Figure 3.

Ventral view of vertebral columns of Pax1^null and the classical undulated homozygous mice at the newborn stage. (A) Wild-type newborn, displaying no phenotypic abnormalities. Vertebral elements from the sixth cervical (C6) to the last sacral (S4) segment are shown. T1–T13, thoracic vertebrae; L1–L6, lumbar vertebrae; S1–S4, sacral vertebrae; pd, pedicle; c, ossification center of vertebral body; i, intervertebral disc. (B) Pax1^un homozygote. The ossification center of C7 is missing (yellow arrowhead). Fusions of pedicles to ossification centers at L3 (large arrowhead) and all caudally following segments. Dual ossification centers are found from L2 to L5, and the completely split centrum of L4 is indicated by an arrow. Note that the proximal ends of the 13th rib pair are ossified (small arrowheads). (C) Pax1^un-ex homozygote. The ossification center of C5 is missing, and of C6 and C7 are reduced (yellow arrowhead). Fusions of pedicles to ossification centers at T5 (large arrowhead) and all caudally following segments. Vertebrae of the lumbar region (L2 to L4) are saggitally split (arrow). The left 13th rib is not connected to its respective vertebra, and the rib heads of rib pair 12 and the single right rib 13 and left rib 11 are ossified (small arrowheads). (D) Pax1^null homozygote with no ossification centers in vertebrae C5 and C6 (yellow arrowhead). Note that pedicles fuse to ossification centers from T5 (large arrowhead) on caudally, and vertebrae of the lumbar region (L3 to L5) are sagittally split (arrow). Note that these malformations lead to skoliosis. The 13th rib pair is not connected to its respective vertebra, and the proximal ends of rib pairs 11 and 12 are ossified (small arrowheads). (E) Pax1^Un-s homozygote. Intervertebral discs and vertebral bodies are missing or strongly malformed all along the vertebral column. In addition, in the cervical region vertebrae are ventrally split (small arrows), whereas thoracic vertebrae display fusions between pedicles and ossification centers (large arrowhead). In the lumbar region all ossification centers are missing (large arrow) and the tail is extremely shortened (double arrowhead). The lower ribs are not connected to the vertebral column (small arrowheads).