CHAPTER 10
Lymphoid System
181
and f ltering lymph, deFending against microbial invasion, and
providing a place For lymphocytes to meet antigens. Each lymph
node has several
afferent lymphatic vessels
and an
efferent lym-
phatic
vessel
. Lymph enters a lymph node through
afferent
vessels
and fl ows into subcapsular sinuses and peritrabecular
sinuses and then into the medullary sinuses and exits the lymph
node through an
efferent lymphatic vessel
(±ig. 10-10). In gen-
eral, a lymph node can be divided into three regions:
cortex
,
paracortex
, and
medulla
. (1) The
cortex
contains a row oF
lym-
phatic
nodules
; most oF these nodules are secondary nodules.
(2) The
paracortex
is located between the cortex and the
medulla. Most T cells are hosted here.
High endothelial venules
(
HEVs
) are Found in this region. HEVs are postcapillary venules,
which have a cuboidal cell lining instead oF the common, fl
at
endothelial cell lining (±ig. 10-12A,B). They are specialized
venules, which allow lymphocytes to pass through their walls
to enter the lymphatic tissue. HEVs can also be Found in other
lymphatic organs such as tonsils. (3) The
medulla
is composed
oF
medullary cords
and
medullary sinuses
(±ig. 10-11B,D).
Medullary cords
are like small islands that are surrounded by
lymphatic channels (medullary sinuses). Medullary cords con-
tain lymphocytes, plasma cells, macrophages, and dendritic
cells.
Medullary sinuses
are lymphatic sinuses. Bacteria and
antigens are trapped and engulFed by antigen-presenting cells
(macrophages and dendritic cells) in the medulla.
Thymus
The
thymus
is the primary lymphoid organ For maturation oF
T cells (±ig. 10-13A–C). It is located in the superior mediastinum.
The thymus continues to grow until puberty and then gradually
atrophies. In elderly individuals, a large portion oF the thymus
tissue is replaced by adipose tissue. The thymus is covered by a
thin layer oF connective tissue (
capsule
) and has two lobes. Each
lobe is composed oF many lobules, and the lobules can be divided
into a
cortex
and
medulla
. Unlike other lymphatic organs, the
thymus does not have lymphatic nodules. Its stroma is com-
posed oF a Framework oF epithelial reticular cells derived From
the
endoderm. (1) The
cortex
contains
thymocytes
(developing
T cells), macrophages, dendritic cells, and
epithelial
reticular
cells
. T-cell maturation occurs in the cortex. (2) The
medulla
contains virgin T cells, which have developed and migrated From
thymocytes in the cortex. The medulla also contains a large
number oF epithelial reticular cells.
Hassall corpuscles
(
thymic
corpuscles
), which are Formed by concentrically arranged
type
VI epithelial reticular cells
, are Found in the medulla. There are
several types oF epithelial reticular cells in the thymus:
Types I
to
type III epithelial reticular cells
are located in the cortex,
type IV
in the junction oF the cortex and medulla, and
types V
and
VI
epithelial reticular cells
in the medulla (±ig. 10-13B,C).
Spleen
The
spleen
is a large and highly vascularized lymphoid organ,
located in the superior leFt quadrant oF the abdomen. It is cov-
ered by a thick layer oF dense connective tissue (
capsule
). The
spleen does not have a cortex and medulla; it is organized into
two regions:
white pulp
and
red pulp
(±ig. 10-14A–C). (1) The
white pulp
is an immune component in the spleen, composed
oF
nodules
,
central arteries
, and a
periarterial lymphatic sheath
(
PALS
).
Lymphatic
nodules
are oFten secondary nodules, which
have germinal centers and are oFten called
splenic nodules
.
Central arteries
pass through the white pulp and give rise to
sinuses in the marginal zone (peripheral region oF the nodule).
The central artery also gives rise to the penicillar arterioles in the
red pulp (±ig. 10-15A). The
PALS
is a sheath oF concentrated
T cells surrounding a central artery. (2) The
red pulp
is the main
region; it
f lters antigens and particulate materials, engulFs aged
erythrocytes, and serves as reservoir For erythrocytes and plate-
lets. It is composed oF
splenic sinuses
and
splenic cords
(
Billroth
cords
).
Splenic sinuses
are venous sinuses (discontinuous capil-
laries) that have large lumens and large gaps between endothe-
lial cells, which permit large proteins and cells to pass through
the walls oF sinuses.
Splenic cords
are Formed by a Framework
oF reticular tissue, which contains lymphocytes, plasma cells,
macrophages, and other blood cells.
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