WISH analysis of Glass and Mox and immunohistochemical localization of serotonergic cells in normal and zinc-treated embryos. Whole-mount in situ hybridization (WISH) analysis identified the transcription factors (a) Glass and (b) Mox as being expressed in single cells of the apical organ. Although Glass expression is eliminated in zinc-treated embryos (b), the expression of Mox is expanded and upregulated in zinc-treated embryos (f) (also see quantitative real-time polymerase chain reaction data in Table 2). Immunohistochemical localization of serotonin in (c, d) normal and (g, h) zinc-treated embryos shows that whereas normal embryos produce four to six serotonergic cells (panel d), the number of serotonergic cells is elevated to at least 30 on average in zinc treated embryos (panel h). Panels d and h are fluorescent photographs of the same embryos depicted in transmitted light in panels c and g, respectively.