The duration of Fgf8 isthmic organizer expression is key to patterning different tectal-isthmo-cerebellum structures.

The isthmic organizer and its key effector molecule, fibroblast growth factor 8 (Fgf8), have been cornerstones in studies of how organizing centers differentially pattern tissues. Studies have implicated different levels of Fgf8 signaling from the mid/hindbrain boundary (isthmus) as being responsible for induction of different structures within the tectal-isthmo-cerebellum region. However, the role of Fgf8 signaling for different durations in patterning tissues has not been studied. To address this, we conditionally ablated Fgf8 in the isthmus and uncovered that prolonged expression of Fgf8 is required for the structures found progressively closer to the isthmus to form. We found that cell death cannot be the main factor accounting for the loss of brain structures near the isthmus, and instead demonstrate that tissue transformation underlies the observed phenotypes. We suggest that the remaining Fgf8 and Fgf17 signaling in our temporal Fgf8 conditional mutants is sufficient to ensure survival of most midbrain/hindbrain cells near the isthmus. One crucial role for sustained Fgf8 function is in repressing Otx2 in the hindbrain, thereby allowing the isthmus and cerebellum to form. A second requirement for sustained Fgf8 signaling is to induce formation of a posterior tectum. Finally, Fgf8 is also required to maintain the borders of expression of a number of key genes involved in tectal-isthmo-cerebellum development. Thus, the duration as well as the strength of Fgf8 signaling is key to patterning of the mid/hindbrain region. By extrapolation, the length of Fgf8 expression could be crucial to Fgf8 function in other embryonic organizers.