Final answer:
Platelets break off from large cells called megakaryocytes in the bone marrow.
Step-by-step explanation:
Platelets are actually cell fragments that break off from large cells called megakaryocytes, which are found in the bone marrow. These megakaryocytes release thousands of platelets that enter the bloodstream. When a blood vessel is injured, platelets stick to the injured area and release chemicals that help form a blood clot.