The zebrafish has recently assumed a central position in the study of vertebrate development. Numerous studies of other fish have shown that their central nervous systems, and especially their visual systems, continue to add new neurons throughout life, which is probably related to their abilities to regenerate axons and whole nervous tissue. Retinal neurogenesis had not been examined in adult zebrafish, and two reports concluded that the optic tectum ceased neurogenesis early in life, so the question arose whether the zebrafish was anomalous in this regard. We labeled embryonic (24- and 48-h postfertilization) and adult zebrafish with the thymidine analog, bromo-deoxyuridine, and, after short and long survivals, examined the retina and brain for labeled cells. They were abundant in both the optic tectum and the retina. Although the rate of retinal growth slows considerably between embryonic and adult stages, the patterns of neurogenesis in both the embryo and the adult are similar to those described in other fish, so these "fish-specific" features of general interest can justifiably be studied in zebrafish.

Brain factor 1 (BF-1) is a winged-helix transcription factor with restricted expression in the anterior optic vesicle and in the telencephalic neuroepithelium of the neural tube. We have previously found that targeted disruption of the BF-1 gene results in hypoplasia of the cerebral hemispheres, which is more severe in structures derived from the ventral telencephalon. Here we show that the loss of BF-1 leads to multiple developmental anomalies of the eyes. The most ventral structure arising from the optic vesicle, the optic stalk, is missing and is replaced by an expanded retina. Ventral closure of the optic cup and choroid fissure does not occur. These dorsal-ventral patterning defects are not limited to the BF-1-expressing (anterior) cells, but also involve the cells of the posterior optic vesicle. Sonic hedgehog (shh) expression within the ventral telencephalic neuroepithelium is specifically lost in the BF-1(-/-) mutant. Taken together, these findings suggest that shh produced at this site plays a role in patterning the developing eye. This localized deficit in shh expression may also contribute to the prominence of the ventral defects in the telencephalon of the BF-1(-/-) mutant.

In animals with binocular vision, retinal ganglion cell (RGC) axons from each eye sort in the developing ventral diencephalon to project to ipsi- or contralateral targets, thereby forming the optic chiasm. Ipsilaterally projecting axons arise from the ventrotemporal (VT) retina and contralaterally projecting axons primarily from the other retinal quadrants. The winged helix transcription factor Foxd1 (previously known as BF-2, Brain Factor 2) is expressed in VT retina, as well as in the ventral diencephalon during the formation of the optic chiasm. We report here that in embryos lacking Foxd1,both retinal development and chiasm morphogenesis are disrupted. In the Foxd1 deficient retina, proteins designating the ipsilateral projection, such as Zic2 and EphB1, are missing, and the domain of Foxg1 (BF-1) expands from nasal retina into the VT crescent. In retina-chiasm co-cultures, VT RGCs from Foxd1 deficient retina are not repulsed by chiasm cells, and in vivo many VT RGCs aberrantly project contralaterally. However, even though the ipsilateral program is lost in the retina, a larger than normal uncrossed component develops in Foxd1 deficient embryos. Chiasm defects include axon stalling in the chiasm and a reduction in the total number of RGCs projecting to the optic tract. In addition, in the Foxd1 deficient ventral diencephalon, Foxg1 invades the Foxd1 domain, Zic2 and Islet1 expression are minimized, and Slit2 prematurely expands, changes that could contribute to axon projection errors. Thus, Foxd1 plays a dual role in the establishment of the binocular visual pathways: first, in specification of the VT retina, acting upstream of proteins directing the ipsilateral pathway; and second, in the patterning of the developing ventral diencephalon where the optic chiasm forms.