MRC-5 cells induce the AER prior to the duplicated pattern formation in chick limb bud

We have previously shown that MRC-5 cells induce the duplication of the chick limb bud following the implantation into the anterior limb bud only during pre-limb-bud stages. We now report the process of duplicated pattern formation caused by MRC-5 cells. The duplicated patterns are also formed following the implantation into the center of the limb bud and an excess apical ectodermal ridge (AER) with Msx2 expression is induced prior to these duplicated pattern formulations. Only after the implantation into the anterior leg bud, the shh gene is expressed additionally in the anterior leg bud and the mirror-symmetric duplication along the anteroposterior (A-P) axis is formed. The map of the polarizing activity in stage 21 embryo suggests that the high polarizing activity of the normal flank region is responsible for the changes in the A-P polarity when MRC-5 cells are grafted into the anterior leg bud. These results indicate that MRC-5 cells induce the AER and that the excess AER produces the duplicated cartilage pattern of the limb bud.