Mutation of the zebrafish lakritz (lak) locus completely eliminates the earliest-born retinal cells, the ganglion cells (RGCs). Instead, excess amacrine, bipolar, and Müller glial cells are generated in the mutant. The extra amacrines are found at ectopic locations in the ganglion cell layer. Cone photoreceptors appear unaffected by the mutation. Molecular analysis reveals that lak encodes Ath5, the zebrafish eye-specific ortholog of the Drosophila basic helix-loop-helix transcription factor Atonal. A combined birth-dating and cell marker analysis demonstrates that lak/ath5 is essential for RGC determination during the first wave of neurogenesis in the retina. Our results suggest that this wave is skipped in the mutant, leading to an accumulation of progenitors for inner nuclear layer cells.

Despite the essential functions of the digestive system, much remains to be learned about the cellular and molecular mechanisms responsible for digestive organ morphogenesis and patterning. We introduce a novel zebrafish transgenic line, the gutGFP line, that expresses GFP throughout the digestive system, and use this tool to analyze the development of the liver. Our studies reveal two phases of liver morphogenesis: budding and growth. The budding period, which can be further subdivided into three stages, starts when hepatocytes first aggregate, shortly after 24 h postfertilization (hpf), and ends with the formation of a hepatic duct at 50 hpf. The growth phase immediately follows and is responsible for a dramatic alteration of liver size and shape. We also analyze gene expression in the developing liver and find a correlation between the expression of certain transcription factor genes and the morphologically defined stages of liver budding. To further expand our understanding of budding morphogenesis, we use loss-of-function analyses to investigate factors potentially involved in this process. It had been reported that no tail mutant embryos appear to lack a liver primordium, as assessed by gata6 expression. However, analysis of gutGFP embryos lacking Ntl show that the liver is in fact present. We also find that, in these embryos, the direction of liver budding does not correlate with the direction of intestinal looping, indicating that the left/right behavior of these tissues can be uncoupled. In addition, we use the cloche mutation to analyze the role of endothelial cells in liver morphogenesis, and find that in zebrafish, unlike what has been reported in mouse, endothelial cells do not appear to be necessary for the budding of this organ.

for anatomical phenotypes. Thirteen recessive mutations in 12 genes were discovered. In one mutant, ddl, the majority of RGCs fail to differentiate. Three of the mutations, vrt, late and tard, delay the orderly ingrowth of retinal axons into the tectum. Two alleles of drg disrupt the layer-specific targeting of retinal axons. Three genes, fuzz, beyo and brek, are required for confinement of the tectal neuropil. Fasciculation within the optic tract and adhesion within the tectal neuropil are regulated by vrt, coma, bluk, clew and blin. The mutated genes are predicted to encode molecules essential for building the intricate neural architecture of the visual system.

The optic tectum is the largest visual center in most vertebrates and the main target for retinal ganglion cells (RGCs) conveying visual information from the eye to the brain. The retinotectal projection has served as an important model in many areas of developmental neuroscience. However, knowledge of the function of the tectum is limited. We began to address this issue using laser ablations and subsequent behavioral testing in zebrafish. We used a transgenic zebrafish line that expresses green-fluorescent protein in RGCs projecting to the tectum. By aiming a laser beam at the labeled retinal fibers demarcating the tectal neuropil, the larval tectum could be selectively destroyed. We tested whether tectum-ablated zebrafish larvae, when presented with large-field movements in their surroundings, displayed optokinetic responses (OKR) or optomotor responses (OMR), two distinct visuomotor behaviors that compensate for self-motion. Neither OKR nor OMR were found to be dependent on intact retinotectal connections. Also, visual acuity remained unaffected. Tectum ablation, however, slowed down the OKR by reducing the frequency of saccades but left tracking velocity, gain, and saccade amplitude unaffected. Removal of the tectum had no effect on the processing of second-order motion, to which zebrafish show both OKR and OMR, suggesting that the tectum is not an integral part of the circuit that extracts higher-order cues in the motion pathway.

Targeting of axons and dendrites to particular synaptic laminae is an important mechanism by which precise patterns of neuronal connectivity are established. Although axons target specific laminae during development, dendritic lamination has been thought to occur largely by pruning of inappropriately placed arbors. We discovered by in vivo time-lapse imaging that retinal ganglion cell (RGC) dendrites in zebrafish show growth patterns implicating dendritic targeting as a mechanism for contacting appropriate synaptic partners. Populations of RGCs labeled in transgenic animals establish distinct dendritic strata sequentially, predominantly from the inner to outer retina. Imaging individual cells over successive days confirmed that multistratified RGCs generate strata sequentially, each arbor elaborating within a specific lamina. Simultaneous imaging of RGCs and subpopulations of presynaptic amacrine interneurons revealed that RGC dendrites appear to target amacrine plexuses that had already laminated. Dendritic targeting of prepatterned afferents may thus be a novel mechanism for establishing proper synaptic connectivity.