Biology · Developmental Biology

The following statements were made regarding the molecular interactions regulating the early development of a tetrapod limb bud involving the Apical Ectodermal Ridge (AER) and the Zone of Polarizing Activity (ZPA): A. When the limb bud grows, Shh creates a new signaling center that induces the posterior-anterior polarity. B. When the concentration of FGFs rises, it can inhibit Gremlin, thus allowing BMPs to begin repressing the AER-FGFs. C. FGFs 4, 9, and 17 from the AER inhibit Shh to stabilize the ZPA. D. Repression of Gremlin synthesis helps maintain the AER. Which one of the following options represents the combination of all correct statements?