On Top of the Alveolar Epithelium: Surfactant and the Glycocalyx

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.

Metabolic Glycoengineering Enables the Ultrastructural Visualization of Sialic Acids in the Glycocalyx of the Alveolar Epithelial Cell Line hAELVi

“…The recently established hAELVi cell line has been described as a model for ATI cells, although molecular characterization has been first, limited to the expression of a few marker genes and second, not been performed using cells harvested at ALI ( Kuehn et al, 2016 ). After 13–14 days of culture at ALI, we report extensive formation of microvilli and a well-marked glycocalyx, both structural features more prevalent in ATII cells ( Brooks, 1969 ; Adamson and Bowden, 1970 ; Roth, 1973 ; Mason, 2006 ; Ochs et al, 2020 ). Thus, the differentiation fate of the cell line might depend on the applied culture conditions.…”

“…The underlying liquid hypophase acts as a space for assembly and recycling of surfactant precursor and degradation products, respectively, but also consists of an apical sugar-rich cell surface coat called the glycocalyx. The latter has recently been proposed to interact with surfactant components ( Ochs et al, 2020 ). The epithelial surface of lumen forming organs is characteristically rich in glycocalyx, providing overlapping but also highly tissue specific functions.…”

“…Despite upcoming interest in the glycocalyx of the alveolar epithelium, its structure and functions remain largely unexplored (reviewed in Ochs et al, 2020 ). This is first, attributable to the lack of physiologically relevant and cost effective in vitro models.…”

Volume electron microscopy: analyzing the lung

“…FIB-SEM does not necessarily require high-contrast embedding, but the single lamellae of LBs are at the edge of possible resolution and hard to discriminate, especially their curvatures and continuity. With classical embedding protocols (see “ Serial sectioning transmission electron microscopy (ssTEM) ”) using en bloc UA and one osmium step, individual intra- and extracellular surfactant lamellae are clearly discernible in a TEM on ultrathin sections (Ochs et al 2020 ). ET also enables resolving the individual lamellae, even the two phospholipid leaflets (Ochs et al 2016 ).…”

Mechanical ventilation-induced alterations of intracellular surfactant pool and blood–gas barrier in healthy and pre-injured lungs

“…In general, the intra-alveolar surfactant pool responsible for biophysical and immunological functions can be differentiated from the intra-cellular surfactant pool within the alveolar epithelial type II (AE2) cells (Ochs 2010 ). Pulmonary surfactant, including both lipids and surfactant proteins, is synthesized, stored and secreted by AE2 cells (Perez-Gil and Weaver 2010 ; Ochs et al 2020 ). AE2 cells possess a unique organelle for storing surfactant lipids and hydrophobic surfactant proteins called lamellar bodies (LB) which are the ultrastructural correlate of the intracellular surfactant pool.…”