Spatio-temporal expression of Pax6 in Xenopus forebrain.

x-Pax6 gene expression has been previously demonstrated in the Xenopus forebrain at early developmental stages and, in particular, it was analyzed in relation to putative migratory cells in the primordial septum and olfactory bulbs. Here we investigated the pattern of x-Pax6 expression during embryonic and larval stages and in the adult. To gain more insight into the exact localization of x-Pax6 labeled cells, we used combinations of x-Pax6 with other telencephalic markers of the Xenopus forebrain such as Islet-1, Nkx2.1, and tyrosine hydroxylase. The results, analyzed within the frame of the current prosomeric model of the vertebrate forebrain, demonstrated robust expression in the pallium and the prethalamic diencephalic segment, from early embryonic stages. In addition, starting at the end of the embryonic period, x-Pax6 was detected in the subpallial mantle (but not in the proliferating cells of the ventricular/subventricular zones). The pattern of distribution in the pallium narrows through larval development and in the adult, whereas in the subpallium the expression was gradually followed and identified into the dorsal part of the septum, corroborating in Xenopus a mixed pallial/subpallial origin of the neurons in this region. The findings show that the strongly conserved features of x-Pax6 expression through forebrain development shared by all amniote vertebrates are also present in the anamniote amphibians as a common characteristic of the forebrain organization of tetrapods.