CHAPTER 8
Blood and Hemopoiesis
137
Figure 8-1.
Overview of peripheral blood cell types (mature blood cells), blood smear.
Wright stain,
3
1,569
Blood
is a specialized connective tissue composed of
blood cells
suspended in intercellular fl uid (or
plasma
). Blood cells include
eryth-
rocytes
(
red blood cells
),
platelets
(
thrombocytes
), and
leukocytes
(
white blood cells
).
Erythrocytes
are the most numerous. They are
biconcave disk–shaped cells without nuclei and are important in transportation of gases.
Platelets
are very tiny cell fragments that have
no nuclei, cannot reproduce, and come from huge cells called
megakaryocytes
(Figs. 8-9B and 8-12). Platelets play an important role in
hemostasis.
Leukocytes
can be classi± ed as either
agranulocytes
or
granulocytes
based on the absence or presence of speci± c granules in
their cytoplasm. Granulocytes are also called
polymorphonuclear leukocytes
because of the multiple lobes of their nuclei, but the term
is often used speci± cally for neutrophils. (1)
Agranulocytes
lack speci± c cytoplasmic granules but have the ability to divide.
Lympho-
cytes
and
monocytes
fall into this category.
Lymphocytes
are the smallest cells in the leukocyte series. Each has a round nucleus and a
small amount of cytoplasm. They can be found outside of the blood stream in lymphoid organs and connective tissues. Lymphocytes
can be classi± ed as
T lymphocytes
,
B lymphocytes
, and
null cells
. They are associated with immunological defense functions. B lym-
phocytes can further differentiate into
plasma cells
(see Figs. 4-2B and 4-3A,B).
Monocytes
are the largest cells in the leukocyte series.
They have large, elongated, and often kidney-shaped nuclei. They can differentiate into
phagocytes
, including
macrophages
,
Kupffer
cells
,
microglia
, and
osteoclasts
. (2)
Granulocytes
contain speci± c granules in their cytoplasm, and their nuclei are segmented. They are
terminal cells without the capability to divide further. Granulocytes include
neutrophils
,
eosinophils
, and
basophils
.
Neutrophils
are
the most abundant leukocytes in circulating blood. Each cell has a multilobed nucleus and a pale pink cytoplasm that contains primary
and secondary (speci± c) granules. Neutrophils play an important role in defense against bacterial infection.
Eosinophils
contain large,
speci± c granules that stain red with eosin dye. They usually have a bilobed, or occasionally a trilobed, nucleus. Eosinophils function
in controlling allergic reactions and in combating parasitic infections.
Basophils
are the rarest of the leukocytes (<1%). They contain
large, speci± c granules that are deep violet with a Wright stain. Each basophil has a nucleus with two to three lobes that are not com-
pletely separated. Basophils, along with mast cells, are instigators of allergic reactions (see the introduction to this chapter).
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Erythrocytes
Platelets
Neutrophils
Monocytes
Eosinophils
Lymphocytes
Basophils
Agranulocytes
Granulocytes
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