Bone

Platelets are produced by megakaryocytes in the bone marrow.

    • Dense granules are storage organelles for serotonin, ATP, ADP, phosphate and calcium.

    • Serotonin is not present in newly produced platelets.

    • Platelets are cytoplasmic fragments of megakaryocytes (MKs).

    • MKs are large polyploid stem cells in the bone marrow.

    • One MK can make 2-3,000 platelets.

    • New platelets have the discoid shape of resting platelets.

    • They are fragmented off MKs without nucleus and serotonin.

    • New platelets contain all granules such as α‐ and dense granules.

    • Platelets are unique to mammals. Animals that evolved before mammals have thrombocytes in their blood.

    • Thrombopoietin (TPO) is a chemical produced by the liver and kidney.

    • It stimulates MKs to produce platelets.

  • Thrombopoiesis is the process of platelet formation.

Healthy megakaryocytes produce discoid, functional platelets.

New platelets contain dense granules without serotonin (5-hydroxytryptamine, 5-HT). Serotonin is produced in the gut, released into the blood, and taken up by platelets.

Serotonin is needed for body temperature regulation – and thus is related to fever – and proper hemostasis as we know from rare disorders like Hermansky-Pudlak syndrome.