GROSS AND MICROSCOPIC FEATURES OF LYMPH NODE.pptx

Histology of lymph node

1. GROSS AND MICROSCOPIC FEATURES OF LYMPH NODE DR SURAJ PD SAH

2. IMMUNE SYSTEM Protects the organism against invading pathogens or antigens ( bacteria , parasites and viruses). The lymphoid system includes all cells, tissues ,and organs that contain aggregates of immune cells called lymphocytes. Lymphoid organs are divided into two major categories – 1. Primary lymphoid organs – Bone marrow and thymus. 2. Secondary lymphoid organs - Lymph node, spleen, tonsils, MALT and Peyer patches.

3. LYMPH NODE GROSS FEATURES: Lymph nodes are bean-shaped structures about 0.1 – 2.5 cm in length . The node is enclosed in a  capsule  and has an indentation on one surface known as the   hilum . The hilum is the point at which arteries carrying nutrients and lymphocytes enter the lymph node and veins leave it.  

4. Afferent lymphatic vessels  enter the lymph node through the capsule peripherally and  efferent lymphatic vessels  leave the node via the hilum. The Afferent takes lymph from peripheral sites to the node, while the efferent takes processed lymph from the nodes back to the circulation.

5. MICROSCOPIC FEATURES A horizontal section through the lymph node reveals that the  dense connective tissue   capsule  (composed of elastin, collagen and  fibroblasts ) Additionally , there is a  pericapsular adipose tissue layer  that surrounds the connective tissue capsule. This layer contains  arterioles  and  venules  that supply the lymph node . Capsule projects into lymph node interiorly is called trabeculae ; giving the lymph node a lobular appearance , The lymph node is divided into an outer cortex and an inner medulla .

6. Histological staining of the node reveals that the cortex stains darker than the medulla with hematoxylin and eosin (H&E) due to its higher cell content. Beneath the fibrous capsule is the  subcapsular sinus . subcapsular sinus communicates with the  cortical sinuses  that travel parallel to the capsular trabeculation . They carry lymph to the medullary sinus The medullary sinuses drain the lymph coming from the cortical sinuses to the efferent lymphatic vessel via the hilum.

7. Cortex The cortex contains spherical collections of lymphocytes is called lymphoid nodules or lymphoid follicles . It primarily contain B- cells , specialized follicular dendritic cells and some supporting T-cells. These can either be primary or secondary follicles depending on their cellular population. Primary lymphoid follicles contain small, dormant lymphocytes. while secondary lymphoid follicles contain a lighter/pale staining area of active lymphocyte proliferation known as a  germinal centre .

8. Paracortex Deep to the cortical layer and  superficial to the medulla is the Paracortex . This region contains mostly  T-cells  of the CD4 (cluster of differentiation) and CD8 subsets . Migrating  dendritic cell  lines (such as Langerhans cells) found in this area present processed antigen to the T-cells. Medulla The cords also contain  plasma cells , small lymphocytes and macrophages . The lymphocytes of the medulla are less organized and form irregular  medullary cords .

9. The germinal centre supports affinity maturation (high affinity antibody production) of B-cells. It is subdivided into a dark zone, light zone and a mantle zone . The B-cells of the  dark zone  are known as  centroblasts . They rapidly replicate, resulting in hypermutation of their antibody molecules. Centroblasts migrate to the  light zone , where they are referred to as  centrocytes . Here they compete for binding with the unprocessed antigens presented on the surface of  follicular dendritic cells . Those centrocytes that successfully bind to the follicular dendritic cells will survive, while the others will die. Small, quiescent cells are peripherally marginalized due to the rapid proliferation of the central cells. These cells form the  mantle zone  of the germinal centre .

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