Fig. 1. Thymus and parathyroid defects in RA-rescued Raldh2^–/– mutants. Coronal histological sections (stained with tetrachrome) of an E18.5 wild-type fetus (WT; A,D) and two Raldh2^–/– fetuses (B,E and C,F) that were rescued by maternal RA supplementation from E7.5 to E12.5. (A) A representative section of the wild-type thymus gland. (D) Sections of the left and right thyroid lobes and the adjacent parathyroid glands. (B) No thymus was seen in the mediastinum of the first mutant (note the presence of a persistent truncus arteriosus). (C) An ectopic cervical thymus rudiment was found in the second animal. (E-F) This rudiment was connected to the thyroid tissue via a pouch-like structure (F, inset). No parathyroid glands were seen in the mutants. At, atrium; PTA, persistent truncus arteriosus; Pth, parathyroid; Th, thymus; Tr, trachea; Ty, thyroid.