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Ninfa Fragale, Satya Srirama Karthik Divvela, Beate Brand‑Saberi

• Atonal homologue 8 \(\it (atoh8)\) is a basic helix-loop-helix transcription factor expressed in a variety of embryonic tissues. While several studies have implicated \(\it atoh8\) in various developmental pathways in other species, its role in zebrafish development remains uncertain. So far, no studies have dealt with an in-depth in situ analysis of the tissue distribution of \(\it atoh8\) in embryonic zebrafish. We set out to pinpoint the exact location of \(\it atoh8\) expression in a detailed spatio-temporal analysis in zebrafish during the first 24 h of development (hpf). To our surprise, we observed transcription from pre-segmentation stages in the paraxial mesoderm and during the segmentation stages in the somitic sclerotome and not—as previously reported—in the myotome. With progressing maturation of the somites, the restriction of \(\it atoh8\) to the sclerotomal compartment became evident. Double in situ hybridisation with \(\it atoh8\) and \(\it myoD\) revealed that both genes are expressed in the somites at coinciding developmental stages; however, their domains do not spatially overlap. A second domain of \(\it atoh8\) expression emerged in the embryonic brain in the developing cerebellum and hindbrain. Here, we observed a specific expression pattern which was again in contrast to the previously published suggestion of \(\it atoh8\)transcription in neural crest cells. Our findings point towards a possible role of \(\it atoh8\) in sclerotome, cerebellum and hindbrain development. More importantly, the results of this expression analysis provide new insights into early sclerotome development in zebrafish—a field of research in developmental biology which has not received much attention so far.