194
UNIT 3
Organ Systems
Figure 10-13A.
Thymus.
H&E,
3
46
The
thymus
is a primary lymphoid organ for T cells where T-cell
maturation takes place. The thymus is large in children and gradu-
ally atrophies to be replaced by fat after puberty. The thymus is
located in the superior mediastinum and is divided into smaller
units called
lobules
by connective tissue
septae
, which extend
inward from the surface of the organ. The thymus does not have
lymphatic nodules; it is organized into
cortex
(peripheral) and
medulla
(center). There are no afferent lymphatic vessels; its effer-
ent lymphatic vessels arise from the corticomedullary junction and
medulla and leave the thymus in company with the blood vessels.
Thymocytes
(developing T cells) are concentrated in the cortex
region, and as they undergo differentiation, they move down to
the medulla. The blood vessels pass through the interlobar septa
and enter the thymus at the junction of the cortex and medulla.
Thymic capillaries are continuous capillaries with thick basement
membranes. They are surrounded by epithelial reticular cells and
form an effective thymic-blood barrier, which prevents foreign
antigens from entering the thymus.
Septum
Septum
Medulla
Medulla
Cortex
Cortex
Medulla
Septum
Cortex
Medulla
Medulla
C
o
rte
r
t
e
x
Medulla
Cortex
M
e
d
u
lla
l
l
a
Cortex
Cortex
C
o
rte
r
t
e
x
Medulla
Cortex
Cortex
Septum
Septum
Septum
A
B
Epithelial
Epithelial
reticular
reticular
cells
cells
Epithelial reticular cells
Epithelial reticular cells
Epithelial reticular cells
Epithelial reticular cells
Macrophage
Macrophage
Macrophage
Macrophage
Epithelial reticular cells
Epithelial
reticular
cells
Epithelial reticular cells
Macrophage
Macrophage
Figure 10-13B.
Thymus, cortex.
H&E,
3
278; insets
3
510
The
cortex region
contains
thymocytes
,
macrophages
,
dendritic
cells
, and
epithelial reticular cells
. The
macrophages
and
dendritic
cells
are antigen-presenting cells; they present self-antigens to
thymocytes
. Only 1% to 2% of thymocytes survive and continue to
develop.
Epithelial reticular cells
are derived from endoderm (lym-
phocytes are derived from mesoderm). They are interconnected with
each other to form a framework to hold T lymphocytes together.
They have large, ovoid nuclei and long processes and make contact
with each other by desmosomes. They contain secretory granules
and produce thymosin, serum thymic factor, and thymopoietin hor-
mone. These hormones play an important role in T-cell maturation.
The epithelial reticular cells can be classiF ed into six types based on
their functions and locations. Types I to III are located in the cortex
region, and type IV in the corticomedullary junction. Types V and
VI are located in the medulla of the thymus.
C
Epithelial reticular cells
Epithelial reticular cells
Epithelial reticular cells
Epithelial reticular cells
Epithelial reticular cells
Epithelial reticular cells
Hassell
Hassell
corpuscle
corpuscle
Hassall
corpuscle
Figure 10-13C.
Thymus, medulla.
H&E,
3
624; insets
3
843
The
medulla region
contains
naive
(
virgin
)
T cells
,
macrophages
,
and
types
V
and
VI epithelial reticular cells
. The
naive T cells
are immunocompetent cells. They mature from thymocytes in the
cortex and migrate from the medulla to secondary organs where
they become effective or memory T cells if they meet with
speciF
c
foreign antigen. The medulla of the thymus is also the place where
T cells are selectively removed by
macrophages
. Both
types V
and
VI epithelial reticular cells
are located in the medulla. The type
VI epithelial reticular cells
show various degrees of keratinization
and are arranged into concentric layers forming a spherical struc-
ture called a
Hassall corpuscle
. Although the function of Hassall
corpuscles is not fully understood, their numbers are increased
in older individuals. Hassall corpuscles can be used as one of the
unique features to distinguish the thymus from other lymphatic
organs during the histological slide examination.
Thymus
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