Transcript Immune System and Lymphatics Anatomy-Histology Correlate By: Michael Lu, Class of ‘07

Immune System and Lymphatics
Anatomy-Histology Correlate
By: Michael Lu, Class of ‘07
- The immune system, which is designed to protect us from foreign particles and organisms, is not localized to any
part of the body. In fact, it is dispersed in bone marrow, lymph nodes, lymphatic duct and vessels, the gut, tonsils,
thymus gland, and the spleen. Here we will focus on 4 of them – the lymph node, spleen, thymus, and tonsils.
- Lymph nodes vary in size, shape, and location. The typical description is that they are kidney-bean shaped
organs enclosed by a connective tissue capsule that forms trabecula within the lymph node. It is further
subdivided into cortex and medulla. Note the germinal centers, sinuses, and postcapillary high endothelial
venules, all which will be discussed later.
- Lymph nodes receive both blood and lymph via different routes. The circulation through a lymph node is detailed
in the bottom right.
- Circulating lymphocytes play an important role in the immune defense. They can enter a lymph node via two
routes – 1) 85 – 90% of lymphocytes, mostly undifferentiated, enter the lymph node from the blood via
postcapillary high endothelial venules; and 2) 10 – 15% of lymphocytes, mostly memory cells, enter via the
afferent lymphatic vessels.
- A histological slide of a lymph node is shown on the
top left. Note the capsule and trabecula. The vertical
line marks the border between the cortex and medulla.
Note at the hilum of the lymph node the efferent
lymphatics and the blood vessels.
- Lymphatic vessels may be difficult to identify. They are
generally very thin-walled and dilated and contain
lymphocytes instead of red blood cells. Shown in the
bottom left is an afferent lymphatic vessel entering
the lymph node through the capsule.
- The afferent lymphatics empty their contents into the
subcapsular sinus (bottom right) lined by endothelial
cells and crossed by reticular cells. The sinus is
readily permeable to both lymph and cells –
lymphocytes, plasma cells, and macrophages.
- In the cortex of the lymph node, there are multiple
follicles. In the middle of a follicle may be a paler
staining germinal center (magnified in bottom left) that
indicates an antibody response to an antigen is
occurring. The left nodule is a primary follicle, while the
one on the right is a secondary follicle.
- The follicles or nodules are B-cell rich. When an
antigen is presented to the B-cells by dendritic cells,
they undergo clonal expansion and differentiate into
antibody secreting plasma cells and memory cells.
- Outside the germinal center (bottom right) is the
diffuse or deep cortical zone which is T-cell rich.
IMPORTANT: The follicles (black bracket) contain
germinal centers (white bracket), which are B-cell rich.
The remainder of the cortex is T-cell rich.
-Within the lymph node cortex, indicated by the blue
arrows here (and the white arrows in the bottom right
panel in the previous slide), are postcapillary high
endothelial venules. They are composed of high
cuboidal cells, which may be difficult to individually
identify because of the extravasation of lymphocytes.
This is one way that lymphocytes in the body enter
lymph nodes, exiting the postcapillary high endothelial
venules and entering the deep cortex.
- The lymph node medulla in shown in the bottom right.
It is filled with medullary sinuses separated by
medullary cords. The medulla then empties into the
efferent lymphatic vessels which exit at the hilum. There
are variable amounts of macrophages, lymphocytes,
and plasma cells.
- Next we will look at the spleen. It is located in the left
upper quadrant of the abdomen, tucked away behind
the stomach and tail of the pancreas. It is connected to
the stomach by the gastrosplenic ligament and to the
left kidney by the splenorenal ligament.
- The spleen is supplied by the splenic artery, which is
a main branch off the celiac trunk from the abdominal
aorta.
- Functionally, the spleen can be divided into red pulp
and white pulp. Note the functions stated below on the
bottom right.
- The red and white pulp are shown here histologically.
