Fig. 2. The 3rd pharyngeal pouches do not separate as buds to form the primordia of thymus/parathyroid. (A,B) Transverse sections showing that in wild-type embryos (A), the 3rd pouches evaginate and then separate as buds to form the primordia of thymus/parathyroid (th/pt) at around E12.5, while the 4th pouches separate to form the rudiments of the ultimobranchial bodies (ub) at the same time; however in Eya1–/– embryos (B), the primordia of thymus/parathyroid failed to form, while the rudiments of ultimobranchial bodies (ub) were formed but with a slight indentation (arrow in B). (C-F) Transverse sections showing Pax1 and Pax9 expression in the rudiments of the thymus/parathyroid and ultimobranchial bodies at E12.0 of wild-type embryos (C,E). In Eya1–/– embryos, the expression of Pax1 and Pax9 was only detected in the rudiments of ultimobranchial bodies (arrows). The absence of Pax1 and Pax9 expression in the prospective region of the thymus/parathyroid rudiments confirms the absence of these structures in Eya1–/– embryos (D,F). Dorsal is up. ph, pharynx; tr, trachea.
