Figure 1.

Epithelial cell types in different regions of the lower respiratory tract. The composition of the epithelial layer varies along the proximal-distal axis of the airways. While “classical” characterization of the epithelial lineage via histochemical methods has led to the identification of the more prominent cell types (basal cells, ciliated cells, goblet cells, club cells), novel approaches using single-cell RNA sequencing have ascertained the presence of less frequent cell types, such as tuft cells, neuroendocrine cells, and ionocytes.