Fig. 3. Homeotic transformations within the vertebral column. (A,B) Ventral views of the cervical and upper thoracic vertebrae from P7 wild-type (A) and a 4C-BAC16 mutant (B) littermates. The clavicle, first sternebrae (manubrium) and ribs have been removed to expose the vertebrae. (B) The left side of the sixth cervical vertebrae, C6, has lost its anterior tuberculum (white arrow), whereas C7 has acquired a rudimentary process (black arrow). (C,D) Left lateral views of the same region in two other 4C animals. (C) Partial loss of C6 anterior tuberculum (white arrow) accompanied by fusion of ribs 1 and 2, and their lack of articulation with the sternum. Note that this lack of articulation results in precocious ossification in the joint separating the manubrium and second sternebrae. (D) The left part of C7 behaves like T1, by extending a rudimentary bone (black arrow) that articulates abnormally with the first rib. (E,F) Ventral views of lower lumbar and upper sacral vertebra of wild-type (E) and 4C-BAC16 (F) littermates. (F) The sixth lumbar vertebrae has acquired an S1 fate and begins to articulate with the adjacent pelvic bones. Scale bars: 2 mm in A,C,D; 3 mm in E,F.