Immediately above the leukocytes in the buffy coat lies a
thin layer of platelets. On Wright's stained peripheral
smear platelets are small (2-5m) round-oval anuclear bits of
pale blue cytoplasm containing azurophilic granules.
Smaller platelets tend to be older; while large platelets
tend to be young.
The peripheral blood platelet count ranges from 150 - 450
x 10 /L.
About 2/3 of platelets circulate, while 1/3 are in the
splenic pool or other extravascular locations.
During its lifespan the average megakaryocyte (MK) gives
rise to approximately 4,000 platelets which live an average
of 9-12 days.
In the steady state, where platelet production = platelet
destruction, daily production is 30,000 - 40,000 /uL.
As with RBCs platelet production can be increased up to
8x normal if needed.
Megakaryocytes undergo endomitosis - a process whereby
DNA is duplicated without cell division. Thus MKs become
polyploid during amplification. Most platelet forming MKs
have a ploidy of 16N, (range 4N - 64N). MKs are the largest
normally occuring cells in the marrow (35-160m)
Increased numbers of MKs in the marrow is known
The hematopoietic factors involved are: ________
Closer inspection of MKs shows a network of platelet
demarcation zones forming fissures throughout the MK
cytoplasm. The membranes of this demarcation system
eventually formcytoplasmic platelet membranes.
Platelet granules are of two types: alpha granules and
Alpha granules contain Platelet Derived Growth Factor,
platelet factor 4, Factors V & XIII and fibrinogen.
Dense bodies contain serotonin,nucleotides (ADP) and
calcium. Lysosomes containing hydrolytic enzyme are also
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