To begin to investigate the effect of the
ptc2 mutation on the postembryonic zebrafish retina, histologic analyses were performed on
ptc2 −/− mutants and their heterozygous and wild-type siblings. Although most
ptc2 −/− fish do not reach adulthood, ∼2% survive to 6 weeks of age (juveniles). Juvenile
ptc2 −/− mutants were smaller than their wild-type siblings and had mild pigmentation defects; overall eye structure appeared normal (
Fig. 1A). Juvenile
ptc2 +/− fish were indistinguishable from their wild-type siblings (data not shown) and when examined histologically, retinal morphology, and lamination also appeared to be normal (
n = 6;
Fig. 1C), when compared with wild-type siblings (
Fig. 1B). In
ptc2 −/− mutants, however, regions of disrupted retinal lamination were detected in the dorsal peripheral retina, as well as morphologic abnormalities in the CMZ and ciliary zone (
Fig. 1D). Of the 10
ptc2 −/− juveniles that were analyzed by histology, four contained ectopic clusters of cells that appeared to be continuous with the CMZ (
n = 4/10;
Figs. 1E,
1F). These ectopic cells were incorrectly incorporated into the retina and disrupted normal retinal lamination. Three other
ptc2 −/− juveniles contained clusters of ectopic cells within the INL; however, lamination in the remainder of the retina in these individuals appeared to be normal (
n = 3/10;
Figs. 1G,
1H). Examination of sequential histologic sections through the depth of the retina indicated that these clusters did not appear to be continuous with the CMZ (data not shown). These ectopic clusters contained photoreceptors, indicated by the presence of photoreceptor outer segments, and they were associated with an ectopic plexiform layer (
Fig. 1H, arrows). Immunohistochemical analyses confirmed the presence of both cone and rod photoreceptors within these dysplastic foci (
Figs. 2A–D). In adult
syu t4+/− mutant fish, which carry a mutant allele of
shh, double-cone loss and outer segment abnormalities have been reported.
30 Higher magnification images, however, show that in the
ptc2 −/− juvenile retina, cone morphology was largely normal, and no significant double-cone cell loss was observed (
n = 8;
Figs. 2E,
2F).