Comparative analysis of olSix3.1, olSix3.2, and olSix6 expression pattern during embryonic development. Medaka embryos at different developmental stages (as indicated in the panels) were hybridized in toto with specific probes, as indicated on the top of each column. (a to r) Anterior dorsal views; (s to u) frontal vibratome sections through the eye. From St16 to St19, only olSix3.1 and olSix3.2 are expressed in the anterior neural plate (panels a to c) and then in the presumptive telencephalon and optic vesicles (panels d to i), although olSix3.1 is more abundant in the optic vesicles (panels d and g) and olSix3.2 in the telencephalic region (arrowheads in panels e and h). From St22 onward, when olSix6 mRNA also becomes detectable, the three genes are co-expressed, albeit at different levels, in the developing neural retina, optic stalk, and pre-optic and hypothalamic area (panels j to r). In addition, olSix3.2 is distributed in the developing lens, olfactory pits (panels k and n; arrow), telencephalon, and anterior and posterior thalamus (panels k, n, and q). During retinal neurogenesis, olSix3.2 and olSix6 are restricted to the retinal ganglion and amacrine cells (panels t and u), whereas olSix3.1 is restricted to the inner nuclear layer (panel s).