Siamois, Goosecoid, and axis essays

Siamois, Goosecoid, and axis essays Axis formation is established very early in the development of vertebrates. Much research has been done in this area, especially with Xenopus, and the consensus is that the dorsal lip plays a key role in establishing the dorsal-ventral and anterior-posterior axes and organizing the develoment of the organism. Pioneering experiments were done by Spemann and Mangold in the 1920's: by transplanting a dorsal lip from one embryo to a point opposite the native dorsal lip on a second embryo, they were able to induce the formation of a second notochord and most of a complete second organism in the manner of a siamese twin. The paper the two published summarized thusly: A piece of the upper blastoporal lip of an amphibian embryo undergoing gastrulation exerts an organizing effect on its environment in such a way that, if transplanted to an indifferent region of another embryo, it causes there the formation of a secondary embryonic anlage. Such a piece can therefore be designated as an organizer. The homeobox gene which is implicated in this activity is known as siamois. A second homeobox gene, goosecoid, was later implicated in the formation of the a-p axis, and both genes are regulated at least indirectly by the $-catenin protein, which is a component of the Wnt signaling pathway. This paper will attempt to explain the relationship among these genes and gene products and summarize the current research in the field. The dorsal lip originates in the grey crescent, a segment of the egg which forms opposite the sperm entry point (SEP) through a 30E subcortical rotation of the cytoplasm with respect to the cortex. This rotation, which is driven by a basketwork of microtubules at the vegetal side, is the actual establishment of the dorsoventral axis, but proper head and tail structures will not emerge without the organizers influence. The pathway begins with the accumulation of $-catenin in the dorsal blastomeres...