Phenotypic determination of epithelial appendages: Genes, developmental pathways, and evolution

Epithelial appendages are derivatives of epithelia that elaborate to form s pecialized structures and functions. The appendage can protrude out, such a s in teeth and feathers, or invaginate in, such as in glands. The epithelia can be ectodermal, such as in hairs, or endodermal, such as in livers. Usi ng feather as a prototype of epithelial appendage, we study the molecular s ignals involved in the successive stages of epithelial-mesenchymal interact ions during morphogenesis. We propose that these form the basics of gene ne tworks, which can be integrated to gene supernetwork and totinetwork, Becau se the unit of development is molecular pathway rather than single molecule , and the unit of morphogenesis is cell group rather than single cell, we m ake the analogy between genes/developmental pathways and words/sentences. T he study of developmental pathways in epithelial appendage organogenesis wi ll help us to understand the grammar of genes and the basic rules in constr ucting regulated new growth. This knowledge may contribute to the study of cancer biology (deregulated new growth) and organ regeneration.