Threading an elephant through the eye of a needle: Where are platelets made?

Abstract

There has been a long-standing controversy of whether megakaryocytes release platelets in the marrow or travel to the lungs and release platelets there. Using two-photon electron microscopy and orthotopic lung transplantation, Lefrançais et al. now document that platelet release occurs physiologically in the lungs of mice from extrapulmonary megakaryocytes and that this release accounts for ∼50% of total platelet production.

Figure 1

Alternative models for the site of thrombopoiesis. (A-C) Schematics showing from left to right, the intramedullary marrow, the fenestrated endothelial lining separating the marrow from the medullar sinuses, the medullar sinus, which through the returning veins (not shown), connects to the pulmonary arteries, followed by the pulmonary microcirculation (lungs) and then the pulmonary veins prior to emptying into the general circulation (not shown). Intramedullar release of platelets (A). Release of platelets as megakaryocytes traverse the medullar endothelial lining (B). Pulmonary release of platelets from megakaryocytes that have traveled from the marrow as supported by Lefrançais et al. (C).