HISTOLOGICAL STRUCTURE AND FUNCTIONS OF
LYMPH NODE, TONSIL, THYMUS AND SPLEEN
Dr. N. MADHUSUDHAN, DEAN
BGS SCIENCE ACADEMY & RESEARCH CENTRE
Jnanaganothri Campus, Chikkaballapura -562101
www:bgsscienceacademy.ac.in
LYMPH
• Lymph is an alkaline fluid that originates as
interstitial fluid in the body and circulates
throughout the lymphatic system.
• The lymph is transported through
larger lymphatic vessels to lymph nodes,
where it is cleaned by lymphocytes.
• Lymph composition continually changes as
the blood and the surrounding cells
continually exchange substances with the
interstitial fluid. It is generally similar
to blood plasma, which is the fluid
component of blood.
FUNCTIONS OF THE LYMPHATIC SYSTEM
Reabsorbs excess interstitial fluid:
• Returns it to the venous circulation
• Maintain blood volume levels
• Prevent interstitial fluid levels from rising out of control.
Transport dietary lipids:
• Transported through lacteals
• Drain into larger lymphatic vessels
• Eventually into the bloodstream.
Immune response
• Lymph may pick up bacteria and bring them to lymph nodes, where they are destroyed.
• Metastatic cancer cells can also be transported via lymph.
Lymphocyte development
• Lymphocytes: are white cells which circulate between blood and lymph. Lymphocytes initially develop in
the bone marrow. Some migrate to the thymus, where they mature into T cells ; others mature in the bone
marrow as B cells. They play an important role in fighting infection.
Monitor body surfaces and fluid compartments
• e.g. epidermis, mucosae*, interstitium
COMPONENTS OF THE
LYMPHATIC SYSTEM
• Lymph
• Lymphatic Vessels
• Lymphatic Capillaries
• Lymphatic Vessels
• Lymphatic Trunks
• Lymphatic Ducts
• Lymphatic Organs
A. Tonsils
B. Thymus
C. Spleen
D. Red Bone marrow
E. Lymph Nodes
• Lymphatic cells
A-Tonsils
E-Lymph nodes
(7 locations):
1.Cervical
B-Thymus
C-Spleen
2.Axillary
3.Thoracic
4.Abdominal
D-Red
bone
marrow
5.Intestinal &
mesenteric
6.Inguinal
7.Popliteal
“Swollen
glands”
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Primary Lymphoid Organs: Primary lymphatic organs are where
lymphocytes are formed and mature. They provide an environment
for stem cells to divide and mature into B- and T- cells.
The bone marrow, the thymus and the Gut-Associated Lymphoid
Tissue (e.g. appendix, terminal ileum) are the initial “education
centers” of the immune system.
In these organs, lymphocytes differentiate into immunocompetent
cells.
This differentiation is said to be antigen-independent.
The lymphocytes then enter the blood and lymph to populate.
 epidermis and mucosae
 connective tissue
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Secondary Lymphoid Organs: Secondary lymphoid tissues are arranged as a
series of filters monitoring the contents of the extracellular fluids, i.e. lymph, tissue
fluid and blood. The lymphoid tissue filtering each of these fluids is arranged in
different ways. Secondary lymphoid tissues are also where lymphocytes
are activated.
 The lymph nodes, lymphatic nodules, tonsils, spleen are the secondary
“education centers” of the immune system
 In these organs, immunocompetent lymphocytes differentiate into immune
effector and memory cells that undergo antigen-dependent activation and
proliferation in these organs.
 These lymphocytes then carry out their functions in the
 connective tissue
 secondary lymphoid organs
 mucosal surfaces lining epithelia
 They participate in
 Cell mediated immunity (mostly “cytotoxic” T cells)
 Humoral responses (production of antibody) (B cells, also requires “helper T”
cells.
LYMPH NODES
• Lymph nodes are small, Bean or round or oval shaped
bodies (organs) of the lymphatic system.
• An estimated
2 total of 1,00,000 Lymph nodes present in
the entire body
4- in clusters. along the blood vessels or
• Typically found
8 side of the joint
the flexural
• Range from being microscopic to the size of a marble
and length from 1 - 25 millimeters
• Lymph nodes are also found individually throughout
the body tissues.
• There are several hundred lymph nodes found mostly
throughout the thorax and abdomen of the body with
the highest concentrations in the axillary (armpit) and
inguinal (groin) regions.
• The outside of each lymph node is made of a dense
fibrous connective tissue capsule. Inside the capsule,
the lymph node is filled with reticular tissue containing
many lymphocytes and macrophages.
