Nerve cell and nematocyte production in Hydra is deregulated by lithium ions.

LiCl, a well-known vegetalising agent, interferes with the commitment of stem cells to nerve cells and nematocytes in Hydra attenuata. Treatment with 20 mM LiCl inhibits commitment to nerve cells, treatment with 1 mM LiCl inhibits commitment to nematocytes. However, LiCl does not prevent stem cells committed to the nematocyte pathway from dividing and differentiating into nests of nematocytes. Following LiCl treatment, determination to nerve cells and nematocytes is triggered again. Commitment to nerve cells is strongly stimulated within the first 3 h following pulse treatment with LiCl if the animals have been fed immediately prior to treatment. In Hydra exposed to LiCl for 10 days the stem cell density is reduced by at least 90% of the initial value, and nematocytes are almost completely missing, whereas the density of nerve cells is within the normal range in animals with normal morphology. Animals which developed a transverse constriction in the middle of the body axis contain a 1.7-fold higher nerve cell density in the lower part than is observed in control animals.