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Microbiology, 2021

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17.3 • Cellular Defenses 701<br />

Hematopoiesis<br />

All of the formed elements of blood are derived from pluripotent hematopoietic stem cells (HSCs) in the bone<br />

marrow. As the HSCs make copies of themselves in the bone marrow, individual cells receive different cues<br />

from the body that control how they develop and mature. As a result, the HSCs differentiate into different types<br />

of blood cells that, once mature, circulate in peripheral blood. This process of differentiation, called<br />

hematopoiesis, is shown in more detail in Figure 17.12.<br />

In terms of sheer numbers, the vast majority of HSCs become erythrocytes. Much smaller numbers become<br />

leukocytes and platelets. Leukocytes can be further subdivided into granulocytes, which are characterized by<br />

numerous granules visible in the cytoplasm, and agranulocytes, which lack granules. Figure 17.13 provides an<br />

overview of the various types of formed elements, including their relative numbers, primary function, and<br />

lifespans.<br />

Figure 17.12<br />

All the formed elements of the blood arise by differentiation of hematopoietic stem cells in the bone marrow.

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