E-Lymph nodes
(7 locations):
1.Cervical
2.Axillary
3.Thoracic
4.Abdominal
5.Intestinal &
mesenteric
6.Inguinal
7.Popliteal
“Swollen
glands”
HISTOLOGY OF LYMPH NODE
• The nodes are covered by a capsule of dense connective tissue, and have capsular extensions, of connective tissue,
called the trabeculae, which provide support for blood vessels entering into the nodes.
• Lymph, containing micro-organisms, soluble antigens, antigen presenting cells, and a few B-cells, enters the lymph
node via afferent lymphatic vessels which enter the subcapsular sinus. It then runs through cortical
sinuses into medullary sinuses and leaves through the efferent lymphatic vessels, at the Hilum as efferent lymph.
This contains lots of T-lymphocytes, B-lymphocytes, plasma cells and antibody.
HISTOLOGY OF LYMPH NODE (contd.)
• All the blood sinuses are lined by a discontinuous layer of simple squamous endothelium, and they also
contain lymphocytes and macrophages. Reticular fibres provide additional support to the matrix/stroma.
• The cortex is divided into an outer and an inner cortex.
• The outer cortex has lymphatic nodules that mostly contain B-cells. Small lymphocytes sit in the spaces
between the reticular fibre meshwork in the cortex.
• The lighter staining areas are germinal centres, where the B-cells proliferate into antibody secreting plasma
cells ( B-and T-lymphocytes).Macrophages are also present in these regions, together with dendritic cells, and
some T-cells. Both the macrophages, and the dendritic cells trap antigens and present them on their surfaces to
B-cells.
• The inner cortex contains mostly T-cells.
• The deep cortical, and medullary cords contain B-cells and plasma cells. Plasma cells live for 3 days, and
make IgG type antibodies.
• Most of the lymphocytes enter the lymph nodes via blood vessels, and about 10% enter through the lymph.
• The structure of the post-capillary venule, in the deep cortex (paracortex) is unusual in that it is not lined by
simple squamous epithelium, but by a simple cuboidal epithelium. These are called high endothelial venules
(HEVs). Lymphocytes recognise and adhere to these endothelial cells, and squeeze through them into the deep
cortical regions of the nodes. This region of the lymph has lots of T-cells, as well as the antigen presenting
dendritic cells.
• T-cells entering here become activated in the cortex, between lymphoid follicles.
Functions of Lymph nodes
1.The lymph nodes function as filters of lymph that enters from
several afferent lymph vessels, thereby promoting lymphocyte
contact with antigen
2.The reticular fibers of the lymph node act as a net to catch any
debris or cells that are present in the lymph. Provides necessary
microenvironment for antigen-dependent differentiation
3.Macrophages and lymphocytes attack and kill any microbes caught
in the reticular fibers.
4.Efferent lymph vessels then carry the filtered lymph out of the
lymph node and towards the lymphatic ducts.
MALT
• The mucosa-associated lymphoid tissue (MALT), also called mucosaassociated lymphatic tissue.
• It is a diffuse system of small concentrations of lymphoid tissue found in
various submucosal membrane sites of the body, such as the gastrointestinal
tract, oral passage, nasopharyngeal tract, thyroid, breast, lung, salivary
glands, eye, and skin.
• There are three types of situations in which MALT is presentTonsils, Appendix, Small Intestinal area (Peyer’s patches).
• MALT protects the digestive and respiratory systems from foreign matter.
TONSILS
• Tonsils are the two lymph nodes located on each side of the back of throat.
• Tonsils are clusters of lymphatic cells and extracellular matrix not completely surrounded by
24a connective
15 tissue capsule.
• Consist of multiple germinal centers and crypts
• Several groups of tonsils form a protective ring around the pharynx.
• One pharyngeal tonsils (or adenoids) is found in the nasopharynx at the posterior end of
the nasal cavity.
• Two palatine tonsils are in the posterior region of the mouth near the pharynx.
• Several lingual tonsils are located at the posterior root of the tongue (one-third of the
tongue) near the pharynx.
• They function as a defence mechanism. They help prevent your body from infection.
• The tonsils contain many T and B cells to protect the body from inhaled or ingested
substances. The tonsils often become inflamed in response to an infection.
• Tonsils protect our throats from the plaque that we swallow. When the plaque reaches the
stomach, acids usually get rid of the bacteria.
HISTOLOGY OF TONSILS
• The luminal surface of the tonsils are
covered with a stratified squamous
epithelium (in common with the oral
epithelia).
