Fig. 9. hydkk1/2/4-expression pattern and neuronal differentiation in Hydra. (A) The release of HyDkk1/2/4 from gland cells in the endoderm is postulated to facilitate stem cell growth and the formation of neuronal precursor cells in the ectoderm. nvp, nerve cell precursors; nm, mature nematocytes; nmb, nematoblast; nmp, nematocyte precursors; nv, neuronal cells. (B) The hydkk1/2/4-expression domain correlates to that of neuronal and pro-neuronal genes in Hydra. Modified, with permission, from Meinhardt (Meinhardt, 2002; Meinhardt, 2004).