Histology of the Immune System (Lymphoid organs) Flashcards
Lymphatic cells are organized into tissues/organs in 3 general categories namely: ________.
(1) Diffuse unencapsulated bundles of lymphatic cells [usually found in the lamina propria of the mucous membranes of the respiratory tract and the gastrointestinal tract]
(2) Discrete, unencapsulated bundles of lymphatic cells [called lymphatic follicles (nodules). These are found in the lamina propria of the mucosa that line the respiratory, gastrointestinal (GALT), urinary and the reproductive systems. These are referred to as mucosa associated lymphoid tissue (MALT).]
(3) Encapsulated organs containing lymphatic nodules and diffuse lymphatic cells surrounded by a capsule of dense connective tissue [These include the spleen, thymus and lymph nodes. The thymus is a primary lymphoid organ in that it supplies other lymphoid organs and tissues with T-lymphocytes, while the spleen and lymph nodes (secondary lymphoid organs) monitor the internal environment of the body.]
Name the prominent supporting fibres in lymph nodes.
reticular fibres
Briefly describe the organisation of the parenchyma of the lymph node.
The parenchyma of the lymph node is divided into the outer cortex and the inner medulla, which is surrounded by the cortex all around except for at the hilum, where the medulla comes in direct contact with the surface.
What is:
(a) a lymph sinus?
(b) the subcapsular sinus?
(a) lymph sinus: This is a channel within the lymph node lined by endothelial cells along with fibroblastic reticular cells. [The walls of the sinuses can be traversed freely by all components of the lymph, which allows lymphocytes to enter/leave the lymphoid tissue (as part of their circulation) or to get in contact with antigens/antigen-presenting cells that may arrive with the lymph.]
(b) subcapsular sinus: This is a sinus immediately deep to the capsule and its endothelium is continuous with that of the afferent lymph vessels.
The lymphocytes in the outer part of the cortex are organized into nodules and are mainly ____(a)____ cells. The cortex adjacent to the medulla (deep cortex, juxtamedullary cortex, paracortical or tertiary cortex) is devoid of granules. This zone is predominated by ____(b)____ lymphocytes. The medulla is the inner part of the node. It consists of cords of lymphatic tissue separated by lymph sinuses called ____(c)____.
(a) B
(b) T
(c) medullary sinuses
Click on Answer to view histological images of a lymph node.
⚚ [Slide 1] [Slide 2]
⚚ [Virtual microscope]
⚚ [6-minute video]: Shotgun Histology
⚚ [4-minute video]: Osmosis from Elsevier
Discuss the red pulp of the spleen.
⚚ It has a red appearance because it contains a large number of red blood cells.
⚚ It consists of splenic sinusoids interposed between sheets and strands or reticular connective tissue, the splenic cords or cords of Billroth.
⚚ The splenic cords consist of a meshwork of reticular cells and reticular fibres containing large numbers of erythrocytes, macrophages, lymphocytes, plasma cells and granulocytes.
⚚ The sinusoids continue into the veins of the pulp, which empty into thin-walled trabecular veins, which eventually coalesce to form the splenic vein.
Further notes:
⚚ Blood cells, which are emptied into the splenic cords, re-enter the blood vessels through the endothelium of the sinusoids.
⚚ The endothelial cells are elongated and oriented along the long axis of the sinusoids. The endothelium of the sinusoids has no junctional complexes and its basement membrane is incomplete.
⚚ Macrophages ingest aged erythrocytes, platelets and other particulate matter as they pass through the splenic cords.
⚚ The iron from degenerating red blood cells is reutilized in the formation of new red blood cells.
Discuss the white pulp of the spleen.
⚚ The white pulp consists mainly of lymphatic tissue, mostly lymphocytes.
⚚ Branches of the splenic artery course through the capsule and the trabeculae of the spleen and then enter the white pulp.
Within the white pulp, the vessel is called the central artery.
⚚ The aggregated lymphocytes around the central artery are designated as the periarterial lymphatic sheath (PALS).
⚚ The lymphocytes within PALS are mainly T-lymphocytes.
⚚ The presence of a central artery distinguishes PALS from nodules.
⚚ Scattered along the course of the sheath, the lymphatic tissue expands to incorporate lymphatic nodules that resemble the cortical nodules of lymph nodes. They represent sites of B-lymphocytes, and many contain germinal centers. The lymphatic nodules of the spleen are called splenic nodules or malphigian corpuscles.
Click on Answer to view histological images of the spleen.
⚚ [Slide 1] [Slide 2] [Slide 3] [Slide 4]
⚚ [6-minute video]: Osmosis from Elsevier
State the distinguishing histological features of the thymus.
(a) septae extending from the thin connective tissue capsule which divide the parenchyma into incomplete lobules
(b) Hassall’s corpuscles [isolated masses of closely packed concentrically arranged epithelial reticular cells]
[Virtual microscope 1] [Virtual microscope 2]
Further notes:
⚚ The capsule and trabeculae contain blood vessels, efferent lymphatics and nerves. In contrast to the lymph nodes, the thymus has no afferent lymphatics.
⚚ Medullary tissue is continuous from lobule to lobule throughout each lobe.
⚚ The cortex of the thymic parenchyma contains a higher concentration of lymphocytes than the inner portion (the medulla).
⚚ The cortex appears extremely basophilic due to the closely packed small lymphocytes containing densely staining nuclei. In contrast, the medulla stains less intensely because it contains large lymphocytes with pale staining nuclei similar to that seen in lympohid nodules.
⚚ The medulla also contains more epithelial reticular cells.
State the necessity of the blood-thymic barrier.
It ensures that lymphocytes being formed and programmed in the thymus to react against foreign antigens are not permanently exposed to them.
State the components of the blood-thymic barrier.
(a) Capillary endothelium
(b) Its basal lamina
(c) A thin perivascular sheath of connective tissue
(d) The epithelial reticular cell sheath
(e) Its basal lamina
List the cell types present in the thymus starting with the most dominant.
lymphocytes, reticular cells, macrophages, mast cells, fat cells, granulocytes, plasma cells
Further notes:
Epithelial reticular cells serve as a framework for the thymic lymphocytes and thus correspond structurally to the reticular cells and their associated reticular fibres in other lymphoid tissues and organs.
Plasma cells are a type of white blood cell that are derived from activated B lymphocytes. They are relatively large, ovoid cells that produce and secrete antibodies.
List three hormones secreted by the thymus.
⚚ Thymopoietin
⚚ Thymulin
⚚ Thymosin
