Fig. 3. Distribution of nNOS mRNA expression (A) and AT immunoreactivity (B) in the zebrafish brain at 55 h.p.f. (parallel cryosections). At this stage the nNOS mRNA-expressing cell populations are present in all major brain areas, coinciding with the differentiation of neuronal structures, mainly represented by AT immunoreactive axons, axon arbors and dense fibre nets at putative termination areas. Scale bar, 100 µm. Pin, pineal organ.