Bronchioalveolar stem cells: the crossroads of lung regeneration

Mammalian lungs are encased in dense endothelial cell networks to support systemic delivery of oxygen to vital organs (1-3), and to provide host pathogen defense by physical or chemical removal of foreign bodies (3,4). The evolutionary-refined architecture of mammalian lungs is primarily comprised of endoderm-derived epithelial cell populations (5); however, the lungs also harbour mesoderm-derived cells which contribute to the renin-angiotensin axis (6), innate immunity (3,4), and platelet production (7). Oronasal to parenchymal, the respiratory system is organized from the trachea, bronchi, bronchioles to the alveoli; the primary site of gas exchange (1,2).