Tentacle morphogenesis in hydra : I. The morphological effect of actinomycin D

Hydra oligactis exposed to 3 μg/ml actinomycin D for 24 hours regenerated only the first pair of tentacles (the mid-laterals). If left uncut, actinomycin D treated animals underwent a reduction of the normal number of tentacles to two or less.Inductive activity was retained in the 2-tentacled hypostomes. However, the tentacles present exhibited reduced capacities to capture and manipulate prey.Histological studies showed that the tentacles of actinomycin D treated hydra were morphologically identical to those of the controls. The interstitial cell (I-cell) population of the treated animals, however, became depleted. Replacing the hypostome of an actinomycin D treated hydra with a normal hypostome reversed the cellular effects of actinomycin D treatment.The modifications in tentacle morphogenesis occurring after actinomycin D treatment are consistent with an impairment of hypostomal function in the animal. It is suggested that the morphological site of this malfunction may be in the nervous system.