**Preface to "The Alveolar Epithelium: Mechanisms of Injury and Repair"**

Alveolar epithelial cells (AECs) of the lung are important contributors to pulmonary immune functions and to pulmonary development and alveolar repair mechanisms following lung injury. AECI, together with the capillary endothelium, form the extremely thin barrier between alveolar air and blood. AECII produce and metabolize the surface-tension lowering and immune-modulating surfactant and are the progenitors of AECI. A great variety of processes rely on their normal functioning, including maintenance of the alveolar barrier; innate immune defense; and processes of differentiation, senescence, apoptosis, and autophagy.

In this issue, the wide range of AEC functions is reflected by the diversity of topics addressed by the four review and eight original articles and the analytical methods used by the authors. The review articles highlight the role of AECs in pathological conditions, such as pulmonary fibrosis and high-altitude edema as well as their role in pulmonary regeneration. The fourth review provides a thorough basis for future research on a relatively new topic in lung research – the glycocalyx of AECs. The original articles cover a delectably wide spectrum of methods, ranging from in vitro analysis on the effects of nanoparticles or the effects of hypercapnia on the Na,K-ATPase of alveolar cells in culture. The in vivo articles address both acute conditions, such as the pulmonary regeneration in infectious diseases or the role of miR21 in a model of acute lung injury, as well as chronic conditions, namely the effects of high-sugar feeding and voluntary activity on the elastic properties of the lung. Three studies investigate aspects of the normal lung, either at a molecular or morphological level or a combination of both. One study addresses the interaction between the P2X7 receptor and JAM-A protein while another one uses a genetically modified mouse model to elucidate the role of surfactant protein B in structure and function of the lung. Finally, a detailed 3D analysis of the AEC ultrastructure provides a thorough basis for future functional or molecular work on the alveolar epithelium.

Thus, this Special Issue sheds light on the broad spectrum of methods and topics in the field of research on the alveolar epithelium.

> **Michael Kasper, Christian M ¨uhlfeld** *Editors*
