Early mesodermal phenotypes in Splotch suggest a role for Pax3 in the formation of epithelial somites

The paired box containing transcription factor Pax3 is a crucial regulator of dermomyotome and muscle development. However, the allelic series of Pax3 /Splotch mutants also displays characteristic vertebral column malformation s, which do not result from defective dorsoventral somite pattern. Rather, vertebral column and sclerotomal phenotypes are reminiscent of the phenotyp es observed in the segmentation/somitogenesis mutants rachiterata and pudgy . Moreover, rostrocaudal somite pattern and somitic boundaries are disturbe d in Splotch as monitored by the expression of Uncx4.1 and Lunatic fringe. Alterations in EphA4, Dll1, and Uncx4.1 expression are evident already in t he condensing paraxial mesoderm, correlating with the first phase of Pax3 e xpression before and during somite formation. This finding suggests an earl y function of Pax3 during the formation of epithelial somites. (C) 2001 Wil ey-Liss, Inc.