Figure 1. Formation of secondary septa in alveolar development.

(A) During the final stages of lung development, cells forming the primary septa – the walls of the saccules – proliferate and migrate to form secondary septa, which divide the saccules into alveoli. The epithelial cells present at this stage of lung development are airway epithelial cells, alveolar epithelial cells type 1 and alveolar epithelial cells type 2. (B) Within the secondary septa are multiple cell types – including alveolar epithelial cells type 1 (blue), alveolar epithelial cells type 2 (green) and fibroblasts and myofibroblasts (brown) – that need to coordinate their movement and proliferation. Alveolar epithelial cells type 2 secrete platelet derived growth factor (PDGF, green dots), which binds to receptors (blue arrowheads) on the surface of fibroblasts and myofibroblasts (dashed arrow 1). This drives the proliferation of the myofibroblasts and fibroblasts (dashed arrow 2), which is required to form secondary septa.

Image credit: figure created using BioRender.