Characteristics of Leukocytes: Although leukocytes and erythrocytes both originate from hematopoietic stem cells in the bone marrow, they are very different from each other in many significant ways. For instance, leukocytes are far less numerous than erythrocytes: Typically there are only 5000 to 10,000 per µL. Bloodand its components
They
are also larger than erythrocytes and are the only formed elements
that are complete cells, possessing a nucleus and organelles. And
although there is just one type of erythrocyte, there are many types
of leukocytes. Most of these types have a much shorter lifespan than
that of erythrocytes, some as short as a few hours or even a few
minutes in the case of acute infection. One of the most distinctive
characteristics of leukocytes is their movement. Whereas erythrocytes
spend their days circulating within the blood vessels, leukocytes
routinely leave the bloodstream to perform their defensive functions
in the body’s tissues. For leukocytes, the vascular network is
simply a highway they travel and soon exit to reach their true
destination. When they arrive, they are often given distinct names,
such as macrophage or microglia, depending on their function. They
leave the capillaries, the smallest blood vessels or other small
vessels through a process known as emigration or diapedesis in which
they squeeze through adjacent cells in a blood vessel wall. Once
they have exited the capillaries, some leukocytes will take up fixed
positions in lymphatic tissue, bone marrow, the spleen, the thymus,
or other organs. Others will move about through the tissue spaces
very much like amoebas, continuously extending their plasma
membranes, sometimes wandering freely, and sometimes moving toward
the direction in which they are drawn by chemical signals.
The plasma membrane
This
attracting of leukocytes occurs because of positive chemotaxis, a
phenomenon in which injured or infected cells and nearby leukocytes
emit the equivalent of a chemical “911” call, attracting more
leukocytes to the site. In clinical medicine, the differential counts
of the types and percentages of leukocytes present are often key
indicators in making a diagnosis and selecting a treatment.
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