smu mutants have defective Hh signaling. (A) In 24 h smub641 mutants (bottom left), nkx2.2 gene expression was reduced compared with wild type (top); nkx2.2 gene expression was less severely affected in smub577 mutants (bottom right). (B) At the 16-somite stage, patched1 expression was essentially absent from smub641 mutants (bottom) compared with wild type (top). (C-E) At 24 h, floor plate and hypochord were present in both mutant (D) and wild-type (C) embryos labeled for col2a1 (Yan et al., 1995). By 27 h (E) gaps appeared in smu mutant floor plate indicated by shh expression. (F) At 20-somite stage, mutants (bottom) lacked lateral floor plate compared with wild type (top), indicated by foxa2. (G) At the 18-somite stage, islet1-expressing primary motoneurons were absent from tail and posterior trunk, and were reduced in anterior spinal cord in smub641 mutants (bottom) compared with wild type (top). (H) In smub641 mutants (bottom), secondary motoneurons were absent at 24 h, indicated by DM-GRASP labeling. (A,C-E,G,H) Side views, dorsal is towards the top, anterior is towards the left. (B,H, insets) Transverse sections of anterior trunk, dorsal is towards the top. (F) Dorsal views, anterior is towards the left. Scale bar:120 µm (A, G); 110 µm (B,H); 100 µm (C-E); 50 µm (F); 82 µm (H, insets).
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