*Review* **On Top of the Alveolar Epithelium: Surfactant and the Glycocalyx**

**Matthias Ochs 1,2,\*,**†**, Jan Hegermann 3,**†**, Elena Lopez-Rodriguez 1, Sara Timm 4, Geraldine Nouailles 5, Jasmin Matuszak 6, Szandor Simmons 6, Martin Witzenrath 2,5 and Wolfgang M. Kuebler 2,6**


Received: 31 March 2020; Accepted: 18 April 2020; Published: 27 April 2020

**Abstract:** Gas exchange in the lung takes place via the air-blood barrier in the septal walls of alveoli. The tissue elements that oxygen molecules have to cross are the alveolar epithelium, the interstitium and the capillary endothelium. The epithelium that lines the alveolar surface is covered by a thin and continuous liquid lining layer. Pulmonary surfactant acts at this air-liquid interface. By virtue of its biophysical and immunomodulatory functions, surfactant keeps alveoli open, dry and clean. What needs to be added to this picture is the glycocalyx of the alveolar epithelium. Here, we briefly review what is known about this glycocalyx and how it can be visualized using electron microscopy. The application of colloidal thorium dioxide as a staining agent reveals differences in the staining pattern between type I and type II alveolar epithelial cells and shows close associations of the glycocalyx with intraalveolar surfactant subtypes such as tubular myelin. These morphological findings indicate that specific spatial interactions between components of the surfactant system and those of the alveolar epithelial glycocalyx exist which may contribute to the maintenance of alveolar homeostasis, in particular to alveolar micromechanics, to the functional integrity of the air-blood barrier, to the regulation of the thickness and viscosity of the alveolar lining layer, and to the defence against inhaled pathogens. Exploring the alveolar epithelial glycocalyx in conjunction with the surfactant system opens novel physiological perspectives of potential clinical relevance for future research.

**Keywords:** lung; alveoli; air-blood barrier; epithelium; air-liquid interface; alveolar lining layer; glycocalyx; surfactant
