Sonic hedgehog pathway
The Hedgehog (Hh) Pathway, originally identified in Drosophila, [1-4] is a highly conserved signaling pathway which orchestrates multiple, disparate aspects of embryogenesis, development and tissue remodeling in a wide spectrum of systems [5-8]. This is usually accomplished by autocrine/paracrine signaling and aims to control the size and localization of Hh-responsive cell populations in response to local/long distance signals [7, 9]. Hh pathway activation typically enhances the growth and viability of Hh-responsive cells, whereas abrogating Hh signal transduction usually triggers apoptosis in such cells, unless other locally available differentiating factors expedite cellular differentiation to a more mature phenotype that no longer requires Hh viability signals [5, 9]. Thus, up-/down-regulation of the Hh pathway provides a selective growth advantage for cell types that are capable of responding to Hh ligands, when compared to neighboring cells that lack Hh receptors. This leads to-expansion/contraction, respectively, of Hh-responsive cells, thereby, orchestrating the cellular composition of several tissues [5-7, 9]. © 2010 Springer-Verlag Berlin Heidelberg.
