Left-right (L-R) asymmetries in neuroanatomy exist throughout the animal kingdom, with implications for function and behavior.
The molecular mechanisms that control formation of such asymmetries are beginning to be understood. Significant progress has
been made by studying the zebrafish parapineal organ, a group of neurons on the left side of the epithalamus. Parapineal cells
arise from the medially located pineal complex anlage and migrate to the left side of the brain. We have found that Fgf8a
regulates a fate decision among anterior pineal complex progenitors that occurs just prior to the initiation of leftward migration.
Cell fate analysis shows that in the absence of Fgf8a a subset of cells in the anterior pineal complex anlage differentiate
as cone photoreceptors rather than parapineal neurons. Fgf8a acts permissively to promote parapineal fate in conjunction with
the transcription factor Tbx2b, but might also block cone photoreceptor fate. We conclude that this subset of anterior pineal
complex precursors, which normally become parapineal cells, are bipotential and require Fgf8a to maintain parapineal identity
and/or prevent cone identity.