There are no conclusive experimental data as yet on the mechanisms of compensated cell enlargement. However, such mechanisms are beginning to be revealed, as illustrated by the specific manipulation of cell size in compensation-exhibiting mutants (Fujikura et al. 2007). We have identified a wide variety of mutants that enable the genetic manipulation of both cell size and cell number (Horiguchi et al. 2006a, 2006b). Moreover, the recent development of tissue-specific inducible gene expression systems (Brand et al. 2006) and an artificial microRNA expression system (Schwab et al. 2006) will allow local or stage-specific modification of cell proliferation or cell expansion in a more sophisticated manner. Whatever the actual mechan-ism(s) involved in compensated cell enlargement, characterization of this phenomenon constitutes a challenging undertaking, one that will further understanding of the control of organ size involving a higher-order regulation that links two different developmental processes.
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