The document summarizes the histology of three lymphoid organs - lymph node, spleen, and thymus. It describes that lymph nodes have an outer cortex containing lymphatic nodules and inner medulla containing medullary cords. The spleen contains white pulp with lymphatic nodules and germinal centers, and red pulp with venous sinuses and splenic cords. The thymus has lobules consisting of a dense cortex and inner medulla containing thymic corpuscles.

1. HISTOLOGY OF LYMPH
NODE, SPLEEN,
THYMUS
DR.SHREYAS
JR PATHOLOGY
IGMC SHIMLA

2. IMMUNE SYSTEM
◦ Protects the organism against invading pathogens or antigens (
bacteria , parasites and viruses).
◦ The lymphoid system includes all cells, tissues ,and organs that
contain aggregates of immune cells called lymphocytes.
◦ Lymphoid organs are divided into two major categories –
◦ 1. Primary lymphoid organs – Bone marrow and thymus.
◦ 2. Secondary lymphoid organs - Lymph node, spleen, tonsils,
MALT and Peyer patches.

3. ◦ Lymph node exhibits an outer cortex and inner
medulla.
◦ Lymphoid nodules characterise the cortex, some of
which exhibit lighter staining areas called germinal
centre’s.
◦ The medulla consists of medullary cords and
medullary sinuses.
◦ Medullary cords are networks of reticular fibres filled
with plasma cells, macrophages and lymphocytes
separated by capillary like channels called medullary
sinuses.
◦ The lymph enters the lymph node via Afferent
lymphatic vessels in the capsule and flows through
cortex and medullary sinuses to exit on the opposite
side via the efferent lymphatic vessels.
Vasculature of the lymph
node

4. Normal Lymph node
◦ Frame work:-
◦ Capsule
◦ Fibrous trabeculae
◦ Reticulin network
◦ Cortical area :-
◦ Lymphoid follicles :- Primary and secondary
◦ Germinal center :-
◦ B cells :- 1. Centroblasts:– dark zones.
◦ 2. Centrocytes:- light zones.
◦ T cells :- small, inactivated
◦ Tingible body macrophages
◦ Dendritic reticulum cells
◦ Mantle zone :- memory B cells
◦ Marginal zone :- monocytoid cells

5. ◦ Paracortical area:–
◦ T cells:- - small, inactivated
◦ -large, immunoblasts
◦ Medullary area :-
◦ Plasma blasts
◦ Plasma cells
◦ Lymphatic sinuses:-
◦ Marginal
◦ Cortical
◦ Medullary
◦ Blood vessels :-
◦ Artery
◦ Vein
◦ Capillaries
◦ High endothelial post capillary venules.

6. Centroblasts
have large, round nuclei with peripherally
located one to three basophilic nucleoli
and a rim of basophilic cytoplasm.
Centrocytes
are small to medium size with angulated,
elongated, cleaved or twisted nuclei.

7. HISTOLOGY
◦ Lymph node shows outer dark staining cortex and
inner light stained medulla.
◦ The lymph node is surrounded by a pericapsular
adipose tissue.
◦ A dense connective tissue capsule surrounds the
lymph node.
◦ The connective tissue trabeculae extend into
node between the lymphatic nodules and also
exhibits major blood vessels.
◦ Afferent lymphatic vessels and valves course in
the connective tissue capsule of the lymph node ,
penetrate the capsule to enter a narrow
subscapular sinus. From here, the sinuses (cortical
sinus) extend along the trabeculae into the
medullary sinuses.

9. ◦ The cortex exhibits
numerous lymphatic
nodules some with lighter
staining germinal center.
◦ Central region is lighter
staining medulla,
characterised by the dark
staining medullary cords
and the light staining
medullary sinuses.

10. Subcortical sinus and lymphatic nodule
◦ The reticular cells are visible in the subscapular sinus , trabecular sinuses and
germinal center of the lymphatic nodules.
◦ A lymphatic nodules with a small section of its peripheral zone and a
germinal center with developing lymphocytes are visible.
◦ Dense peripheral zone — contains small lymphocytes.
◦ Germinal center — contains medium sized lymphocytes.
◦ The largest cells with less condensed chromatin are the lymphoblasts.

12. Paracortex of the
lymph node
◦ Contains postcapillary venules with an
unusual morphology to facilitate the
migration of lymphocytes from blood
into the lymph node .
◦ The image shows high endothelial
venule lined by tall cuboidal
endothelium.
◦ Several migrating lymphocytes are
seen moving through venule wall
between the big endothelium into the
paracortex.
◦ Surrounding the high endothelial
venule are lymphocytes, medullary
sinus and venule .

