ABSTRACT
Many recent experimental findings on heterogeneity, flexibility, and plasticity of tissue
stem cells are challenging the classical stem-cell concept of a pre-defined, cell-intrinsic
developmental program. Moreover, a number of these results are not consistent with the
paradigm of a hierarchically structured stem-cell population with a unidirectional deve-
lopment. Nonhierarchical, self-organizing systems provide a more elegant and compre-
hensive alternative to explain the experimental data.
