Autocrine Regulation of the Micromere by Wnt from the Micromere
Another way micromeres retain their specification is to secrete an autocrine factor, Wnt8 (Angerer and Angerer 2000; Wikramanayake et al. 2004). As soon as the micromeres form, maternal β-catenin and Otx activate the Blimp1 gene, whose product (in conjunction with more β-catenin) activates the Wnt8 gene. Wnt8 protein is then received by the same micromeres that made it (i.e., autocrine regulation), activating the micromeres’ own genes for β-catenin. Because β-catenin activates Blimp1, this autocrine regulation sets up a positive feedback loop between Blimp1 and Wnt8 that establishes a source of β-catenin for the micromere nuclei. Equally important, this cross-regulatory loop serves to lock both genes “on” and can amplify their expression levels.