Nerve cell differentiation in nerve-free tissue of epithelial hydra from precursor cells introduced by grafting. I. Tentacles and hypostome.

The differentiation of hydra nerve cells in the nerve-free tissue of epithelial hydra was examined in Hydra magnipapillata. Nerve cell precursors, the interstitial cells, were introduced into the upper half of epithelial hydra by grafting it onto the lower half of normal hydra. In the tentacles of grafted epithelial hydra, a small number of RF+ ganglion cells first appeared in the proximal area at 1.5 days after grafting, followed by the appearance of NV1+ sensory cells in the same area about a day later. In the following days, both neuron types appeared more numerously in more distal positions. The front boundary for each type moved gradually from the base to the tip of the tentacles in about 7 days. In the hypostome, a small number of RF+ ganglion cells first appeared in the apex at 1.5 days. More nerve cells appeared in the following days, eventually forming a cluster of RF+ sensory cells at the apex surrounded by numerous RF+ ganglion cells in the adjacent tissue. These results show that nerve cells do not differentiate randomly in the epithelial hydra host. Instead, differentiation occurs in a strongly region-specific manner in the same way as in normal hydra, suggesting that epithelial cells in each region provide different cues or signals to produce region-specific nerve cell distribution in normal hydra tissue.