CHAPTER 10
Lymphoid System
195
Figure 10-14A.
Spleen.
H&E,
3
60
The
spleen
is a large lymphoid organ (about 140–180 g in
humans) located in the left superior quadrant of the abdomen
(Fig. 10-6). It is covered by a thick,
dense connective tissue
(
capsule
), which extends into the organ to form trabeculae.
Trabeculae
provide structural support for arteries and veins,
which supply the compartments (white and red pulp) of the
spleen. The spleen is not organized into a cortex and medulla as
are lymph nodes and the thymus but is divided into
white pulp
associated with a central artery and
red pulp
associated with
a vein and venous sinusoids. Functions of the spleen include
(1) an immune component (white pulp) to activate lymphocytes
and promote antibody production by plasma cells, (2) ± ltration
of blood and destruction of aged erythrocytes in red pulp, and
(3) serving as reservoir for erythrocytes and platelets.
Capsule
Capsule
Capsule
White pulp
White pulp
Red pulp
Red pulp
White pulp
Red pulp
Red pulp
Red pulp
Red pulp
Trabeculae
Trabeculae
Red pulp
Red pulp
Trabeculae
Red pulp
A
Germinal
Germinal
center
center
Germinal
center
Marginal zone
Marginal zone
Marginal zone
G
Mantle
Mantle
zone
zone
Mantle
zone
Central
Central
artery
artery
Central
artery
Primary
Primary
follicle
follicle
Primary
follicle
PALS
PALS
PALS
PALS
PALS
PALS
B
Figure 10-14B.
White pulp, spleen.
H&E,
3
194; inset
3
748
White pulp
and
red pulp
are the two basic components of the
spleen.
White pulp
is composed of a
central artery
, a
PALS
,
and a
lymphatic nodule
. The nodules with
germinal centers
are
secondary nodules (follicles) where B cells actively differentiate
into large cells (
lymphoblasts
and
lymphocytes
). The dark ring
region around the germinal center is the
mantle zone
where
small inactive B cells are hosted. The mantle zone stains dark
because of densely packed lymphocytes. The nodules without
the germinal centers are primary nodules, which contain most
of the inactive B cells. The region that surrounds the white
pulp is the
marginal zone
, which contains marginal sinuses.
(G, germinal center.)
Splenic
Splenic
cord
cord
Lymphatic
Lymphatic
nodule
nodule
Splenic
cord
Lymphatic
nodule
Venous
Venous
sinuses
sinuses
Venous
sinuses
Splenic
Splenic
cord
cord
Splenic
cord
Venous sinuses
Venous sinuses
Venous sinuses
C
Figure 10-14C.
Red pulp, spleen.
H&E,
3
256; inset
3
385
Red pulp
(red because it is rich in blood) stains light and con-
tains
splenic cords
and
venous sinuses
that are ±
lled with blood.
Venous sinuses are discontinuous capillaries, which have large
lumens, incomplete basal laminae, and gaps between endothe-
lial cells. These special features allow blood cells to pass
through the capillary wall (see Fig. 9-14A,B). The
splenic cord
is a framework of reticular tissue that contains B cells, T cells,
plasma cells, macrophages, and other blood cells.
Macrophages
in the splenic cord often extend their processes into the lumen
of the sinuses to reach and engulf foreign substances, microbes,
and aged erythrocytes. The red pulp of the spleen also serves as
a reservoir for platelets (Fig 10-16).
Spleen
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