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Learning Objectives:
Histology of Lymphoid tissue
1. Appreciate that the lymphoreticular system is divided into primary and secondary lymphoid organs.
2. Recognise that the structure of the BONE MARROW and THYMUS provides an ideal
environment for B cell and T cell differentiation.
3. Describe how the structure of the LYMPH NODE is well adapted for filtering antigens from the
tissue fluid and initiating immune responses.
4. Describe the structure of the SPLEEN and appreciate that this is related to its role in initiating
immune responses to antigens in the blood.
5. List the important components of the MUCOSAL ASSOCIATED LYMPHOID TISSUES (MALT)
and describe how their structure relates to their function in protecting mucosal surfaces.
1 BONE MARROW : See blood & haemopoiesis (year 1 of course)
2 THYMUS : SLIDE 147
Draw a simple low power sketch of a section through the thymus and label capsule and thymic lobules
with cortical and medullary zones. You should also examine the cortex and medulla at a higher
magnification and identify epithelioreticular cells, lymphocytes and Hassal’s corpuscles.
Q 1. What are the major differences in the histological structure of the thymus in the cortical and
medullary zones?
Q 2. In which part of the thymic lobule would you expect to see mature lymphocytes and can immature
and mature lymphocytes be distinguished morphologically?
Q 3. What is the function of epithelioreticular cells and how can you identify them in this section?
Q 4. What is the distinguishing feature of a Hassal’s corpuscle and are they found anywhere other than
in the thymic medulla?
3 LYMPH NODE : SLIDE 125 (dog) stained for reticular fibres
(larger and darker of the two sections)
SLIDE 143 (cat) H & E stain
Examine the sections and draw a low power sketch to highlight the distinctive landmarks such as
cortex (outer layer), paracortex, medulla (deeper layer) and sinuses that characterise the structure of a
lymph node. Also note the fibrous capsule and other connective tissue support represented by trabeculae
and reticular fibres (stained black in section 125). Under higher magnification (slide143), distinguish
different cell types in the germinal centre and peripheral corona of follicles and diffuse paracortex.
Also examine the medulla with medullary cords and sinuses lined with an endothelium containing
lymphocytes, plasma cells and macrophages.
Try to visualise how the lymph carrying the antigen enters the subcapsular sinus and flows through the
lymph node via cortical and medullary lymph sinuses.
Q 5. What are the major cell types found in the paracortex and how does this differ from the cells
present in the follicles or medulla?
4 SPLEEN : SLIDE 145 (rat)
SLIDE 144 (mouse) carbon injected to label macrophages
Examine the sections and draw a low power sketch to represent the spleen that distinguishes it from
a lymph node and all other lymphoid tissues. Label the connective tissue support represented by a capsule,
trabeculae and reticular fibres. Also note the random scattering of lymphatic tissue (white pulp) centred
around the branches of a central artery in the blood filled parenchyma called red pulp. Note that the red
pulp is composed of elongated structures called splenic cords that lie between the sinusoids.
The lymphatics around a central arteriole are called a periarterial sheath (PALS). The lymphatic follicles
are also present at the edges of the PALS.
Q 6. How does the antigen arrive in the spleen and how does this differ from a lymph node?
Q 7. What role do macrophages play in lymphoid tissue and what additional role do they have in
the spleen?
Q 8. Did you find any megakaryocytes in the mouse spleen and what would that indicate?
5 DUODENUM : SLIDE 45
The lymphatic tissue here shows unorganised
accumulation of scattered lymphocytes in the
mucosal layer of the intestinal wall.
The lymphocytes here appear as dark staining
small cells against the paler staining epithelial
cells.
Note that the lymphocytes are smaller and
darker than the elongated nuclei of columnar
epithelial cells.
6 PEYER’S PATCH : SLIDE 142
First look at the slide without the microscope to observe the Peyer’s patches represented by collections
of lymphatic nodules present in ONLY the wall of the small intestine.
The remaining areas of wall lacks these swellings.
Now confirm the presence of lymphatic nodules by looking under the microscope.
Draw a simple sketch to show the structure of a Peyer’s patch.
Q 9. Do you see both primary and secondary follicles?
Q 10. Where would you find the T and B lymphocytes in a Peyer’s patch?
7 PHARYNGEAL TONSIL : SLIDE 141 (cat)
Tonsil is a partly encapsulated lymphoid tissue characterised by the presence of crypts.
Draw a simple sketch to illustrate its structure.
Q 11. What does the large number of follicles with active germinal centres tell you?
Q 12. Why is the nature of the epithelium unclear in some places?