Page 203 - Atlas of Histology with Functional Correlations

in their cytoplasm, whereas neutrophils contain less obvious and smaller

granules when viewed with the light microscope. Agranulocytes are the
monocytes and lymphocytes. Leukocytes perform their major functions outside
the blood vessels. They migrate out of the blood vessels through capillary walls
and enter the connective tissue, lymphatic tissue, and bone marrow.

The primary function of leukocytes is to defend the body against bacterial

invasion or the presence of foreign material. Consequently, most leukocytes are
concentrated in the connective tissue of different organs.

PLATELETS

Platelets (thrombocytes) are the smallest, nonnucleated membrane-bound
cytoplasmic fragments or remnants of megakaryocytes, which are the largest
cells in the red bone marrow. The platelets are also called thrombocytes, which

is a misnomer because platelets are not whole cells. Platelets are formed when
small, uneven portions of the cytoplasm separate or fragment from the
peripheries of the megakaryocytes and enter the bloodstream. Like the

erythrocytes, platelets perform their major functions within the blood vessels.
Their main function is to continually monitor the vascular system and detect any
damage to the endothelial lining of the vessels. If the endothelial lining breaks,
the platelets adhere to the damaged site and initiate a highly complex chemical
process that produces a blood clot.

Supplemental micrographic images are available at
www.thePoint.com/Eroschenko13e under Blood Cells.

FUNCTIONAL CORRELATIONS 6.1 ■ Platelets

The platelets serve an important function. They repair minor tears in the
walls of the blood vessels and promote blood clotting to prevent blood loss.
Normally, intact endothelial cells of the blood vessels do not cause platelet
aggregation to form the blood clot because laminin of the basement

membrane and collagen fibers are not exposed. Instead, the endothelial cells
produce prostacyclin, a chemical that inhibits platelet aggregation. Damage
to the endothelial wall causes platelet aggregation and their adherence to

exposed collagen and basement membrane proteins at the site of damage.
This action activates the platelets to form a platelet plug to occlude the

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