Liver tetraploidization is controlled by a new process of incomplete cytokinesis.

Cytokinesis is precisely controlled in both time and space to ensure equal distribution of the genetic material between daughter cells. Incomplete cytokinesis can be associated with developmental or pathological cell division programs leading to tetraploid progenies. In this study we decipher a new mechanism of incomplete cytokinesis taking place in hepatocytes during post-natal liver growth. This process is initiated in vivo after weaning and is associated with an absence of anaphase cell elongation. In this process, formation of a functional contractile actomyosin ring was never observed; indeed, actin filaments spread out along the cortex were not concentrated to the putative site of furrowing. Recruitment of myosin II to the cortex, controlled by Rho-kinase, was impaired. Astral microtubules failed to contact the equatorial cortex and to deliver their molecular signal, preventing activation of the RhoA pathway. These findings reveal a new developmental cell division program in the liver that prevents cleavage-plane specification.