Fig. 8. The role of Hyzic in neuronal differentiation. (A) Schematic of the stem cell proliferation, nerve and nematocyte differentiation pathway in Hydra [modified from David and Gierer (David and Gierer, 1974)]. Determination of stem cells to neuron or nematocyte differentiation can occur at any proliferating stage from 2 to 8 cells (white circles in the middle). The probability of a determination event is represented by the thickness of arrows (the thicker the arrow the more likely the event). After determination to nematocyte differentiation, nematoblasts undergo a variable number of cell divisions leading to nests of 2, 4, 8, 16 and 32 cells. At any of these stages, nematoblast cells can undergo a final mitosis and enter the differentiation program, which is mainly characterized by the development of the nematocyst (shown by the increasing black color). The red labeled circles represent Hyzic-expressing proliferating cells already committed to the nematocyte pathway. The onset of Hyzic expression is not sharply defined, so that additional proliferating nests could be detected that might also give rise to nerve cell precursors or enter the nematocyte pathway at later stages (see also Fig. 6D). Post-mitotic differentiating nematocytes express Cnash and Nowa, as indicated by the blue circles. (B) Genetic pathway leading to nematocyte differentiation. It is unclear whether Chordin also acts upstream of Hyzic in Hydra. (C) Genetic regulation of neural differentiation in vertebrates, showing that at least some homologous genes are involved in neural differentiation in both Hydra and vertebrates.