-SAYAN BANERJEE

Lymph node

Lymph nodes are completely encapsulated ovoid structures, in contrast to the lymphatic tissue previously described, and are the immunologic filters of the lymph. The capsule admits afferent lymphatic vessels containing valves that provide one-way flow into the subcapsular sinus. The lymph circulates through sinuses located in the cortex (containing the lymph nodules) and the medulla (containing lymphatic cords), and leaves the node via larger but fewer efferent lymphatic vessels. These also contain valves and emerge from a specific region of the node, the hilus. Lymph nodes, which vary in size from 1 to 25 mm, receive their blood supply only at the hilus of the node. The arterial vessels enter both the trabeculae formed from the capsular connective tissue and the medullary cords, and they regionally supply the node by giving off capillaries; they continue to the cortex, where an arterial branch penetrates each cortical lymph nodule and forms a capillary plexus around the germinal center. From the capillary beds, blood is carried by veins, which follow a pathway similar to the arteries, leaving the node at the hilum along with efferent lymphatic vessels.

•Situation

Nodes are small bodies placed at intervals along the lymphatic vessels, and structured so that the lymph has to pass through them. Afferent lymphatics bring lymph from a drainage area. The node is responsible for combating intruders and confining infection to that area, by sending out antibodies and cells via efferent lymphatics.

•Structure

1. A CT capsule, with some smooth muscle cells, sends in thin CT trabeculae, supporting a network of reticular fibres, and reticular cells of fibroblastic and the accessory dendritic kinds.2. A denser outer cortex and a looser, inner medulla are present.3. Efferent lymphatics leave at a hilus: the point of entry for blood vessels, serving a mostly cortical microvasculature.4. Afferent lymphatics open through the capsule at several places to feed a system of 'sinus' channels running so:subcapsular/marginal sinus --> cortical/intermediate sinuses --> medullary sinuses --> efferent lymphatics.5. Sinuses are lined by reticular cells, accompanied by macrophages.6. Denser masses of lymphoid tissue, extensive and follicular/nodular in the cortex, and continuing into the medulla as widely spaced medullary cords, have packed cells: lymphocytes, lymphoblasts and antigen-trapping dendritic reticular cells with processes. 7. Lymphoblasts/centroblasts occur in the paler germinal centres of the cortical follicles.8. The follicular zone contains B lymphocytes separated by follicular dendritic cells (FDCs).9.The deeper lying paracortical region has mostly T lymphocytes, and dendritic APCs wrapping so intimately around lymphocytes that they received the name interdigitating reticular cells (IRCs).

Functions

1. Mechanical filtration of lymph in the sinuses, trapping, for instance, soot carbon particles. Tumour cells are not so easily stopped.

2. Phagocytosis of materials in lymph by macrophages along the sinuses or lodged across them. The materials taken up include antigens, e.g., on bacteria. APCs and MØs process antigens for lymphocyte activation.

3. Proliferation of sensitized lymphocytes to become lymphoblasts, large, with little GER, but many ribosomes, stainable with pyronin. These lymphoblasts are the source of:.. (a) plasma cells, and hence humoral antibodies, or.. (b) cytolytic lymphocytes, which set out for their distant target -antigen in the drainage area - in the sinus lymph.

4.Recirculation of mature lymphocytes from venule blood to sinus lymph by migration through the cuboidal endothelium of the venules (high-endothelial venules – HEVs).

Bibliography

Di-Fiore’s atlas of histology with functional correlations 11th edition .

William’s histology .

Meyer’s histology.

Google images.

www.histoslides.tk

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