Budded from giant bone-marrow cells, each of which sheds 2,000–3,000 of them
Platelet — fragments of megakaryocyte cytoplasm.
Anatomy · Cells & tissues
Platelet
The smallest formed element of the blood
Blood · cell fragment
Tiny, disc-shaped, nucleus-free fragments of cytoplasm, only about 2–4 micrometres across, that circulate in the blood and seal breaks in vessel walls. Strictly they are not whole cells but pieces budded off giant bone-marrow cells called megakaryocytes, each of which sheds thousands. There are normally about 150,000–450,000 per microlitre of blood, and each lives around a week to ten days. When a vessel is damaged they stick, change shape, clump and help the clotting cascade to build a stable clot.
How to spot them
- ▸Tiny purple-staining granular specks, much smaller than red cells, in a blood smear
- ▸No nucleus; pale cytoplasm with small dense granules
- ▸Often seen in clumps where they have started to stick together
- ▸In a scanning electron micrograph, a rounded disc when resting and a spiky, spread-out star when activated
- ▸Shown among red cells and fibrin strands in a clot
What to know
A tiny anucleate fragment about 2–4 micrometres across, present at some 150,000–450,000 per microlitre of blood
Platelet — also called the thrombocyte.
The first step of stopping a bleed: it sticks to exposed collagen through von Willebrand factor and becomes spiny
Platelet — adhesion and activation, forming the platelet plug.
A hormone made by the liver and kidney, thrombopoietin, controls how many are made
Platelet — thrombopoietin drives megakaryocyte development.
Low-dose aspirin blocks their production of thromboxane A2 for the rest of their week-long life
Platelet — aspirin irreversibly inhibits COX-1, so effects last until new ones are made.
The Italian pathologist Giulio Bizzozero recognised them in 1882 as a distinct element of the blood that plugs vessel breaks (1882)
Platelet — Bizzozero's discovery.
A shortage of them causes easy bruising and pinpoint skin haemorrhages called petechiae
Platelet — thrombocytopenia, a count below about 150,000 per microlitre.
Key ideas
- Haemostasis has three steps: vessel spasm, formation of a platelet plug, and coagulation. Platelets are the main players in the second and give a surface for the third.
- Activated platelets release ADP and thromboxane A2 to recruit others, and link together through fibrinogen bridges between their GPIIb/IIIa receptors.
Quick-fire facts
- •Life span in the circulation is about 7–10 days; the spleen holds about a third of them in reserve
- •A megakaryocyte is huge — 50–100 micrometres across with a lobed nucleus — and stays in the marrow, extending platelet-forming projections into blood vessels
- •They contain alpha and dense granules with clotting factors, ADP, calcium and serotonin
- •Giulio Bizzozero (1846–1901) described them in 1882; James Homer Wright showed in 1906 that they come from megakaryocytes
- •Donated platelets are stored at room temperature with gentle agitation and last only about five to seven days
- •A normal count is about 150,000–450,000 per microlitre
- •Von Willebrand disease, the commonest inherited bleeding disorder, involves a fault in the protein that helps platelets stick
- •Platelets also release growth factors that aid wound healing, which is the idea behind platelet-rich plasma treatments
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