Objectives Introduction Procedure Results Figure Discussion Acknowledgements and References

		Introduction Sonic hedgehog (SHH) is one of the hedgehog genes, which code for paracrine factors important to cell-cell interactions (Perron et al., 2002). SHH has the greatest number of functions of the hedgehog proteins found in vertebrates and thus influences several different areas of the developing embryo, including limb and spinal cord patterning, and lung, tooth, hair and eye development (Perron et al., 2002). Its secretion from the notochord and the floor plate of the neural tube causes ventral neural tube patterning (Tanabe and Jessell 1996, as cited by Incardona et al., 1998). Cyclopamine disrupts ventral neural tube patterning, mediated by SHH, showing that it interferes with the SHH signaling pathway (Incardona et al., 1998). Cholesterol is essential to SHH protein signaling because it binds the active amino-terminal end to the cell membrane (Porter et al.,1996, as cited by Incardona et al., 1998). It is also important to the function of the Patched SHH receptor protein (Incardona et al., 1998). This protein's sterol-binding domain could be the location at which cyclopamine inhibits the SHH pathway, although this region has not been shown to be involved in SHH binding (Incardona et al.l, 1998). Their research, showing that cyclopamine induced effects are not identical to those of known cholesterol synthesis inhibitors, suggests that the inhibition of cholesterol synthesis is not the major means of SHH function interruption. They also suggested that cyclopamine interruption of SHH signaling occurred at much lower concentrations than did cyclopamine interruption of protein formation (Incardona et al., 1998), thus reinforcing the hypothesis that inhibition of cholesterol crucial to SHH amino-terminal modification is not how cyclopamine predominantly inhibits SHH. Cyclopamine is sometimes used to knockout SHH function to study its role in development. Cyclopamine has been shown to interrupt chick neural tube and somite patterning (Incardona et al., 1998); and in zebrafish, cyclopamine exposure disrupted or prevented the normal differentiation of muscle pioneers, the fibers that organize somite myotome into its chevron shape (Wolff, Roy and Ingham 2003). In Drosophila and vertebrates, SHH has been found to be responsible for neurogenesis in the retina (Neumann 2001). Interruption of SHH's role in forebrain patterning results in eye deformities as well. Cyclopia is a birth defect caused by the absence of SHH where holoprosencephaly, an undivided forebrain, leads to the central location of normally lateral facial structures, most notably the eye, and the absence of median ones (Siebert et al., 1990, as cited by Incardona et al., (1998)). This condition and brain defects were observed in lambs whose mothers ingested the lily Veratrum californicum, in which cyclopamine naturally occurs (Incardona et al., 1998). Zebrafish lend themselves to study as they are easily obtainable, can be harvested year round, can be induced to mate at selected times, and develop quickly. They are good for embryological studies because they are optically clear, making it easy to note any malformations. SHH is known to influence their development, thus making them an appropriate organism for this study.