The development of MNP and GC in air sacs of CSF1R-transgenic birds. Whole mount images show that CSF1R-mApple+ cells are distributed throughout the air sac on ED18 (A) and begin to form a pattern 1 day post-hatch (B). By 1–2 weeks of age the air sacs have developed an organised distribution that consist of MNP lining the vessels and early lymphoid aggregate formations at the bifurcations of the capillaries (arrows C, D). By 4 weeks of age these lymphoid aggregates have developed into characteristic GC (arrows E, F). w = week/s of age.