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We treated primitive streak stage chick embryos with cyclopamine, a steroidal alkaloid purified from the lily Veratrum californicumwhich has been shown to cause cranial malformations such as holoproencephaly and cyclopia10(Figure 1).

Figure 1. Chick embryos treated with cyclopamine after 14 hours of incubation.

(A-C) The head of a 6 day embryo exposed to 5µl of 45% HBC/PBS after 14 hours of incubation. All three images are of the same control embryo and illustrate the typical morphology of a developing six day chick. The FNP, MXPs, and nasal placodes are visible. (D-F) The head of a 6 day chick embryo treated with 5 µl of cyclopamine/HBC complex after 14 hours of incubation. All three images are of a single embryo. The loss of midline tissue results in the fusion of the optic vessels and lenses and the nasal placodes. The maxillary and mandibular processes are also fused. The profile images (E-F) illustrate the exaggerated proboscis extending over the cycloptic eye.

Cyclopamine is thought to block SHH protein signaling by interfering with the receipt of the signal by the responding cell rather than by interfering with the expression of shh itself.10,17We then examined the expression of shhand fgf8in the treated embryos. We found that the levels of both shh(Figure 2) and fgf8(Figure 3) are decreased in the craniofacial region, but were unaffected in their other expression domains in the embryos.

@Cebra-Thomas, 2001

[Cebra-Thomas ]