Fig. 9. Model demonstrating how FGFs originating from sensory primordia regulate semicircular canal and common crus formation in adjacent epithelium. (A) Progression of canal pouch development from E3.5 to E5.5. FGF3 and FGF10 emanating from sensory regions (black ovals) promote canal outgrowth by inducing a canal genesis zone (blue stars), possibly by activating Bmp2 expression (orange) and, potentially, other factors (X, Y and Z). Either through physical distance from the sources of FGFs or through other uncharacterized mechanisms, a FGF-negative, prospective common crus domain is established (light blue). This prospective common crus domain is Bmp2 and Bmp7 negative in the dorsal region. Two resorption domains (light gold ovals surrounded by gold dashes) are established on both sides of the common crus region in which the epithelia eventually disappear leaving behind two canals and the common crus (B). (C) A transient, exogenous dose of FGFs applied before E5 expands the BMP territory and prevents the normal resorption process. As the level of exogenous FGF diminishes over time, the resorption process resumes, and includes the rest of the epithelia in the common crus domain that failed to be properly specified. (D) Sustained FGF overexpression by RCAS blocks the resorption process so that the pouches remain open and a canal-rim fate is adopted.