Your blood is composed of a number of different types of cells. There are the red blood cells whose primary job is to carry oxygen throughout the body. You also have the white blood cells which are important in your immune system. Finally, your blood has small cell fragments known as platelets which play an important role in clotting to prevent blood loss. Platelets contain no nucleus but instead are small pieces of a cell released from a larger cell known as a megakaryocyte. Blood, including megakaryocytes, is normally made in the bone marrow. The platelets are shed from the megakaryocytes into the circulation where they perform their job. This is the traditional thinking but recently a team of researchers from California have turned this notion and our understanding of platelets and lung biology on its head.
The team had previously witnessed data suggesting that megakaryocytes may circulate throughout the body and set up shop in the lungs. To test this idea further, the researchers used a type of microscopy that can be done on living animals to look at the small blood vessels within the lungs of mice. Looking at mice who had been engineered to have their platelets fluoresce green, the researchers noticed a striking number of large green glowing megakaryocytes in the lung vasculature of mice. Monitoring the activity of these megakaryocytes the researchers witnessed them making around 10 million platelets per hour, that’s nearly half the mouse’s platelets being made in the lungs. The researchers also noticed a large number of blood stem cells within the lung vasculature indicating that not only does the lung produce platelets, but it also houses stem cells that may repopulate the bone marrow if it is damaged. To test this theory, the researchers took the lung of a mouse whose cells were made to glow green and transplanted it into a mouse who had its bone marrow destroyed. They noticed that the bone marrow in this mouse began to regrow, only this time with cells that glowed green from the donor lung. Additionally, they noted in mice who had the transplanted green lung there was a large increase in the number of platelets in the blood that glowed green indicating that the transplanted megakaryocytes were making platelets for the new mouse.
This research is fascinating and provides a number of new and very interesting avenues of exploration for human health. First, do these megakaryocytes contribute to lung diseases like COPD or asthma? What happens to these megakaryocytes in people who get lung transplants? Can these lung blood stem cells be used to help someone with bone marrow defects? Lots of interesting research is sure to come from this paper.
Reference: Lefrançais et al., 2017. The lung is a site of platelet biogenesis and a reservoir for haematopoietic progenitors. NATURE | LETTER doi:10.1038/nature21706.