Formation
The formation of the primitive streak relies on a complex network of signaling pathways that work together to ensure that this process is highly-regulated. Activation of various secreted factors (Vg1, Nodal, Wnt8C, FGF8 and Chordin) and transcription factors (Brachyury and Goosecoid) adjacent to the site of streak formation is required for this process. In addition, structures, such as the hypoblast, also play an important in the regulation of streak formation. Removal of the hypoblast in the chick results in correctly patterned ectopic streaks, suggesting that the hypoblast serves to inhibit formation of the primitive streak.
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