• The tonsils have many invaginations
which form blind crypts.
• Below the epithelium, there are many
lymphoid follicles beneath which have
germinal centres like the lymph nodes.
• The epithelial cells are able to
phagocytize bacteria, and transfer them
to macrophages, which then present the
foreign antigens to B-cells, which are
activated (with the help of T cells).
THYMUS GLAND
• It is a large organ in the fetus
• The thymus gland is the heart of
the immune system.
• The thymus is a primary
lymphoid organ where T
lymphocytes develop and
undergo maturation.
• it is the site of maturation of T- lymphocytes, secretes hormones -thymopoietin
and thymosins, and plays Critical role in childhood.
• It consists of two lateral lobes, situated partly in the thorax above the heart, partly
in the neck.
• Each lobe is surrounded by a capsule and is divided into lobules, which are
separated from each other by strands of areolar connective tissue called
trabeculae.
HISTOLOGY OF THYMUS
• Under low power view of a young thymus. Note that
the gland is organized into numerous lobules.
• Each lobule contains a dark-staining outer cortex and
inner medulla.
• Also note the loose collagenous capsule that extends
into the thymus to form the interlobular septa that
separate the lobules.
• The capsule and septa contain blood vessels,
lymphatics and nerves.
• The outer cortex contains densely packed developing
T-lymphocytes and epithelioreticular cells that provide
a structural framework for development. Due to high
cell density and small cell size, this zone is deeply
stained.
• The medulla contains mature thymocytes that are
larger and contain more cytoplasm. As a result, this
zone is lightly stained.
• The thymic cortex is heavily filled with developing Tlymphocytes. At the outer cortex, it is common to find
mitotic figures. These are dividing lymphoblasts in the
process of producing clones of smaller mature T-cells. The
epithelial cells in the cortex express class I and class II
MHC and serve to positively select immature T-cells. At the
corticomedullary junction, one can find pale-stained
macrophages. These macrophages remove lymphocytes that
have undergone apoptosis because they failed to develop
properly.
• The medulla of the thymus contains T-lymphocytes and
increased numbers of epithelial cells with pale-staining
nuclei. The epithelial cells provide structural support to the
medulla and negatively select self-reactive T-cells to
generate tolerance against self-antigens. A distinguishing
feature of the medulla is the presence of Hassall’s
corpuscles. They are concentric arrangements of flattened
epithelioid cells that are acidophilic. One may occasionally
observe keratinization of the structure. The purpose of these
structures is currently unknown.
SPLEEN
5inches wide 6 ounce weight
• Largest lymphatic organ in the body.
• Can vary considerably in size and weight
• Location
Protected by ribs 10-12
Left hypochondriac region
Dorsolateral to the stomach
Fits between the diaphragm, stomach, and kidney;
The spleen has gastric area, renal area, and colic area
• Served by splenic artery and vein, which enter and exit at the hilum
• Functions
• Site of lymphocyte proliferation and immune surveillance and response
• Cleanses blood of aged cells and platelets, macrophages remove debris
• Spleen is regarded as the graveyard of RBC / The Belly of The Beast/ A living
Hell for intruders
Structure of the Spleen
Histology of the Spleen
•
•
•
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Thymic lobules contain outer cortex and inner medulla
Most thymic cells are lymphocytes
Cortex contains rapidly dividing lymphocytes and scattered macrophages
Medulla contains fewer lymphocytes and thymic corpuscles involved in regulatory T cell
development (prevent autoimmunity)
• Two distinct areas
• White pulp around central arteries: surrounds the branches of the splenic artery, forming a
periarteriolar lymphoid sheath (PALS) populated mainly by T lymphocytes.
• Mostly lymphocytes on reticular fibers; involved in immune functions
• Red pulp in venous sinuses and splenic cords: consists of a network of sinusoids populated by
macrophages and numerous red blood cells (erythrocytes) and few lymphocytes;
• it is the site where old and defective red blood cells are destroyed and removed.
• Rich in RBCs and macrophages for disposal of worn-out RBCs and blood borne pathogens
• Composed of splenic cords and sinusoids
II Semester M.Sc. Degree Unit -1- Tutorial
ZOOLOGY
HCT 204: HISTOLOGY AND HISTOCHEMISTRY
WRITE A NOTE ON THE FOLLOWING
1. Formation of Bone
2. Bone remodelling
3. Fracture healing
4. Differences between Thick and thin skin
5. Structure and function of skin
6. Haemopoesis
7. Thymus
8. Spleen
9. Tonsils
10. Lymph node
8 M each