H ISTOLOGICAL S TRUCTURE OF L YMPHOID O RGANS DR RANIA GABR

O BJECTIVES 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

L YMPHOID T ISSUE 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.

L YMPHOID S YSTEM B ASICS Two main tissue architecture types: Diffuse Diffuse : uniform appearance Follicular Follicular : consists of lymphoid follicles Two types of lymphoid tissues: Encapsulated : connective tissue capsule spleen, thymus, lymph nodes Unencapsulated (or partly encapsulated) Tonsils, Peyer’s patches, lymphoid nodules in GI tract, respiratory tract, urinary & reproductive tracts

O RGANS 2 T YPES OF L YMPHOID O RGANS Central (primary) lymphoid organ Central (primary) lymphoid organ : where lymphoid cells undergo maturation T cells in thymus B cells in bone marrow Peripheral (secondary) lymphoid organ 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

L YMPHOID F OLLICLES Nodules of densely packed lymphocytes located in all peripheral lymphoid tissues. Most lymphocytes are B cells. Two distinct areas: Mantle 1- Mantle – darker stained, mainly small, resting lymphocytes Germinal center – 2- Germinal center – (defines “secondary” or “reactive” lymphoid follicles): lighter stained, larger, activated B cells

Lymph follicle: - Mantle = cap (dark) - Germinal center (light)

L YMPH N ODES 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

Covered by a capsule which extends to form Trabeculae. Divided into outer cortex and inner medulla. OUTER CORTEX OUTER CORTEX contains: Lymphatic nodules with germinal center INNER MEDULA INNER MEDULA contains: Medullary Cords and Medullary Sinus

M EDULLARY 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

Lymph node CT --- Connective tissue C --- Cortex M --- Medulla P --- Paracortex LN --- Lymph Node T ---Trabeculae MS --- Medullary Sinus MC --- Medullary Cords