Figure 915

Platelet electron micrograph and diagram, a. High-magnification electron micrograph of a platelet situated between an erythrocyte on the left and an endothelial cell on the right. Visible organelles include a mitochondrion, microtubules, a single profile of the surface-connected open canalicular system, profiles of the dense tubular system, the moderately dense a granules, a single very dense 8 granule, and glycogen particles. The microfilaments are not evident against the background matrix of the platelet, b. Diagram of a platelet showing the components of the four structural zones.

derived growth factor. The contents of these granules play an important role in the initial phase of vessel repair, blood coagulation, and platelet aggregation. The smaller, denser, and less numerous 8 granules mainly contain adenosine diphosphate, adenosine triphosphate, serotonin, and histamine, which facilitate platelet adhesion and vasoconstriction in the area of the injured vessel. The A granules are similar to lysosomes found in other cells and contain several hydrolytic enzymes. The contents of A granules function in clot resorption during the later stages of vessel repair. • Membrane zone. This zone consists of two types of membrane channels. The open canalicular system (OCS) is the first type of membrane channel. The OCS is a developmental remnant of the platelet demarcation channels and is simply membrane that did not participate in subdividing the megakaryocyte cytoplasm. In effect, they are invaginations into the cytoplasm from the plasma membrane. The dense tubular system (DTS) is the second type of membrane channel. The DTS contains an electron-dense material originating from the rough endoplasmic reticulum of the megakaryocyte, which serves as a storage site for calcium ions. DTS channels do not connect with the surface of the platelet; however, both the OCS and DTS fuse in various areas of the platelet to form membrane complexes that are important in regulation of the intraplatelet calcium concentration.

Platelets function in continuous surveillance of blood vessels, blood clot formation, and repair of injured tissue

Platelets are involved in several aspect of hemostasis. They continuously survey the endothelial lining of blood vessels for gaps and breaks. When a blood vessel wall is injured or broken, platelets adhere to the exposed connective tissue at the damaged site. The adhesion of the platelets triggers a complex process that results in aggregation of platelets into a clot called a primary hemostatic platelet plug. Extravasation of blood is then stopped by the mass of aggregated platelets. The glycocalyx of the platelets provides a reaction surface for the conversion of soluble fibrinogen into fibrin, which stabilizes the initial plug. At the same time, the activated platelets release their a and 8 granules, which contain among other substances coagulation factors and serotonin. Serotonin is a potent vasoconstrictor that causes the vascular smooth muscle cells to contract, thereby reducing local blood flow at the site of injury. In addition, tissue factors secreted by the damaged blood vessel cells aid in the formation of a definitive clot known as a secondary hemostatic plug.

After the definitive clot is formed, platelets cause clot retraction, probably as a function of the actin and myosin found in the structural zone of the platelet. Contraction of the clot permits the return of normal blood flow through the vessel. Finally, after the clot has served its function, it is lysed by plasmin, a fibrinolytic enzyme that circulates in the plasma in an inactive form known as plasminogen. The hydrolytic enzymes released from the A granules assist in this process. The activator for plasminogen conversion, tissue plasminogen activator (TPA), is derived principally from endothelial cells. It is currently used as an emergency treatment to minimize the damage caused by strokes due to clots.

An additional role of platelets is to help repair the injured tissues beyond the vessel itself. Platelet-derived growth factor released from the a granules stimulates smooth muscle cells and fibroblasts to divide and allow tissue repair.

o formation of blood cells (hemopoiesis)

Hemopoiesis (hematopoiesis) includes both etythro-poiesis and leukopoiesis, as well as thrombopoiesis (development of platelets) (Table 9.4). Blood cells have a limited life span; they are continuously produced and destroyed. Both the human erythrocyte (life span of 120 days) and the platelet (life span of 10 days) spend their entire life in the circulating blood. WBCs, however, migrate out of the circulation shortly after entering it from the bone marrow and spend most of their variable life spans (and perform all of their functions) in the tissues.

In the adult, erythrocytes, granulocytes, monocytes, and platelets are formed in the red bone marrow; lymphocytes are also formed in the red bone marrow and in the lymphatic tissues. To study the stages of blood cell formation, a sample of bone marrow is prepared as a stained smear in a manner similar to that described on page 216 for the preparation of a smear of blood.

Hemopoiesis is initiated in early embryonic development

During fetal life, both erythrocytes and leukocytes are formed in several organs before the differentiation of the bone marrow. The first or yolk sac phase of hemopoiesis begins in the third week of gestation and is characterized by the formation of "blood islands" in the wall of the yolk sac of the embryo. In the second or hepatic phase, early in fetal development, hemopoietic centers appear in the liver (Fig. 9.16). Blood cell formation in these sites is largely limited to erythroid cells, although some leukopoiesis occurs in the liver. The liver is the major blood-forming organ in the fetus during the second trimester. The third or bone marrow phase of fetal hemopoiesis and leukopoiesis involves the bone marrow (and other lymphatic tissues) and begins in the second trimester of pregnancy. After birth, hemopoiesis takes place only in the red bone marrow and lymphatic tissues, as in the adult (Fig. 9.17). The precursors of both the blood cells and germ cells arise in the yolk sac.

232 CHAPTER 9 I Blood TABLE 9.4. Hemopoiesis*

(CFU-Meg)

''This table includes the most recent concepts of a pluripotential stem cell, multipotential colony-forming units (CFUs), and restricted CFUs. Cytokines (including hemopoietic growth factors) may and do act individually and severally at any point In the process from the first stem cell to the mature blood or connective tissue cell.

"Mature functional cells In blood, bone marrow, or connective tissue.

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