The white pulp is lymphocyte rich, and therefore is
more basophilic and stains more darkly (black arrows in
bottom left). The red pulp contains a series of cords
and sinuses reflecting the large number of red blood
cells.
- Similar to a lymph node, the spleen also contains a
connective tissue capsule that extends into the
parenchyma and forms trabecula.
- Within the splenic tissue, we can also find larger
vessels known as trabecular arteries and veins. Note
the circulation of blood within the spleen detailed in the
bottom right.
- Arterial branches arising from the trabecular arteries are
surrounded by mostly T lymphocytes. This region of white pulp,
also the site of T-cell help, is known as the periarterial lymphatic
sheath (PALS), as shown by the bracket in the top panel below.
The artery in the middle of PALS is called the central artery (CA).
- The spleen also contains lymphatic nodules that are rich in B
lymphocytes. They are randomly distributed along the side of the
PALS (bottom panel) and may also have germinal centers (GC).
Surrounding the follicle and separating it from the red pulp (RP) is
the marginal zone (MZ), which will be discussed next.
- The marginal zone (bracket in top left) contains many
reticular fibers, reticular cells, and macrophages
(major antigen-presenting cells). Functionally, the
marginal zone is very important because blood is
emptied into this space by arterioles and capillaries,
known as open splenic circulation.
- Outside the marginal zone are the typical splenic
cords and sinuses within the red pulp. Blood is emptied
directly into the splenic cords via open circulation
(unique to the spleen), flows across the walls of the
splenic sinuses, and enters the venous drainage
system of the spleen. Note the walls of the splenic
sinuses formed by the rod cells.
- The thymus gland can be found in the superior
mediastinum of the thoracic chest cavity. It is located
superior to the heart and anterior to the roots of the
great vessels.
- Shown on the bottom left is a low power view of the
thymus gland. Note the abundance of thymic lobules
(L) separated by septa (Se). The dark and light staining
areas are the thymic cortex (C) and medulla (M),
respectively. The light staining of the medulla reflects
the fact that it contains less lymphocytes. Note also
within the medulla the presence of Hassell’s
corpuscles (HC). They are aberrantly keratinized
epithelial cells that serve no functional purpose, but
help in the identification of thymic tissue.
- As shown in the top left, antigen-independent
differentiation of T-lymphocytes takes place in the
thymic cortex. Note the lack of germinal centers. The
most newly formed cells are near the capsule, while the
immunocompetent T cells exit the thymus by entering
venules in the medulla. A Hassell corpuscle is also
labeled.
- Within the cortex, the capillaries exhibit a bloodthymus barrier. Functionally, this barrier is very
important to ensure the proper differentiation of T cells
and avoid reacting against any possible self antigens.
The epithelial reticular cells that make up the barrier
play an important role in the positive selection of Tcells.
- The thymic tissue is slowly replaced by fat with increasing age via a process called thymic involution.
Immunocompetent T cells exit the thymus during childhood and enter other lymphatic organs in the body. At older
ages, the thymus is no longer necessary for T-cell related immune functions.
- Finally, we will look at the palatine tonsils. Note the
location of the tonsils at the posterior of the oral cavity.
- The palatine tonsil is a dense accumulation of
lymphatic tissue that lies within the connective tissue
underneath the oral epithelium. It is poorly
encapsulated and sits adjacent to mucous glands,
skeletal muscle, and large vessels and nerves.
- Stratified squamous epithelium lines the crypts of the
oral cavity.
- Note the germinal centers of the tonsils. There are
both diffuse and nodular lymphatic tissue.
- The lightly staining spots within the germinal center
are macrophages, which are the major antigenpresenting cells of the palatine tonsil. They give the
tonsil its distinct “starry sky” appearance, which helps in
the identification of the tissue.
- The macrophages in the bottom right panel are
indicated by the big black arrows. Note the large
vesicular nucleus with a conspicuous nucleolus.