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Thrombocytes are also known as platelets. They are
metabolically active cell fragments that are important in
hemostasis.
Thrombocytes develop from CFU-GEMM, BFU-M (burst forming
unit-megakaryocyte), and CFU-M (colony forming
unit-megakaryocyte).
Megakaryocytopoiesis proceeds through mitotic division of
progenitor cells followed by endoreduplication (mitosis
without subsequent division of cytoplasm into identical
daughter cells).
- megakaryoblast
- promegakaryocyte
- megakaryocyte
The megakaryocyte sheds platelets from the cytoplasm.
This takes place in bone marrow. There will be pink areas in
the blue cytoplasm signifying platelet development.
Increased granularity is also seen. One megakaryocyte
produces an average of 1000 platelets, which enter the
spleen and remain for 2 days. After that they either enter
the circulating blood or remain in the active splenic
pool.
The megakaryocyte
- large (up to 160 um) cell
- nucleus is purple with 2 or more lobes
- nucleoli are not visible
- abundant cytoplasm is pinkish-blue with an irregular
border
Mature platelet is small (2-4 um)
- anucleate
- cytoplasm is light blue with scattered red-purple
granules.
Platelets have specific roles in the coagulation
(hemostasis) process. These roles are dependent on adequate
numbers of platelets present and normal platelet function.
There are three specific platelet functions:
1. Maintenance of vascular integrity. Platelets
help nurture the endothelial cells lining the vascular
system. The platelets attach to the endothelium and release
endothelial growth factor into the endothelial cells. If
platelets are not present in adequate numbers, large numbers
of RBCs can migrate through the endothelial wall and produce
petechiae or purpura in the skin.
2. Platelet plug formation. Exposure to
subendothelial connective tissue attracts platelets, which
adhere to the tissue irreversibly (platelet adhesion). The
platelets adhere to the injured area as well as to each
other to try and stop hemorrhage. Absence of platelet
adhesion can result in bleeding disorders. Once the
platelets have adhered to the site of the injury they change
shape to intertwine with each other (platelet aggregation).
As the platelets squeeze together, they release platelet
factors (PFs) which are necessary for completion of the
coagulation process.
3. Stabilization of the hemostatic plug by
contributing to the process of fibrin formation. Various
clotting factors and enzymes are necessary for successful
hemostasis. All of these factors lead to the generation of
large quantities of thrombin on the aggregated platelet's
surface. Thrombin then converts fibrinogen to fibrin, one of
the final stages of clot formation.
Platelets can be evaluated on a peripheral blood smear.
Using the same fields you use for a WBC differential and RBC
morphology evaluation, determine the approximate number of
platelets per field. A normal blood smear should have
between 8 and 20 platelets per field in this area of the
smear. An platelet estimate is obtained by finding the
average number of platelets in 10-15 fields and multiplying
this result by 20,000. A normal platelet estimate is between
200,000-399,999/microliter.
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