Repairing or replacing damaged cells keeps our organs in tip top working condition. For a long time, we thought that only the incredibly rare stem cells in adult organs were able to create brand new cells to replace injured ones and fix damaged areas. Yet some tissues definitely don’t conform to this autocratic model: following liver damage, for example, mature hepatocyte cells that normally exist in a semi-retired state re-engage their cell cycle and undergo a huge amount of cellular proliferation to patch up affected sections.
For other organs, such as the heart, it’s still unknown if stem cells or mature cardiomyocyte cells are responsible for performing such repairs. One team of researcher’s recently got to grips with this question by supplying cells in the heart with thymidine, one of the building blocks of DNA, tagged with a stable isotope of nitrogen, 15N. Since DNA is duplicated during cell division, heart cells that are actively dividing incorporate the 15N-thymidine and can be tracked. After suffering a heart-damaging myocardial infarction, mature cardiomyocyte cells immediately next to the affected area incorporated the trackable thymidine. Around 15% of these mature cells came out of semi-retirement and actively re-entered the cell cycle, dividing to generate fresh new cells. The other 85% simply got bigger to take on more work, compensating for the lost cells and maintaining cardiac output. No stem cells were observed to incorporate 15N-thymidine in a substantial way.
So, it’s not just jazzy, energetic stem cells that can support the biological business of tissue repair. The heart, like the liver, contains established, mature cells capable of producing lovely new cells. This knowledge could help to identify new ways of speeding up the healing process.
Senyo SE, Steinhauser ML, Pizzimenti CL, Yang VK, Cai L, Wang M, Wu TD, Guerquin-Kern JL, Lechene CP, & Lee RT (2013). Mammalian heart renewal by pre-existing cardiomyocytes. Nature, 493 (7432), 433-6 PMID: 23222518