COURTESY OF MARK LOONEY, UCSF
E. Lefrançais et al., “The lung is a site of platelet biogenesis and a reservoir for haematopoietic progenitors,” Nature, 544:105-09, 2017.
Platelets form when megakaryocytes release bits of their specialized cytoplasm, a process long thought to occur primarily in the bone marrow. Mark Looney, a pulmonologist at the University of California, San Francisco, says human studies had reported that blood leaving the lungs contained fewer megakaryocytes and more platelets than blood entering it—suggesting the lungs were producing platelets. “But it’s never been visualized, it’s never been caught in the act.”
In a recent study, Looney and colleagues observed fluorescently labeled megakaryocytes in mice releasing platelets within the lung microvasculature—catching “the act” on video. The researchers estimated that a mouse’s lungs produce about 10 million platelets per hour—half of the animal’s total platelet production.
When the researchers transplanted a normal lung into a mouse with labeled megakaryocytes, glowing megakaryocytes and platelets soon appeared in the transplanted lung. Moving a lung with fluorescently labeled cells into a normal mouse, however, did not yield glowing platelets. This result indicates that the productive megakaryocytes originated outside the lung—in the bone marrow, the researchers believe.
It’s unclear if human lungs are significant platelet sources, says UCSF hematologist Jack Levin. If so, “one would have to anticipate that at least some pulmonary disorders would be associated with a reduced platelet count,” or thrombocytopenia. Yet Levin knows of no such examples. Looney counters that the lung damage in common pulmonary conditions is too small to cause thrombocytopenia, but that the condition is associated with surgeries in which a machine takes over for the heart and lungs temporarily and lung circulation is bypassed.