Plasticity in the nervous system of adult hydra. II. Conversion of ganglion cells of the body column into epidermal sensory cells of the hypostome.

Due to the tissue dynamics of hydra, every neuron is constantly changing its location within the animal. At the same time specific subsets of neurons defined by morphological or immunological criteria maintain their particular spatial distributions, suggesting that neurons switch their phenotype as they change their location. A position-dependent switch in neuropeptide expression has been demonstrated. The possibility that ganglion cells of the body column are converted into epidermal sensory cells of the head was examined using a monoclonal antibody, TS33, whose binding is restricted to a subset of epidermal sensory cells of the hypostome, the apical end of the head. When animals devoid of interstitial cells, which are the nerve cell precursors, were decapitated and allowed to regenerate, they formed TS33+ epidermal sensory cells. As this latter cell type is not found in the body column, and the interstitial cell-free animals contained only epithelial cells and ganglion cells in the part of the ectoderm that formed the head during regeneration, the TS33+ epidermal sensory cells most likely arose from the TS33- ganglion cells. The observation of epidermal sensory cells labeled with both TS33 and TS26, a monoclonal antibody that binds to ganglion cells, in regenerating and normal heads provides further support. The double-labeled cells are probably in transition from a ganglion cell to an epidermal sensory cell. These results provide a second example of position-dependent changes in neuron phenotype, and suggest that the differentiated state of a neuron in hydra is only metastable with regard to phenotype.