FIG. 5.

Cervical skeletons of Af9 mutant mice. The cervical regions of skeletons of newborn Af9LZ mice, stained with Alizarin red and Alcian blue, were examined after removal of the forelimbs and shoulder blades. (A) Wild-type (+/+) control mouse; (B to D) homozygous Af9 mutant specimens. The atlases (C1) of Af9 knockout mice were severely deformed; each had the appearance of two vertebrae partially fused, with a single vertebral body (vb) extending along both substructures. Moreover, most Af9 mutants exhibited a complete, supplementary cervical vertebra, bringing the total number of cervical vertebrae to eight, as opposed to the normal seven (B and D). Occasionally, the third vertebra (C3) was partially fused to C2, but with a vertebral body of its own (C). In addition, Af9 knockout mice generally exhibited small protruding rib anlagen (ra) on vertebra C8, whereas wild-type controls usually had a pair of cervical ribs on C6. Furthermore, the spinous process (sp) normally found on the second thoracic vertebra was observed on T3 instead. C1 to C8, cervical vertebrae; T1 to T3, first thoracic vertebrae.