CHAPTER 4
Connective Tissue
63
Figure 4-4A.
Mast cells.
EM,
3
42,000;
inset toluidine blue
3
3,324)
The contents of the
granules
that F
ll the
cytoplasm of a
mast cell
are electron
dense.
Mitochondria
are the only other
prominent constituent of the cytoplasm.
These granules are not the only source of
signaling molecules released by activated
mast cells. The
plasma membrane
and
outer nuclear membrane
are labeled here
to highlight their roles in the generation of
eicosanoids
, such as
prostaglandins
and
leukotrienes
. These potent mediators of
infl
ammation are not stored but are syn-
thesized from fatty acids of membranes
when the mast cell is stimulated.
The
inset
shows a mast cell in par-
afF
n section stained with toluidine blue.
The purple color of the mast cell granules
is an example of metachromatic stains.
Mitochondrion
Mitochondrion
Granules
Granules
Mitochondrion
Granules
Plasma
Plasma
membrane
membrane
Plasma
membrane
Outer
Outer
nuclear
nuclear
membrane
membrane
Outer
nuclear
membrane
A
D. Cui
IgE binds to Fc receptor
IgE binds to antigen
Mast cell degranulates
Release of histamine
IgE
Fc receptor
IgE
Fc receptor
B
Figure 4-4B.
A representation of a mast cell in an allergic reaction (anaphylaxis)
.
Mast cells
derive from bone marrow and migrate into connective tissue where they function as mediators of infl
ammatory reactions
to injury and microbial invasion. The cytoplasm of mast cells contains many granules, which contain
heparin
and
histamine
and
other substances. In most cases, when the body encounters a foreign material (
antigen)
, the result is clonal selection and expansion
of those
lymphocytes
that happen to synthesize an
antibody
that recognizes the antigen. Some of the stimulated lymphocytes will dif-
ferentiate into
plasma cells
that secrete large amounts of soluble antibody, which enter circulation. Those antibodies that are of the
IgE
class bind to
Fc receptors
on
mast cells
and
basophils
. The
IgE-Fc
receptor complexes
can act as triggers that activate the mast
cell or basophil if the antigen is encountered again. Binding of the antigen leads to cross-linking of the ±c receptors, which initiates
a series of reactions culminating in discharge (
exocytosis
) of the contents of the granules of the mast cell or basophil. The histamine
and heparin that are released from the granules contribute to infl
ammation at the allergic reaction site.
Histamine
stimulates many types of cells to produce a variety of responses, depending on where the
allergic
reaction
takes place.
Effects on blood
vessels include dilation due to relaxation of smooth muscle cells (redness and heat) and fl
uid
leakage from venules
(
edema
) due to loosening of cell-to-cell junctions between endothelial cells. Histamine can stimulate some smooth
muscle
cells to
contract, as occurs with
asthma
in the respiratory tract, and it can cause excessive secretion in glands. Extremely strong
mast cell–
mediated allergic reactions
(also called
allergic
or
type 1 hypersensitivity reactions
) result in
anaphylactic shock
, which can happen
very quickly and often requires emergency attention. It can sometimes be fatal.
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