The study of tissue P systems was initiated in [6], inspired from the way neurons cooperate, processing impulses in the complex net established by synapses. These systems use multisets of objects for processing, and it was shown that computational completeness can be achieved using a small number of cells and states. In this paper, we use string objects as the underlying data structure. The control structure used is a restricted form of contextual rules and rewriting rules. We obtain two characterizations of recursively enumerable languages using these systems: tP systems having 2 states and 2 cells as well as tP systems having 4 states and a single cell generate all recursively enumerable languages. We also discuss the relationships with ET0L and E0L languages. © Springer-Verlag Berlin Heidelberg 2003.