Retinal patterning in the zebrafish mutant cyclops. 
Determination of cell fate in the vertebrate retina has been shown to be largely independent of lineage.  After cell fates are determined, retinal neurons become organized in a precise laminar pattern.  The mechanisms for this patterning could involve morphogens distributed in gradients or, alternatively, direct cell-cell interactions.  In the zebrafish mutant cyclops (Cyc(b16)), most embryos have two partial retinas joined in the ventral midline.  This presents developing retinal cells near the midline with abnormal cellular environments, whereas laterally the pattern of developing cells is normal.  We examined the consequences of this for patterning in the mutant's retina.  We found that the retinas are joined in the midline at the apical surfaces of the photoreceptor layers.  A laminar pattern emerges in the midline that preserves normal positional relationships between retinal cell types locally but is abnormal with respect to patterning over the entire retina.  Lateral to the midline, retinal patterning appears normal.  Metabolic labeling experiments showed that late rounds of DNA synthesis precede the emergence of the novel pattern in this midline region.  We conclude that these observations in the cyclops mutant are compatible with mechanisms of pattern formation in the retina involving local cell interactions.