Towards a generalized model of encystment (cryptobiosis) in ciliates: a review and a hypothesis.

In this work we propose a generalized model to explain encystment (cryptobiosis) in ciliates. The model is elaborated from both structural and physiological studies previously reported, and some hypothetical considerations. The main features of this model are the following: (a) starvation is considered as the most important inducer of ciliate encystment, and it constitutes the first trigger of this cell differentiation process; (b) the "switch on" of encystment genes involves the appearance of several new transcripts; (c) starvation involves cell autophagy and protein turnover, which provides material to synthesize the new cystic proteins; (d) cytoplasmic dehydration has an important role during the resting cyst formation; (e) a gradual loss of intracellular water can inactivate cell metabolism, leading to an ametabolic state (cryptobiosis); (f) as a late precystic event, the encysting cell forms a barrier (cyst wall), that isolates the cell from the hostile environment.