| During evolution, a great variety of organs have developed to fulfill specific functions during the life cycles of the different organisms. Morphological and molecular comparisons of organs with similar features have provided insights into the mechanisms of growth, morphogenesis and differentiation during organ formation. For example, comparisons of epidermal organs such as teeth and hair have shed considerable light on the epithelial-mesenchymal interactions that govern their development. Sonic Hedgehog (Shh ) encodes a secreted signaling peptide that mediates epithelial-mesenchymal interactions during tooth and hair development. Its function is well conserved, as in both organs loss of Shh function leads to a reduction in size and to specific disruptions in morphogenesis. However, cell differentiation which leads to the production of keratins in the hair, enamel and dentin in the tooth, proceeds normally despite the abnormal morphologies. Thus, the Shh pathway is a highly conserved pathway during epidermal development which plays a fundamental role in regulating organ growth and morphogenesis. |
