Chapter 17
Blood
645
17
Platelets
Describe the structure and function of platelets.
Platelets
are not cells in the strict sense. About one-fourth
the diameter of a lymphocyte, they are cytoplasmic fragments
of extraordinarily large cells (up to 60 μm in diameter) called
megakaryocytes
(meg
0
ah-kar
9
e-o-sītz). In blood smears, each
platelet exhibits a blue-staining outer region and an inner area
containing granules that stain purple. Te granules contain an
impressive array of chemicals that act in the clotting process, in-
cluding serotonin, Ca
2
1
, a variety of enzymes, ADP, and platelet-
derived growth factor (PDGF).
Platelets are essential for the clotting process that occurs
in plasma when blood vessels are ruptured or their lining is
injured. By sticking to the damaged site, platelets form a tem-
porary plug that helps seal the break. (We explain this process
shortly.) Because they are anucleate, platelets age quickly and
degenerate in about 10 days if they are not involved in clotting.
In the meantime, they circulate freely, kept mobile but inactive
by molecules (nitric oxide, prostacyclin) secreted by endothelial
cells lining the blood vessels.
A hormone called
thrombopoietin
regulates the formation
of platelets. Teir immediate ancestral cells, the megakaryocytes,
are progeny of the hematopoietic stem cell and the myeloid stem
cell, but their formation is quite unusual
(Figure 17.12)
. In this
line, repeated mitoses of the
megakaryoblast
(also called a stage
I megakaryocyte) occur, but cytokinesis does not. Te final result
is the mature (stage IV) megakaryocyte (literally “big nucleus
cell”), a bizarre cell with a huge, multilobed nucleus and a large
cytoplasmic mass.
A±er it forms, the megakaryocyte presses against a sinu-
soid (the specialized type of capillary in the red marrow) and
sends cytoplasmic extensions through the sinusoid wall into
the bloodstream. Tese extensions rupture, releasing the plate-
let fragments like stamps being torn from a sheet of postage
Leukemias
Te term
leukemia
, literally “white blood,” refers
to a group of cancerous conditions involving overproduction
of abnormal white blood cells. As a rule, the renegade leuko-
cytes are members of a single
clone
(descendants of a single
cell) that remain unspecialized and proliferate out of control,
impairing normal red bone marrow function. Te leukemias
are named according to the cell type primarily involved. For
example,
myeloid leukemia
involves myeloblast descendants,
whereas
lymphocytic leukemia
involves the lymphocytes.
Leukemia is
acute
(quickly advancing) if it derives from stem
cells, and
chronic
(slowly advancing) if it involves proliferation
of later cell stages.
Te more serious acute forms primarily affect children.
Chronic leukemia occurs more o±en in elderly people. Without
therapy, all leukemias are fatal, and only the time course differs.
In all leukemias, cancerous leukocytes fill the red bone mar-
row and immature WBCs flood into the bloodstream. Te other
blood cell lines are crowded out, so severe anemia and bleeding
problems result. Other symptoms include fever, weight loss, and
bone pain. Although tremendous numbers of leukocytes are
produced, they are nonfunctional and cannot defend the body
in the usual way. Te most common causes of death are internal
hemorrhage and overwhelming infections.
Irradiation and antileukemic drugs can destroy the rapidly
dividing cells and induce remissions (symptom-free periods)
lasting from months to years. Stem cell transplants are used in
selected patients when compatible donors are available.
Infectious Mononucleosis
Sometimes called the “kissing
disease,”
infectious mononucleosis
is a highly contagious viral
disease most o±en seen in young adults. Caused by the Epstein-
Barr virus, its hallmark is excessive numbers of agranulocytes,
many of which are atypical. Te affected individual complains
of being tired and achy, and has a chronic sore throat and a
low-grade fever. Tere is no cure, but with rest the condition
typically runs its course to recovery in a few weeks.
Stem cell
Developmental pathway
Hematopoietic stem
cell (hemocytoblast)
Megakaryoblast
(stage I megakaryocyte)
Megakaryocyte
(stage II/III)
Megakaryocyte
(stage IV)
Platelets
Figure 17.12
Formation of platelets.
The hematopoietic stem cell gives rise to cells
that undergo several mitotic divisions unaccompanied by cytoplasmic division to produce
megakaryocytes. The plasma membrane of the megakaryocyte fragments, liberating the
platelets. (Intermediate stages between the hematopoietic stem cell and megakaryoblast
are not illustrated.)
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