Figure 1.

Developmentally regulated polyploidy. Normal mitotic cell cycles results in two diploid mononucleated daughter cells with each nucleus containing two copies of each homologous chromatid (2N). Re-replication of DNA during S-phase is an aberrant event that produces giant nuclei and apoptosis. However, developmental signals can induce cells to become polyploid either by completing mitosis in the absence of cytokinesis [(C^-), acytokinetic mitosis], or by melding two G0-phase cells into a single cell containing two G-phase nuclei (cell fusion), or by arresting cells in G2-phase and then inducing another S-phase (endoreduplication), or by arresting cells in M-phase (M*) in the absence of cytokinesis (endomitosis). Multiple rounds of acytokinetic mitosis produce multinucleated giant cells. Multiple cell fusion events produce multinucleated myotubes in skeletal muscle. Multiple rounds of endoreduplication (endocycles) produce mononucleated giant cells, whereas multiple rounds of endomitosis produce a single multilobular nucleus.