ALTERATIONS IN SOMITE PATTERNING OF MYF-5-DEFICIENT MICE - A POSSIBLEROLE FOR FGF-4 AND FGF-6

Mice carrying a targeted mutation in the gene for the myogenic factor Myf-5 fail to form major parts of the ribs, which leads to an unstable thorax and perinatal death. Here, we report that somites of Myf-5-def icient mice lack the expression of FGF-4 and FGF-6 while TGF beta-2 is expressed normally. Early sclerotomal markers, such as Pax-1 reveal n o substantial reduction of sclerotome size. At E11.5 the condensing me senchyme of the rib anlagen is considerably reduced in size in Myf-5 m utant mice. This may be caused by the lack of Myf-5-positive, FGF-expr essing cells which normally are in close contact with the lateral scle rotome generating the rib progenitors. The potential role of FGFs and TGF beta on sclerotome formation is demonstrated in micromass cultures of early somites. Combinations of FGF-4 or FGF-6 with TGF beta-2 pote ntiate chondrogenesis suggesting that these growth factors emanating f rom early myotomal and dermomyotomal cells may have instructive or per missive effects on differentiation or outgrowth of sclerotomal cells.