Presentation on theme: "Histological Structure of Lymphoid Organs"— Presentation transcript:
1 Histological Structure of Lymphoid Organs DR RANIA GABR
2 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
3 Lymphoid TissueLymphoid tissue is connective tissue chch by rich supply of lymphocytes.It is found either Free in regular CT2-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.
4 Lymphoid System Basics Two main tissue architecture types:Diffuse: uniform appearanceFollicular: consists of lymphoid folliclesTwo types of lymphoid tissues:Encapsulated: connective tissue capsulespleen, thymus, lymph nodesUnencapsulated (or partly encapsulated)Tonsils, Peyer’s patches, lymphoid nodules in GI tract, respiratory tract, urinary & reproductive tracts
5 2 Types of Lymphoid Organs Central (primary) lymphoid organ:where lymphoid cells undergo maturationT cells in thymusB cells in bone marrowPeripheral (secondary) lymphoid organ: where functional lymphocytes go including:1- lymph nodes spleen,3- Peyer’s patches, lymphoid nodules of GI and other tracts
7 Lymphoid FolliclesNodules 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 lymphocytes2- Germinal center –(defines “secondary” or “reactive” lymphoid follicles): lighter stained, larger, activated B cells
8 Lymph follicle:Mantle = cap (dark)Germinal center (light)
10 Lymph Nodes Present throughout the body, along lymph vessels Numerous in axilla, groin, cervical area and thoracic/abdominal mesenteriesFilter lymph before it returns to vasculatureHilum: concave side, arteries, nerves enter; veins and efferent lymph vessels leave the organAfferent lymph vessels enter convex surface
12 Covered by a capsule which extends to form Trabeculae. Divided into outer cortex and inner medulla.OUTER CORTEX contains:Lymphatic nodules with germinal centerINNER MEDULA contains:Medullary Cords and Medullary Sinus
13 Medullary cordsAre 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 sinusesThey contain lymph , lymphocytes, macrophages, sometimes granulocytes if the lymph node is draining an infected organ
15 Lymph node CT --- Connective tissue C Cortex M Medulla P Paracortex LN --- Lymph Node T Trabeculae MS --- Medullary Sinus MC --- Medullary Cords
16 Spleen It is an encapsulated lymphoid organ The capsule extends to form Trabeculae which contain the trabecular arteries and trabecular veins.It consists of stroma and parenchyma.The parenchyma of the spleen consists of:1.White Pulp:lymphoid follicles or nodules with germinal centre.A Central artery passes through lymphatic nodules.
18 2.Red Pulp:surrounds the white pulp and is also divided to splenic cords and splenic sinuses.Splenic Cords:Known as Splenic Cords of BillrothThey consist of all WBCs, macrophages, RBCs ,B & T Lymphocytes.Splenic Sinuses:Full of blood because the spleen acts as a filter of blood. Splenic sinuses are lined by endothelium.The stroma of the spleenis similar to that of the lymph node with a capsule, trabeculae & reticular fibers
23 ThymusThe thymus is a primary lymphoid organ in that it supplies other lymphoid organs and tissues with T-lymphocytes.The thymus is enclosed by a thin C.T capsule from which numerous septa extend into the thymus subdividing the 2 lobes into numerous lobules.
24 Each lobule is divided into: Cortex :darker peripheral zone with densely packed lymphocytes (No lymphatic nodules).Medulla:lighter central zone with fewer lymphocytes but more epithelial reticular cells.
25 Thymus Medulla also contains Thymic (Hassall’s) Corpuscles. Thymic (Hassall’s) Corpuscles are oval structures consisting of round whorls of flattened epithelial reticular cells.Thymus
27 Palatine TonsilSurface of the Tonsil is covered by Stratified Squamous nonkeratinized epithelium.Tonsil is invaginated by deep grooves called Tonsillar Crypts.Below epithelium, lymphatic nodules are present in the connective tissue.Dense connective tissue underlies the palatine tonsil and forms capsule.