Platelets are produced in the bone marrow by highly specialized cells called megakaryocytes. The arrow in the image on the right points to a megakaryocyte. The area enclosed by the red circle contains platelets that are created within the megakaryocyte cytoplasm and eventually released. The average lifespan of a platelet is 9-12 days.
About 70-80% of a person's total platelets are circulating in the vasculature at any given time. Approximately 20-30% of the total platelets are pooled in the spleen.