13. SPLEEN
◦ Largest lymphoid organ with extensive blood supply ; filters blood and
serves as a blood reservoir.
◦ T cells surrounds the central arteries, whereas B cells are mainly in the
lymphatic nodules.
◦ APCs and macrophages are found in white pulp.
◦ Breaks down hemoglobin from erythrocytes and recycles iron to bone
marrow.
◦ Degrades heme from hemoglobin, which is then excreted in bile.
◦ During fetal life spleen is an important hematopoitic organ and in
adults serves as blood barrier.

14. HISTOLOGY
◦ The spleen is surrounded by a dense connective tissue capsule, from which arise connective tissue
trabeculae.
◦ Located within the trabeculae are trabecular arteries and trabecular veins .
◦ Spleen is divided into white pulp and red pulp .
◦ Spleen is characterised by numerous lymphatic nodules that constitute the white pulp , included
in the white pulp are the germinal centers and blood vessels called central arteries.
◦ Periarterial lymphatic sheaths (PALS) form the lymphatic nodules of the white pulp.
◦ The diffuse cellular meshwork that makes up the bulk of the organ and constitutes the red or
splenic pulp.
◦ Present in the red pulp are the pulp arteries, venous sinuses and splenic cords( of billroth).

17. ◦ White pulp:-
◦ The lymphatic nodule exhibits a peripheral zone — periarterial lymphatic sheath with densely
densely packed small lymphocytes.
◦ Central artery in the lymphatic nodule has a peripheral or eccentric position, however the
artery occupies the center of the PALS, it’s called the central artery.
◦ The cells in the PALS are mainly T cells.
◦ Germinal center present.
◦ Red pulp :-
◦ Contains the splenic cords (of billroth) and venous sinuses .
◦ Splenic cords – contain small lymphocytes, various blood vessels.
◦ Venous sinuses– are dilated vessels lined with modified endothelium.
◦ Pulp arteries – branches of the central artery.

19. ◦ White pulp consists
of lymphocytes and
lymphatic nodules
with a germinal
center, and a central
artery .
◦ Surrounded the
white pulp is the red
pulp, primarily
composed of venous
sinuses and splenic
cords .

20. THYMUS GLAND
◦ Lobulated lymphoepithelial organ.
◦ Most active in childhood and has important role early in life in immune
system.
◦ Site where immature lymphocytes from bone marrow mature into T
cells, Helper T cells, Cytotoxic T cells .
◦ Blood – thymus barrier prevents developing lymphocytes contacting
blood borne antigens.
◦ Sends mature T cells to populate the lymph node and spleen.

21. Histology
◦ Lobulated lymphoid organ enclosed by a connective tissue capsule from which
arise connective tissue trabeculae that extend into the organ and subdivide the
thymus gland into incomplete lobules .
◦ Each lobule consists of dark staining cortex and a light staining inner medulla .
◦ The cortex of each lobule contains densely packed lymphocytes .
◦ The medulla contains numerous Thymic (Hassall) corpuscles that characterise
the thymus gland.
◦ In addition, parenchyma or cellular portion of the gland is gradually replaced by
loose connective tissue and adipose cells.

23. ◦ The Thymic lymphocytes in the
cortex form dense aggregations. In
contrast, the medulla contains only a
few lymphocytes but more of
epithelial reticular cells.
◦ The Thymic (hassall) corpuscles are
oval structures consisting of round or
spherical aggregations ( whorls) of
flattened epithelial cells.
◦ Thymic corpuscles also exhibit
calcification or degeneration centers
that is eosinophilic.
◦ Blood vessels and adipose cells are
present .

24. ◦ A connective tissue trabeculae subdivides the gland into incomplete lobules.
◦ Each lobule consists of the darker staining cortex and light staining medulla.
◦ Thymic (Hassall) corpuscles is present in the center of the medulla in one of the lobules.

25. Positive stains
◦ B cells :- CD19, CD20, CD22, CD79
◦ Germinal center B cells :- BCL6 and CD10
◦ B cells in primary follicles and mantle zones:– IgD, IgM, CD21, CD23
◦ T cells :- CD2, CD3, CD4, CD8
◦ Follicular helper T cells :- CD4
◦ Premature B and T cells :- TdT ( terminal deoxynucleotidyl tranferase)
◦ Follicular dentritic cells :- CD21, CD23, CD35
◦ Macrophages:- CD68, lysozyme
◦ NK cells :- CD56, CD57

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