Xenopus OrganizerComments

Xenopus Organizer

A key function of the Nieuwkoop center is to induce the formation of dorsal mesoderm. It turns out that the most dorsal mesoderm also functions as a signaling center, known as the Spemann Organizer. The difference between the Nieuwkoop center and the Spemann Organizer can be seen in transplantation experiments. Both signaling centers induce a new D/V axis. However, cells in the Nieuwkoop center are incorporated only into the endoderm, whereas the cells of the Organizer are found in the mesoderm of the newly formed axis. Thus, the Organizer is the dorsal mesoderm tissue induced by the Niewkoop center. It is called the “organizer” because it can “organize” host cells into a new D/V axis. This implies that the organizer (dorsal mesoderm) produces factors that can pattern the mesoderm along the D/V axis.

Transplantation experiments tell us that the Organizer becomes the dorsal mesoderm—if it is removed, the embryo becomes completely ventralized. What are the signals produced by the Spemann organizer that influence differentiation along the D/V axis? It has become clear that the secreted dorsalizing proteins of the organizer are actually antagonists of the ventralizing growth factors BMP-4 (a Dpp homolog) and Wnt-8 (a Wingless homolog). The secreted proteins Noggin, Chordin (a Sog homolog) and Follistatin are all thought to act by binding to BMP-4, preventing its binding to its receptor, and thus preventing its ventralization function. The noggin gene was cloned because injection of its RNA is capable of dorsalizing (i.e., partially rescuing) UV-irradiated embryos. The Frzb gene encodes an antagonist of Wnt-8.

The transcription factors VegT and TCF/LEF (for which b-catenin is a co-activator) are thought to be responsible for bringing about the transcription, in a graded pattern along the prospective DV axis, of the Xnrs. What transcription factors are responsible for expression in the Spemann organizer of the BMP and Wnt antagonists? By a variety of techniques, organizer-specific transcription factor encoding genes have been identified. These include the three homeobox genes Goosecoid, Lim1, and Xnot, and the fork head protein HNF3-ß. These transcription factors then cause expression of Noggin, Chordin, Follistatin, and and Frzb in the cells of the "organizer".