Figure 3.

Orchestrated induction of DER ligands in the ventral ectoderm. In the embryonic ventral ectoderm, activation of the DER pathway is triggered by simultaneous expression of the Spitz precursor, Rho and Star in the midline cells, giving rise to a localized source of secreted Spitz. The cells adjacent to the midline encounter maximal levels of secreted Spitz. In these cells, induction of argos expression through Pointed P1 provides a negative-feedback loop, restricting the expansion of ventral-most fates (dark gray). Parallel induction of vein expression through Pointed P1, assures the induction of lateral cell fates (light gray), even under conditions in which the levels of secreted Spitz from the midline may be compromised.