Signal sequences

contain one or more positively charged amino acids adjacent to a continuous stretch of 6–12 hydrophobic residues (known as the hydrophobic core)

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Two major modes of inductive interaction:

 Instructive interaction

 Permissive interaction

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In instructive interaction, 

a signal from the inducing cell is necessary for initiating new gene expression in the responding cell.

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Instructive interaction

It occur where the responding cell has a choice of fates and will follow one developmental pathway following induction and an alternative pathway in absence of the inductive signal. 

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In Permissive interaction

The responding tissue has already been specified and needs only an environment that allows the expression of these traits. 

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Epithelial-mesenchymal interactions

the interactions of sheets of epithelial cells with adjacent mesenchymal cells.

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Properties of epithelial-mesenchymal interactions:

Regional specificity of induction

Genetic specificity of induction