Platelet Production

The production of platelets is called thrombopoiesis because platelets used to be called thrombocytes.9 The latter term is now reserved for nucleated true cells with a blood-clotting function in animals such as birds and reptiles. Thrombopoiesis begins when a hemocytoblast develops receptors for the hormone thrombopoietin, which, like erythropoietin, is produced by the liver and kidneys. With these receptors in place, the hemocytoblast has become a committed cell called a megakaryoblast. In response to thrombopoietin, the megakaryoblast replicates its DNA repeatedly without undergoing nuclear or cytoplasmic division. The result is a gigantic cell (up to 100 ^m in diameter) called a megakaryocyte10 (meg-ah-CAR-ee-oh-site), with a huge multilobed nucleus and multiple sets of chromosomes (fig. 18.7). Most megakaryocytes live in the bone marrow, but some of them colonize the lungs.

A megakaryocyte exhibits infoldings of the plasma membrane that divide its marginal cytoplasm into little compartments. The cytoplasm breaks up along these lines of weakness into tiny fragments that enter the bloodstream. Some of these are functional platelets, while others are larger particles that break up into platelets as they pass through the lungs. About 25% to 40% of the platelets are stored in the spleen and released as needed. The remainder circulate freely in the blood and live for about 10 days.

thrombo = clotting + cyte = cell 10mega = giant + karyo = nucleus + cyte = cell thrombo = clotting + cyte = cell 10mega = giant + karyo = nucleus + cyte = cell

Megakaryocyte Breakup

Figure 18.7 A Megakaryocyte Producing Platelets. Several red and white blood cells are shown for size comparison.

scerulo = blue-green, the color of oxidized copper + plasm = blood plasma

Figure 18.7 A Megakaryocyte Producing Platelets. Several red and white blood cells are shown for size comparison.

Saladin: Anatomy & I 18. The Circulatory System: I Text I © The McGraw-Hill

Physiology: The Unity of Blood Companies, 2003 Form and Function, Third Edition

Before You Go On

Answer the following questions to test your understanding of the preceding section:

9. List the fetal tissues and organs that produce blood.

10. How do the sites of hemopoiesis differ between children and adults?

11. Distinguish between lymphoid and myeloid hemopoiesis.

12. How is a hemocytoblast different from a committed hemopoietic cell?

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Responses

  • sebhat welde
    How do the sites of hemopoiesis differ between children and adults?
    8 years ago
  • Franciszek
    How to draw platelets?
    3 years ago

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