Platelets budding off the mothership

Ace microscopists have captured the bizarre process of platelet formation in real time and in color (Science 317, 1767–1770).

Boo! A megakaryocyte (red; darker areas are presumably nuclei) associated closely with a bone marrow sinusoid (green). Inset: pseudopodia extending into the vessel. Credit: Tobias Junt and Ulrich von Andrian

Platelets bud off of a massive precursor cell, the megakaryocyte. Ulrich von Andrian and his colleagues—noted for their movies of immune cells in action—observed that megakaryocytes associate closely with sinusoids, nascent blood vessels in the bone marrow (shown here). The megakaryocyte then extends protrusions, dubbed 'plump perivascular pseudopodia', which enter the blood vessels. These protrusions are sheared off by the force of blood moving through the vessel and shed platelets into the bloodstream. The findings solidify conclusions from previous observations in culture and with still microscopy.

The human body creates approximately 1 × 10¹¹ new platelets each day.