Stem cell plasticity in mammals and transdetermination in Drosophila: Common themes?

Stem cells have been identified in a number of mammalian tissues (e,g,, bon e marrow, muscle, gut, skin, and neural tissues). Until recently, it was ge nerally believed that the differentiation potential of a mammalian somatic stem cell is restricted to one tissue only, as in the case of hematopoietic stem cells differentiating into hematopoietic cells. In this sense, somati c stem cells are limited in their differentiation potential. Several lines of evidence now challenge the idea of unilateral development. New reports s how mammalian somatic stem cells can, in the course of regeneration, repopu late heterologous cell systems and therefore possess a surprisingly broad s pectrum of differentiation potential. Thus, mammalian stem cells are appare ntly capable of fate changes between stem cell systems, although the mechan isms leading to such changes are unclear. Mechanistic models for fate changes have been proposed in Drosophila, speci fically for transdetermination of imaginal discs. Imaginal discs of the lar va are the primordia of the adult exoskeleton and appendages, for example, legs, and antennae. Transplantation experiments of imaginal discs have show n that discs are determined for their disc identity. Transdetermination in Drosophila refers to cases when, after regenerative cell divisions, imagina l disc cells change from one state of determination to another, initiating a pathway of differentiation leading to structures other than those corresp onding to the initial state or determination; for example, an antennal imag inal disc transdetermines to a leg imaginal disc, A fate change is thus pos sible in both mammalian somatic stem cells and Drosophila imaginal discs fo llowing transplantation and subsequent proliferation. Here we summarize and compare observations made in such cases of stem cell and imaginal disc dif ferentiation.