Midline signals regulate retinal neurogenesis in zebrafish

In zebrafish, neuronal differentiation progresses across the retina in a pa ttern that is reminiscent of the neurogenic wave that sweeps across the dev eloping eye in Drosophila. We show that expression of a zebrafish homolog o f Drosophila atonal, ath5, sweeps across the eye predicting the wave of neu ronal differentiation. By analyzing the regulation of ath5 expression, we h ave elucidated the mechanisms that regulate initiation and spread of neurog enesis in the retina. ath5 expression is lost in Nodal pathway mutant embry os lacking axial tissues that include the prechordal plate. A likely role f or axial tissue is to induce optic stalk cells that subsequently regulate a th5 expression. Our results suggest that a series of inductive events, init iated from the prechordal plate and progressing from the optic stalks, regu lates the spread of neuronal differentiation across the zebrafish retina.