ABC of clinical haematology: Platelet disorders

Introduction

Platelets are produced predominantly by bone marrow megakaryocytes as a result of budding of the cytoplasmic membrane. Megakaryocytes are derived from the haemopoietic stem cell, which is stimulated to differentiate to mature megakaryocytes under the influence of various cytokines, including thrombopoietin.

Once released from the bone marrow, young platelets are trapped in the spleen for 36 hours before entering the circulation, where they have a primary haemostatic role.

Normal haemostasis

The platelet membrane has integral glycoproteins essential in the initial events of adhesion and aggregation, leading to formation of the platelet plug during haemostasis.

Glycoprotein receptors react with aggregating agents such as collagen on the damaged endothelial surface (for example, blood vessels), fibrinogen, and von Willebrand's factor to facilitate platelet-platelet and platelet-endothelial cell adhesion. Storage organelles within the platelet include the “dense” granules, which contain nucleotides, calcium, and serotonin, and α granules containing fibrinogen, von Willebrand's factor, platelet derived growth factor, and many other clotting factors. After adhesion, platelets are stimulated to release the contents of their granules essential for platelet aggregation. Platelets also provide an extensive phospholipid surface for the interaction and activation of clotting factors in the coagulation cascade.

Congenital abnormalities

Congenital abnormalities can be disorders of platelet production or platelet function. All are very rare.

Fanconi's anaemia is an autosomal recessive preleukaemic condition that often presents as thrombocytopenia with skeletal or genitourinary abnormalities. The cardinal feature is abnormal chromosomal fragility. The condition can be cured only with bone marrow transplantation.

Amegakaryocytic thrombocytopenia presents with severe neonatal thrombocytopenia (platelet count <2x10⁹/l), though this often corrects itself after the …