Fig. 3. mespb gene dosage effect revealed by the skeletal morphology at 18.5 dpc (A-F) and gene expressions at 11.5 dpc (G-R) reflecting RC polarity and subsequent resegmentation. (A-F) The lumber regions of the vertebral columns stained with Alcian Blue-Alizarin Red. The wild-type embryo (A) exhibits clear separation of the lamina (l), pedicle (p) and transverse process (t). (B-E) Varying degrees of fusion of the pedicles and laminas are observed in the mespb or neo-menpb fetuses. (F) A Mesp2-null fetus with a completely fused pedicle and lamina. (G-R) The segmental pattern is prefigured by the expression pattern of Uncx4.1 (G-L) in segmented somites and Dll1 (M-R) in the anterior PSM. Expression of these genes, normally localized in the caudal half of each somite and CPM, is expanded rostrally in mespb, or neo-mespb embryos.