Which of the following correctly explains regional specificity of induction?
Epithelial-mesenchymal recombination experiments revealed that regional identity resides predominantly in mesenchyme rather than epithelium. When salivary, mammary or cutaneous mesenchyme is combined with simple epithelium, resulting branching pattern, histology and cytodifferentiation follow mesenchymal origin, not epithelial source. Mesenchyme secretes instructive signals such as FGF10, BMPs and Wnts in region-specific combinations that activate unique transcription programs in competent epithelium. This instructive interaction explains why ureteric mesenchyme induces tubules while lung mesenchyme induces buds, overriding any autonomous tendency of responding tissue during organogenesis.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Epithelial-Mesenchymal Interactions and Regional Specificity.