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Development 129, 2473-2481 (2002)
© 2002 The Company of Biologists Limited

Hypomorphic Mesp allele distinguishes establishment of rostrocaudal polarity and segment border formation in somitogenesis

Aya Nomura-Kitabayashi1, Yu Takahashi1, Satoshi Kitajima1, Tohru Inoue1, Hiroyuki Takeda2 and Yumiko Saga3,*

1 Cellular and Molecular Toxicology Division, National Institute of Health Sciences, 1-18-1 Kamiyohga, Setagaya-ku, Tokyo 158-8501, Japan
2 Division of Early Embryogenesis,
3 Division of Mammalian Development, National Institute of Genetics, Mishima, Shizuoka 411-8540, Japan

*Author for correspondence (e-mail: ysaga{at}lab.nig.ac.jp)

Accepted 19 February 2002

A bHLH-type transcription factor, Mesp2, plays an essential role in somite segmentation in mice. Zebrafish mespb (mesp-b), a putative homologue of mouse Mesp2, is transiently expressed in the rostral presomitic mesoderm similarly to Mesp2. To determine whether zebrafish mespb is a functional homologue of mouse Mesp2, zebrafish mespb was introduced into the mouse Mesp2 locus by homologous recombination. Introduced mespb almost rescued the Mesp2 deficiency in the homozygous mespb knockin mouse, indicating that mespb is a functional homologue of mouse Mesp2. Segmented somites were clearly observed although the partial fusion of the vertebral columns still occurred. Interestingly, however, the nature and dosage of the mespb gene affected the rescue event. A mouse line, which has a hypomorphic Mesp2 allele generated by the introduction of neo-mespb, gave rise to an epithelial somite without normal rostrocaudal (RC) polarity. RC polarity was also lacking in the presomitic mesoderm. The defects in RC polarity were determined by the altered expressions of Uncx4.1 and Dll1 in the segmented somites and presomitic mesoderm, respectively. In contrast, the expression of EphA4 (Epha4), lunatic fringe or protocadherin, thought to be involved in segment border formation, was fairly normal in hypomorphic mutant embryos. These results suggest that the Mesp family of transcription factors is involved in both segment border formation and establishment of RC polarity through different genetic cascades.

Key words: Somitogenesis, Mesp2, mespb, Rostrocaudal polarity, Segment border, Resegmentation, Mouse, Zebrafish


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© The Company of Biologists Ltd 2002