Reciprocal Dependence of Ectoderm and Mesoderm During Chick Embryo Limb Development

Abstract

An hypothesis is presented which postulates that limb bud ectoderm and mesoderm have a reciprocal dependence. According to this view the chief function of the limb bud''s apical ectodermal ridge is to promote continued outgrowth of the mesoderm. In addition to responding to this stimulus the mesoderm contains a factor for continued persistence of the ectodermal ridge. Asymmetries of the limb develop because of the asymmetrical distribution of the maintenance factor in the mesoderm. This determines which regions of the ectodermal ridge will become thick and active and thereby controls many aspects of limb pattern. The action of two mutations which have been studied seems to be primarily upon the maintenance factor. The distribution is altered in poly-dactylous limbs while the wing buds of a "wingless" mutant appear to be deficient in the maintenance factor.