Tentacle morphogenesis in hydra: I. The role of head activator

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Abstract
Stimulation of tentacle-specific cell differentiation by the neuropeptide head activator was investigated in Hydra magnipapillata. Tentacle-specific sensory nerve cells were identified by a monoclonal antibody, NV1. Treatment of hydra with 1 pM head activator for 18 h stimulated differentiation of NV1+ nerve cells and tentacle epithelial cells in tissue from the distal gastric region. Head tissue and tissue from the proxmial gastric region did not respond to head activator treatment with increased NV1+ differentiation. Hence the distal gastric region appears to be the site of tentacle formation in hydra. Tentacle precursors in head tissue seem to be committed since they fail to respond to head activator or to changes in tissue size with altered amounts of tentacle formation. We suggest that NV1 precursors form a complex with tentacle epithelial cell precursors, which then moves distally through the head region into the tentacles. The signal for NV1+ differentiation appears to be formation of this complex.