Ectodermal-mesodermal interactions in the growth of limb buds in the chick embryo: constancy and temporal limits of the ectodermal induction.

Abstract Heterochronous recombinations were made using chick embryo limb-bud mesoblasts and ectodermal hulls of differing developmental stages. Any functional apical ectodermal ridge (AER) was found to induce outgrowth of limb parts in the responding mesoderm in their proper proximodistal sequence, regardless of the relative ages of the ectoderm and mesoderm. It is concluded, therefore, that the inductive signal emanating from the AER remains qualitatively constant from stage to stage and from level to level during limb-bud outgrowth; information for proper sequencing of level-specific patterns in the limb must therefore be programmed intrinsically within the mesoderm. The AER loses the capacity to induce complete limbs from young limb-bud mesoblasts during stage 29. Recombination of a young ectodermal hull with older mesoderm does not accelerate the loss of function by the AER and, conversely, it is unlikely that prolonged association with young mesoderm can prolong the period of the AER's inductive potency. Loss of function thus occurs on a schedule that is determined intrinsically within the ectoderm and that is affected little or not at all by the stage of the underlying mesoderm.

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