Fig. 8. Model for functions of wnt3a, wnt8 and sp5l in tail development. (A) wnt3a and wnt8 both regulate anterior-posterior (A/P) patterning and dorsal-ventral (D/V) patterning during gastrulation. The thicker arrow from wnt8 represents the larger contribution to early patterning events by wnt8 relative to wnt3a. Specification of the tail organizer is separately listed for clarity, but is probably tightly linked to D/V patterning. During somitogenesis, both genes are required for maintenance of presomitic mesoderm, but later roles for Wnt signaling in promoting mesoderm formation versus floor plate are more wnt3a dependent (represented by the thicker arrow). (B) wnt3a and wnt8 both regulate sp5l expression in the tailbud. Overlap of sp5l expression with Wnt activity (visualized by TOPGFP reporter expression) in the tailbud is represented in green. When wnt3a and wnt8 function is blocked, Wnt activity and sp5l expression in the tailbud is dramatically reduced, resulting in a failure in tail formation.