Fig. 8. Model for the differentiation of hypothalamic DA neurons. Our data suggest a model for the specification of hypothalamic DA neurons. An early patterning step promotes expression of Six3+ cells, potentially affecting ambient levels of Wnt activity (Lagutin et al., 2003; Kapsimali et al., 2004). Shh acts to ventralise Six3+ cells, downregulating Pax6 and upregulating Shh. Cells at this point are specified to a forebrain-like/incomplete hypothalamic DA neurotransmitter identity, but do not exhibit definitive aspects of hypothalamic regional identity. Subsequently, Bmp7 acts to induce hypothalamic regional identity on Shh+ Six3+ cells, upregulating Nkx2.1 and Msx1/2.