar growth preserves normal structure-function relationships, experimental manipulation of selected metabolic pathways can distort these relationships. Finally, PNX widens the disparity between the rapidly adapting acini and slowly adapting conducting airways and blood vessels, leading to disproportionate airflow and hemodynamic dysfunction and secondary hypertrophy of the right ventricle and respiratory muscles that limits overall organ function despite regeneration of gas exchange tissue. These are key concepts to consider when formulating approaches to stimulate or augment compensatory growth in chronic lung disease.