Download - Histological Structure of Lymphoid Organs
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HISTOLOGICAL STRUCTURE OF LYMPHOID ORGANSDR RANIA GABR
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OBJECTIVES
Understand the location of lymphatic organs.
Discuss the microscopic features of Lymph Node.
Discuss the microscopic features of Spleen.
Discuss the microscopic features of Thymus.
Discuss the microscopic features of Tonsils
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LYMPHOID TISSUE
Lymphoid tissue is connective tissue chch by rich supply of lymphocytes.
It is found either 1- Free in regular CT
2-Surrounded by capsules, forming the “lymphoid organs” Very little cytoplasm so stain dark blue with
H&E. Rich network of reticular fibrils produced by
fibroblasts.
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LYMPHOID SYSTEM BASICS Two main tissue architecture types:
Diffuse: uniform appearance Follicular: consists of lymphoid follicles
Two types of lymphoid tissues:Encapsulated: connective tissue capsule
spleen, thymus, lymph nodesUnencapsulated (or partly
encapsulated)Tonsils, Peyer’s patches, lymphoid nodules in GI tract, respiratory tract, urinary & reproductive tracts
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2 TYPES OF LYMPHOID ORGANS
Central (primary) lymphoid organ: where lymphoid cells undergo maturation
T cells in thymus B cells in bone marrow
Peripheral (secondary) lymphoid organ: where functional lymphocytes go including:
1- lymph nodes 2- spleen,3- Peyer’s patches, 3- lymphoid nodules of GI and other tracts
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LYMPHOID FOLLICLES
Nodules of densely packed lymphocytes located in all peripheral lymphoid tissues. Most lymphocytes are B cells.
Two distinct areas:1- Mantle – darker stained, mainly small, resting lymphocytes 2- Germinal center – (defines “secondary” or “reactive” lymphoid follicles): lighter stained, larger, activated B cells
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Lymph follicle:
- Mantle = cap (dark)- Germinal center
(light)
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LYMPH NODES
Present throughout the body, along lymph vessels
Numerous in axilla, groin, cervical area and thoracic/abdominal mesenteries
Filter lymph before it returns to vasculature
Hilum: concave side, arteries, nerves enter; veins and efferent lymph vessels leave the organ
Afferent lymph vessels enter convex surface
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• Covered by a capsule which extends to form Trabeculae.
• Divided into outer cortex and inner medulla.• OUTER CORTEX contains:
Lymphatic nodules with germinal center• INNER MEDULA contains:
Medullary Cords and Medullary Sinus
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MEDULLARY CORDS
Are branched, cordlike extensions of lymphoid tissue arising from the paracortex. They contain primarily B- lymphocytes and often plasma cells and macrophages.
Medullary cords are separated by dilated spaces , frequently bridged by reticular cells and fibers , called Medullary sinuses
They contain lymph , lymphocytes, macrophages, sometimes granulocytes if the lymph node is draining an infected organ
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Lymph node
CT --- Connective tissueC --- CortexM --- MedullaP --- ParacortexLN --- Lymph NodeT ---TrabeculaeMS --- Medullary SinusMC --- Medullary Cords