Development of BALT regions in the intrapulmonary primary and secondary bronchi of CSF1R-transgenic birds. Whole mount analysis of the intrapulmonary bronchi of a CSF1R-mApple chicken. BALT structures are associated with each opening of the secondary bronchi and increase in size and number with age (A–L). Intrapulmonary bronchi of older CSF1R-eGFP animals show the presence of BALT (K–L) and the number of lymphoid follicles seemingly reduces in older animals (K–L). Primary bronchus (PB) is indicated by dashed lines, lymphoid follicles indicated by a dashed circle and openings to the secondary bronchi are marked by an asterisk. w = week/s of age.