Stimulation of head-specific nerve cell formation in Hydra by pulses of diacylglycerol.

In Hydra magnipapillata, repeated pulses of diacylglycerol (DG) induce a lengthening of the body column and the formation of ectopic head structures (Müller, 1989). In the present study, seven pulses given on 7 successive days led, in the gastric region, to a 1.87-fold increase in the number of epithelial cells from 16,200 to 30,400; a 4.6-fold increase in the total number of nerve cells from 2900 to 13,400; and an 18-fold increase of RF-amide immunopositive nerve cells from 100 to 1800. This subset of neurons, which is normally distributed in the form of a density gradient having its high point around the mouth and ending below the tentacle whorl in the upper gastric column, displays an altered pattern in DG-treated animals. While the density peak in the original head persisted, a second peak developed in the lower gastric region at the site of imminent ectopic tentacle formation, reflecting the local increase in positional value. Thus, the temporal sequence in which DG-induced ectopic apical body structures arise is a function of the rise in positional value and reflects the normal spatial sequence of these structures along the body column: first RF-amide neurons appear, then tentacles, and finally hypostomes.