Fig. 3. Defects in the formation of the nose, thymus, kidney and skeletal muscles. Histological analyses of the wild-type (A,C,E,G) and the Six1^–/– (B,D,F,H) neonates. (A,B) Transverse sections of the nasal region. Nasal cavities form complex, branched structures with nasal epithelia in the wild type (A), and a pair of simple round cavities with no nasal epithelia is seen in the Six1^–/– neonates (B). (C,D) The thymus is prominent anterior to the heart in the wild type (C) but completely absent in the Six1^–/– neonate (D, asterisk). (E,F) Kidney defects in Six1^–/– mice. Note the bilateral renal aplasia (F, asterisks). (G,H) Abdominal transverse sections. Note severe reduction of skeletal muscle mass in Six1^–/– neonates (H). More than five pairs of wild-type and Six1^–/– neonates were analyzed and virtually the same results obtained except for the kidney (see text). ad, adrenal gland; kd, kidney; nc, nasal cavity; sc, spinal cord; th, thymus; ur, ureter. Scale bars: 1 mm.