Roads to polyploidy: The megakaryocyte example

Polyploidy, recognized by multiple copies of the haploid chromosome number, has been described in plants, insects, and in mammalian cells such as, the platelet precursors, the megakaryocytes. Several of these cell types reach high ploidy via a different cell cycle. Megakaryocytes undergo an endomito tic cell cycle, which consists of an S phase interrupted by a gap, during w hich the cells enter mitosis but skip anaphase B and cytokinesis. Here, we review the mechanisms that lead to this cell cycle and to polyploidy in meg akaryocytes, while also comparing them to those described for other systems in which high ploidy is achieved. Overall, polyploidy is associated with a n orchestrated change in expression of several genes, of which, some may be a result of high ploidy and hence a determinant of a new cell physiology, while others are inducers of polyploidization. Future studies will aim to f urther explore these two groups of genes. (C) 2002 Wiley-Liss, Inc.