Fig. 3. Ectopic Hox gene expression and skeletal analysis in eedxBmi1 mutants. Genotypes are indicated above the panels. (A,B) Representative images of sectioned wild-type and mutant embryos following mRNA in situ hybridization with Hoxa5 (A) and Hoxb4 (B) antisense probes. Arrows denote the anterior Hox gene expression boundary and the first prevertebra. All images were captured at 50 x magnification. (C-G) Homeotic transformations and vertebral abnormalities in cervical (C-E), thoracic (F) and lumbar regions (G). (C) Two ectopic ossification centers (arrowheads) and broadening of the neural arch (asterisk) constitute Bmi1-specific defects (van der Lugt et al., 1994). (D) The first cervical vertebra reveals a significantly broader ventral arch in eed and Bmi1 mutant skeletons (arrow), which is likely to result from an incomplete regression of the vertebral body during embryogenesis (Verbout, 1985). As all vertebrae posterior to C1 contain a body, incomplete regression of the body in C1 represents a posterior homeotic transformation (C1C2). (D,E) The rudimentary vertebral body also broadens the anterior arch, which is visible in both rostral and lateral views of C1 (arrowheads). (E) The presence of ribs transforms the seventh cervical vertebra toward the identity of the first thoracic vertebra (C7T1^*). (F) Posterior transformation of the seventh thoracic vertebra is evident from lack of sternal fusion of the ribs (T7T8). (G) Fusion with the ilial bones represents a homeotic transformation of the sixth lumbar vertebra toward a sacral identity (L6S1^*). Images were captured at 20x (C,D), 16x (E), 12x (F) and 18x magnification (G). pv, prevertebra.