Neuron differentiation in hydra involves dividing intermediates

The neuron differentiation pathway in hydra is usually assumed to be the following. A multipotent stem cell among the large interstitial cells becomes committed to neuron differentiation and divides. The two daughter cells, which are postmitotic small interstitial cells, subsequently differentiate into neurons. Herein the neuron pathway of the lower peduncle of Hydra oligactis was examined in some detail. In this region a substantial amount of neuron differentiation takes place, but very few large interstitial cells are present. It was found that small interstitial cells, which are capable of dividing, differentiate into neurons. The minimum time required to traverse the pathway from S phase of the last proliferating intermediate to a neuron is 18 hr. Thus, the neuron differentiation pathway in the lower peduncle involves dividing intermediates and is therefore more complex than usually assumed. Evidence for dividing small interstitial cells in the head, where the highest rate of neuron differentiation occurs, suggests that this more complex pathway may be common to all regions of the animal. A consequence of this finding is that the body of evidence concerning the commitment of multipotent stem cells to neurons and the control of this commitment requires reinterpretation.