Disrupting the establishment of polarizing activity by teratogen exposure

Between days 9.5 and 10, the forelimb buds of developing murine embryos pro gress from stage 1 which are just beginning to express shh and whose poster ior mesoderm has only weak polarizing activity to stage 2 limbs with a dist inguishable shh expression domain and full polarizing activity. We find tha t exposure on day 9.5 to teratogens that induce the loss of posterior skele tal elements disrupts the polarizing activity of the stage 2 postaxial meso derm and polarizing activity is not subsequently restored. The ontogeny of expression of the mesodermal markers shh, ptc, bmp2, and hoxd-12 and 13, as well as the ectodermal markers wnt7a,fgf4,fgf8, cx43, and p21 occurred nor mally in day 9.5 teratogen-exposed limb buds. At stage 3, the treated limb apical ectodermal ridge usually possessed no detectable abnormalities, but with continued outgrowth postaxial deficiencies became evident. Recombining control, stage matched limb bud ectoderm with treated mesoderm prior to ZP A grafting restored the duplicating activity of treated ZPA tissue. We conc lude that in addition to shh an early ectoderm-dependent signal is required for the establishment of the mouse ZPA and that this factor is dependent o n the posterior ectoderm. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.