A. Anterior foregut region at E9.5. The thyroid and lung primordia can be distinguished as discrete regions of the ventral endoderm expressing Nkx2-1, located in close vicinity of the developing heart. B. At E10.5 the thyroid primordium forms a caudally directed bud that is closely apposed to the aortic sac. C. The lung buds at E10.5 form hollow, blind-ended epithelial tubes surrounded by concentric layers of mesenchyme. Sagittal sections. As, aortic sac; h, heart; lu, lung primordium; th, thyroid primordium; ve, ventral endoderm. Scale bars = 100 μm in A, 75 μm in B and C.