The parabronchus region in CSF1R-transgenic birds. Tertiary bronchi or parabronchi represent the functional unit of gas exchange and consist of a lumen (L), atria (solid yellow line) and infundibulum (dashed white line) surrounded by connective tissue that contains blood vessels and lymphoid follicles (indicated by dotted line). Scattered CSF1R-transgene+ are located throughout the parabronchus and co-express CD11 (A) and MHC II (B). LAMP1 is expressed in the majority of the CSF1R-transgene+ cells and in the pneumocytes in the interatrial septa (C). Enlargements of the interatrial septa (D–F).