1. Dr. ANAND SRINIVASAN

2.  Made of tissues and organs made of lymphocytes.  Concerned with immunity  Made of :  Immunoglobins  Lymphocytes  Natural Killer cells  Macrophages & cells of mononuclear phagocytic  Antigen presenting cells  Neutrophils, mast cells & eosinophils

3. LYMPHOID TISSUE DiffuseDenseMALTDiscrete Thymus Lymph node SpleenTonsil

4.  Made of connective tissue & lymphocytes  Covered by capsule  Trabeculae / interlobular septum carrying neurovascular structures.  Reticulum supporting the cellular parenchyma

5.  Central lymphoid organ  Bilobed, unequal size  Superior & Anterior mediastinum  Dual origin “Lympho – epithelial organ”  Lymphocytes  Mesoderm  Epithelial reticular cells  Endoderm  Well developed in foetus & early childhood  Peak development – puberty; later involutes  Only Efferent lymphatics; No Afferent lymphatics

6.  Presence of Capsule, Inter lobular septa with blood vessels  Darkly stained cortex and lightly stained medulla  Presence of “Hassal’s bodies” made of central hyaline material and surrounded by flat epithelial cells  Number & calcification of Hassal corpuscles increases with age  Presence of Cellular cytoplasmic reticulum / Epithelial Reticular cells (This is different from reticulum of other lymphoid organs which are formed by reticular fibers) which secrete Thymosin and thymopoietin.

8.  Oval or bean shaped structures  Serve as filters  Usually found in groups  Afferent and efferent vessels

9.  Presence of thin capsule and trabeculae  Presence of subcapsular sinus (made of reticular fibers and cells) continuous with trabecular sinus which in turn continuous with medullary sinus  Presence of lymphatic nodule with / without germinal centers(primary & secondary nodules).  Presence of medullary cord and medullary sinus

12.  Largest lymphoid organ  Upper left part of abdominal cavity  Functions  Foetal life : Blood formation  Postnatal : destroys old RBC, filters blood from antigens & microorganism.

13.  Capsule covered by mesothelium (peritoneum)  Capsule contains few smooth muscle fibers (Spleen as reservoir of blood)  Thick trabeculae carrying neurovascular bundles  Presence of “white pulp” and “Red pulp”

14.  WHITE PULP (Malpighian bodies )  Lymphoid aggregation (PALS)around central artery.  Central A. then enters red pulp as “Penicillar arteries”  RED PULP  Presence all blood cells  Composed of  Splenic cords of Bilroth  Splenic venous sinuses

18.  GENERAL FEATURES:  Located in oropharynx between palatoglossal and palatopharyngeal arches.  Medial surface – crypts.  HISTOLOGICAL FEATURES:  Crypts are lined by stratified squamous epithelium  Lymphatic nodules in the lamina propria

20.  Chapter 9 – Microanatomy Workbook, RAKMHSU