Thymus-forming potential of the second pharyngeal pouch and its regulation by local mesenchyme in avian embryos

The thymus derives from the endoderm of pharyngeal pouches (PPs). The number and location of PPs with thymus-forming potential differ among jawed vertebrates, and ectopic thymus locations in mice and humans suggest a broader thymus-forming potential in PP endoderm than previously ascribed. We used the quail-chick chimera system to test if non-canonical pouches could form a thymus and examined the role of pharyngeal arch (PA) mesenchyme in this process. After testing several tissue associations, we identified thymus-forming potential in both non-canonical second PP and canonical third/fourth PP endoderm. We found the 3/4PA and the ventral region of 2PA mesenchyme to be capable of positively regulating this potential, while the dorsal region of 2PA exerts an inhibitory effect. Transcriptomic analysis revealed a shared genetic program associated with thymic potential in PP endoderm and uncovered distinct signaling pathways mediating cellular interactions between PP endoderm and PA mesenchyme, which modulate this thymic potential.