Transcript Slide 1

HISTOLOGY &
EMBRYOLOGY
Teaching PPT
Dept. of Anat., Hist. & Embry.
School of Medicine
Xi’an Jiaotong University
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Dr. Jinsong Zhou
Immune System
I. Lymphocytes: Divided into 3 types
based on the surface antigens.
A. Thymus derived lymphocytes (T
cells): Formed in thymus. Participate
in cell-mediate immune responses.
Divided into Tc, Th, Ts and memory T
cells.
B. Bone derived lymphocytes (B
cells): Formed in bone marrow?.
Participate in humoral immune
responses.
C. NK cells.
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II. Mononuclear phagocyte
system (MPS): Composed
of the cells that are derived
from monocytes in blood
and as meanwhile have
high capability to
phagocytose. MPS includes
macrophages, osteoclasts,
microglia, Kupffer cells,
dust cells and Langerhans
cells, etc.
III. Lymphoid tissue:
Reticular tissue containing
rich macrophages,
lymphocytes and plasma
cells, etc.
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A. Divided into 2 types based on function:
1. Central lymphoid tissue: Irrelevant to
immune response.
a. develops earlier and perishes earlier.
Makes up the central lymph organ, the
thymus and bone marrow?.
b. the site where the B lymphocytes (in
bone marrow?) and T lymphocytes (in
thymus) proliferate and differentiate into
different types.
2. Peripheral lymphoid tissue:
a. develops later and retains forever.
b. the site where immune cells perform
their functions. Makes up of peripheral
lymph organs, or distributes as tissue in
other organs, e.g. the mucosa of
digestive and respiratory tract, etc.
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B. Divided into 3 types based
on the composition and
appearance:
1. Diffuse lymphoid tissue:
Mainly contains T cells and
has obscure boundary with
surrounding CT. Has post
venule capillaries, which are
lined by high endothelium and
are the important channels for
lymphocyte recirculation. It is
the place where the lymphoid
nodule is formed when
stimulated by antigens.
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2. Lymphoid nodule: Ball-like
structure with distinct boundary,
mainly contains B cells.
a. primary lymphoid nodule:
smaller.
b. secondary lymphoid nodule:
the primary one enlarges when Th
cells and antigens exist. It has a
cap and less densely stained
region known as germinal center,
where the particular lymphocytes
proliferate and differentiate into
particular plasma cells to respond
to the particular antigens. It
disappears when the antigens
disappear.
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3. Lymph cords: Cord-like lymphoid tissue. Mainly contains B
cells and surrounded by rich capillaries.
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IV. Lymph organs:
A. Thymus: A central lymphoid organ.
1. Capsule: Composed of CT, which protrudes into parenchyma to
form the supporting trabeculae that divide the parenchyma into
incomplete lobules.
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The incomplete lobule is composed of cortex and continuous
medulla. The cortex is densely stained because the cells in it
are closely arranged.
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2. Cortex: divided into
outer and inner cortex.
a. thymocytes: the T cells
in different developing
stages. The cells in outer
cortex are larger, and in
inner cortex are smaller
but much more.
b. epithelial reticular cells:
provide micro support.
Divided into 2 types:
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The cells that support the thymocyte differentiation are called
thymus stromal cells, which include epithelial reticular cells,
macrophages, eosinophils, mast cells, fibroblasts, etc. Thymus
begins to perish in puberty. The thymus in adult contains a lot
of fat cells instead of thymocytes.
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3. Medulla: Contains
mature T cells,
macrophages and 2 types
of epithelia:
a. medullary epithelial
cells: larger,
desmosomes can be
found among them.
Secrete thymosin.
b. thymic corpuscle
epithelial cells: exist in
Hassall’s corpuscles.
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Hassall’s corpuscles: The acidophilic characteristic structures of
thymus, which are composed of concentrically arranged, flattened
thymic corpuscle epithelial cells that become filled with keratin
filaments, degenerate and sometimes calcify. Function unknown.
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4. Blood thymus barrier:
Composed of
endothelial cells and
their basal lamina,
space surrounding
capillary in which exist
macrophages,
endothelial reticular
cells and their basal
lamina. Prevents
circulating antigens
from reaching the
thymus where T
lymphocytes are being
formed.
