Junt, et al. Observed How Mature Megakaryocytes in Mouse Bone Marrow Routinely Extend Protrusions (Proplatelets) into Blood Vessels. Sheer stress from blood flow fragments these protrusions, generating platelets. Legend and figure from Geddis & Kaushansky, Science 317:1689 (2007). Illustration: Preston Huey. Reprinted with permission from AAAS.