Determination of cell fate in the vertebrate retina has been shown to
be largely independent of lineage. After cell fates are determined, re
tinal neurons become organized in a precise laminar pattern. The mecha
nisms for this patterning could involve morphogens distributed in grad
ients or, alternatively, direct cell-cell interactions. In the zebrafi
sh mutant cyclops (cyc(b16)), most embryos have two partial retinas jo
ined in the ventral midline. This presents developing retinal cells ne
ar the midline with abnormal cellular environments, whereas laterally
the pattern of developing cells is normal. We examined the consequence
s of this for patterning in the mutant's retina. We found that the ret
inas are joined in the midline at the apical surfaces of the photorece
ptor layers. A laminar pattern emerges in the midline that preserves n
ormal 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 emer
gence of the novel pattern in this midline region. We conclude that th
ese observations in the cyclops mutant are compatible with mechanisms
of pattern formation in the retina involving local cell interactions.
(C) 1997 Wiley-Liss, Inc.