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B. Lymph node: A
peripheral lymphoid
organ. Locates in the
pathway of lymph
circulation.
1. Capsule: Composed of
CT. Contains one efferent
lymph vessel that leaves
at hilum and several
afferent lymph vessels.
Trabeculae, which have
the function of support,
are the protrusions of
capsule into the
parenchyma.
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2. Cortex: Beneath
the capsule.
a. superfacial cortex
(outer cortex): locates
at periphery, usually
contains a lot of
lymphoid nodules.
b. paracortex zone
(inner cortex):
Composed of diffuse
lymphoid tissue. Also
called deep cortex
unit. It is the thymus
dependent area in
lymph node.
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Deep cortex units have semi-spherical appearance. The plane
surface is opposite to an afferent lymphatic vessel, and the other
surface is continuous to medulla. The inner part of the unit is
occupied by T cells and its periphery contains many post
capillary venules and small blind sinus, which are the beginnings
of medullary sinus.
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The post capillary
venules are lined by
simple cuboidal/columnar
epithelium, from which
the lymphocytes in blood
enter the lymphoid tissue.
They, together with small
blind sinus, are the
important channels for
lymphocyte re-circulation.
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3. Cortical sinus:
a. subcapsular sinus
exists between the
outer cortex and
capsule, and receive
lymph from the
afferent lymph
vessels.
b. intermediate sinus:
usually have blind
ends and are parallel
to trabeculae. The
ones between deep
cortex units are called
thin channels, which
communicate the
medullary sinus. 19
In the sinus, there
are satellite
epithelial cells that
provide support,
lymphocytes,
veiled cells and
more macrophages.
The slowly running
lymph gives more
chances for
immune cells to
detect antigens.
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3. Medulla: Located in the center
and connects hilum directly.
a. medullary cords: cord-like
lymphoid tissue in which B
lymphocytes predominate. They
are the branched extensions of
inner cortex.
b. medullary sinus: connect the
thin channels and contain more
macrophages.
Recirculation of lymphocyte:
Lymphocytes in inner cortex →
small blind sinus → efferent vessel
→ right lymphatic duct and
thoracic duct → blood vessels →
post Cap venules → inner cortex.
One circulation circle needs 1~221
days.
Results in a
constant
monitoring of all
parts of the body by
recirculating
lymphocytes, which
can inform the
immune system of
the presence of
foreign antigens
ASAP.
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5. The pathway of
lymph fluid: Afferent
vessels →
subcapsular sinus
→thin channel
→medullary
sinus→hilum
→efferent vessel.
6. Function:
Participates in filtration
of lymph, immune
response and
lymphocyte recirculation.
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C. Spleen: A peripheral lymphoid organ. Situates in the blood
circulation pathway.
1. Capsule: Thick serous membrane containing DCT and smooth
muscles. Penetrates into parenchyma to form trabeculae, which
have support function and contain trabecular arteries/veins.
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2. White pulp: Isolated and randomly distributed.
a. periarterial lymphatic sheath (PALS): diffuse lymphoid tissue
surrounding the central artery, which is companied by a lymphatic
vessel.
b. splenic corpuscles: lymphoid nodules that append to the PALS.
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3. Red pulp: Among white
pulp.
a. splenic cords: lymphoid
tissue cords containing
many blood cells.
b. splenic sinus: sinusoidal
capillaries. Occupy the
spaces among the splenic
cords. Lined by rod-shaped
endothelia with their long
axis parallel to the long axis
of sinus and surrounded by
circular reticular fibers.
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4. Marginal region:
Locates at the
boundary of white and
red pulp, and is the
unique way for
lymphocytes in blood
and in white pulp to
exchange. Plays a
major role in splenic
immune responses.
Between the marginal
region and white pulp,
there are many dilated
ends of branches of
central atrery, the
marginal sinus.
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5. Blood circulation pathway: Spleen artery → trabecular arteries
→ central arteries (white pulp) → splenic cords (& sinusoid) →
sinusoid → trabecular veins → spleen vein.
6. Function of spleen: filtration of blood. Immune response.
Formation of blood when as fetus.
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