Platelets (thrombocytes)
The blood platelets are the smallest cells of the blood,
averaging about two to four micrometres in diameter.
Although much more numerous (150,000 to 400,000 per
cubic millimetre) than the white cells, they occupy
a much smaller fraction of the volume of the blood because
of their relatively minute size. Like the red cells
they lack a nucleus and are incapable of cell division,
but they have a more complex metabolism and internal
structure than have the red cells. When seen in fresh
blood they appear spheroid, but they have a tendency
to extrude hairlike filaments from their membranes.
They adhere to each other but not to red cells and white
cells. Tiny granules within platelets contain substances
important for the clot-promoting activity of platelets.
The function of the platelets is related to hemostasis,
the prevention and control of bleeding. When the endothelial
surface (lining) of a blood vessel is injured, platelets
in large numbers immediately attach to the injured surface
and to each other, forming a tenaciously adherent mass
of platelets. The effect of the platelet response is
to stop the bleeding and to form the site of the developing
blood clot, or thrombus. If platelets are absent, this
important defense reaction cannot occur, and protracted
bleeding from small wounds (prolonged bleeding time)
results. The normal resistance of capillary membranes
to leakage of red cells is dependent upon platelets.
Severe deficiency of platelets reduces the resistance
of the capillary walls, and abnormal bleeding from the
capillaries occurs, either spontaneously or as the result
of minor injury. Platelets also contribute substances
essential for the normal coagulation of the blood, and
they cause the shrinking, or retraction, of a clot after
it has been formed. (For additional information about
the causes, consequences, and treatment of thrombosis,
see bleeding and blood clotting.)
Platelets are formed in the bone marrow by segmentation
of the cytoplasm (the cell substance other than the
nucleus) of cells known as megakaryocytes, the largest
cells of the marrow. Within the marrow the abundant
granular cytoplasm of the megakaryocyte divides into
many small segments that break off and are released
as platelets into the circulating blood. After about
10 days in the circulation, platelets are removed and
destroyed. There areno reserve stores of platelets except
in the spleen, in which platelets occur in higher concentration
than in the peripheral blood. Some platelets are consumed
in exerting their hemostatic effects, and others, reaching
the end of their life span, are removed by reticuloendothelial
cells (any of the tissue phagocytes). The rate of platelet
production is controlled but not so precisely as the
control of red cell production. A hormonelike substance,thrombopoietin,
which has not been identified chemically, is believed
to be the chemical mediator that regulates the number
of platelets in the blood by stimulating an increase
in the number and growth of megakaryocytes, thus controlling
the rate of platelet production.
