Expression pattern of the homeobox protein NKX2-1 in the developing Xenopus forebrain.

Although morphological data suggest that the amphibian forebrain contains similar subdivisions to those observed in birds and mammals, it is presently unclear whether the same patterning mechanisms are conserved among all three classes of tetrapods. Here we report that NKX2-1, a transcription factor that is essential for the ventral patterning of the forebrain in birds and mammals, is expressed in corresponding (homologous) domains in the developing Xenopus forebrain. NKX2-1 expression is restricted to two domains in the amphibian forebrain: (1) a ventral diencephalic domain, with expression limited to hypothalamic structures; and (2) a telencephalic domain, with expression in the medial ganglionic eminence, preoptic area and part of the septum. Thus, the detailed analysis of the distribution of NKX2-1 provides the first unequivocal evidence for distinct progenitor zones within the amphibian forebrain through embryonic and larval development.