Roles of the Hedgehog Signaling Pathway in Epidermal and Hair Follicle Development, Homeostasis, and Cancer.

The epidermis is the outermost layer of the skin and provides a protective barrier against environmental insults. It is a rapidly-renewing tissue undergoing constant regeneration, maintained by several types of stem cells. The Hedgehog (HH) signaling pathway is one of the fundamental signaling pathways that contributes to epidermal development, homeostasis, and repair, as well as to hair follicle development and follicle bulge stem cell maintenance. The HH pathway interacts with other signal transduction pathways, including those activated by Wnt, bone morphogenetic protein, platelet-derived growth factor, Notch, and ectodysplasin. Furthermore, aberrant activation of HH signaling is associated with various tumors, including basal cell carcinoma. Therefore, an understanding of the regulatory mechanisms of the HH signaling pathway is important for elucidating fundamental mechanisms underlying both organogenesis and carcinogenesis. In this review, we discuss the role of the HH signaling pathway in the development and homeostasis epidermis and hair follicles, and in basal cell carcinoma formation, providing an update of current knowledge in this field.