Alveolar epithelial type II cells from embryonic stem cells: knights in shining armour?

T oday, the pulmonary epithelium is generally accepted to possess unique properties that go far beyond the role as a mere physical barrier protecting an organism’s interior against the detrimental effects exerted by airborne environmental challenges, such as gases, particles, allergens or microorganisms. The specific tasks respiratory epithelial cells have to fulfil differ along the bronchiotracheal tract down to the alveoli [1, 2]. Accordingly, there is a continuous change in the cellular composition of the epithelium along the airway tract reflecting remarkable structural and functional differentiation. Besides long-known roles of the epithelium, such as mucociliary clearance, lung fluid balance and surfactant homeostasis, which assure the removal of particles and cells, the avoidance of oedema and the proper adjustment of surface tension, respectively, are additional important functions that have been identified in recent years related to the regulation of innate and adaptive immune responses [3–6].