CHAPTER 8
Blood and Hemopoiesis
149
Thrombopoiesis
Figure 8-12A.
Promegakaryocytes (immature megakaryo-
cytes), bone marrow smear.
Wright stain,
3
754; inset
3
1,605
Promegakaryocytes
develop from megakaryoblasts in the bone
marrow.
Megakaryoblasts
lose their ability to undergo cytokine-
sis but undergo a series of incomplete cell cycles called “endomi-
tosis” that results in replication of DNA up to 64N without
division of nuclei or cytoplasm. Each megakaryoblast has a
large nucleus, multiple nucleoli, and basophilic cytoplasm (Fig.
8-9B).
Promegakaryocytes
can be recognized by their large size,
round (or oval) nuclei, and large amount of cytoplasm. Devel-
opment of a
demarcation membrane system
is an important
feature of thrombopoiesis and begins at a very early stage. The
demarcation membrane system is produced by the invagination
of plasma membranes to form branched interconnected chan-
nels through the cytoplasm. This system may play an important
role in later subdivision of the cytoplasm into platelet zones.
Megakaryoblast
Megakaryoblast
Megakaryoblast
A
CLINICAL CORRELATION
Figure 8-12C.
Essential
Thrombocytosis,
Bone
Marrow Smear.
Wright stain,
3
304
Essential thrombocytosis
, also called
essential throm-
bocythemia
, is one of the myeloproliferative disorders,
characterized by overproduction of platelets by mega-
karyocytes in the bone marrow without an identi± able
cause. The platelet counts exceed 600,000/
μ
L, but the
platelets do not function properly. Symptoms and signs
include headache; bleeding from gums, nose, and gas-
trointestinal tract; throbbing and burning pain of the
hands and feet caused by thrombosis of small arteri-
oles; and
splenomegaly
(enlarged spleen). Treatment
includes using low-dose aspirin to control headache
and other vasomotor symptoms and anticancer agents
such as
hydroxyurea
to maintain proper platelet count.
Bone marrow smears show increased numbers of mega-
karyocytes. Large platelets, similar in size to red blood
cells, may be found in the peripheral blood.
Megakaryocytes
Megakaryocyte
C
Nucleus of the
megakaryocyte
Incipient
platelets
Neutrophils
B
Figure 8-12B.
Megakaryocytes, bone marrow smear.
Wright
stain,
3
754; inset
3
2,827
Because of their very large size,
megakaryocytes
are sometimes
called
giant cells
. They have a large indented (partially lobulated)
nucleus and an extremely voluminous cytoplasm containing a
variety of granules. In this stage, the
demarcation membrane
system
is complete, and the megakaryocytes are ready to release
platelets. Megakaryocytes tend to move toward and attach to
the
marrow sinusoids
(capillaries in the bone marrow) after
they become mature. The platelets are released into the blood
circulation through the wall of the marrow sinusoids. Note the
neutrophils on the surface of the megakaryocyte for size com-
parison. The
inset
shows
incipient platelets
(fragments of cyto-
plasm beginning to become platelets) in the surface region of the
megakaryocyte that are ready to be released.
Thrombopoietin
,
a humoral factor produced in the liver, is believed to regulate
megakaryocytes and the production of platelets.
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