THE ECTODERMAL CONTROL IN CHICK LIMB DEVELOPMENT - WNT-7A, SHH, BMP-2AND BMP-4 EXPRESSION AND THE EFFECT OF FGF-4 ON GENE-EXPRESSION

We have manipulated the chick limb bud by dorsoventrally inverting the ectoderm, by grafting the AER to the dorsal or ventral ectoderm and b y insertion of an FGF-4 soaked heparin bead to the mesoderm. After dor so-ventral reversal of the ectoderm, Wnt-7a expression is autonomous f rom an early stage of limb development in the original dorsal ectoderm . Exogenous FGF-4 causes ectopic Wnt-7a expression and induces ectopic Shh. In addition, exogenous FGF-4 increases the thickness of cartilag es and also shortens them, and both Bmp-2 and Bmp-4 may mediate this e ffect. The ectoderm outside the AER can regulate not only the dorso-ve ntral polarity of the underlying mesenchyme cells but also the cartila ge formation, and both Bmp-2 and Bmp-4 may mediate this control.