Pathology > Basic Hematology > Normal Hematopoiesis > Platelets & Megakaryocytes

Platelets & Megakaryocytes

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 as: _______

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 dense bodies.

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 present.

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