Figure 2: Selective expansion of muscle regenerative cells. The figure depicts two different strategies to obtain cultures enriched with muscle regenerative cells: a ‘two-step’ approach and the selective expansion of highly regenerative cells. These procedures start with the establishment of a muscle culture from a small biopsy. The culture is heterogeneous and contains a subpopulation of cells capable of engraftment. Two-step approach: The cultures may be cultured under ‘regular’ conditions in order to expand the cellsrapidly and extensively. Subsequently, strategies, such as preconditioning or exposing the cultures to hypoxic conditions (see text for details), can be appliedto boost the regenerative potential of the expanded cellsprior to transplantation (referred to as a two-step approach in the text; red arrows). The efficiency of this strategy may be low, because of the extensive loss of regenerative potential in the first step and the modest restoration (~2-fold) of the regenerative potential in the second step of the protocol. Selective expansion: Alternatively, the mechanisms of stem cell self-renewalin vivomay are applied to cultured cells (e.g. use of PEG-based hydrogel-based culture substrates or use of immobilized Notch ligand; see text). We hypothesize that such well-defined culture conditions promote the selective (self-renewing) expansion and, consequently, enrichment of engraftment-competent cells. It is expected that limited numbers of cells from such enriched cultures are required for efficient engraftment and regeneration.