Lymph Nodes - Anatomy & Physiology
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The body contains hundreds of lymph nodes of varying size and these are located along the routes of lymphatic vessels. Lymph nodes act as a filter for the lymph removing antigens and releasing immune-competent cells and immunoglobulins.
Development
Lymph nodes develop from lateral plate mesoderm in paired sacs from lymphatic vessels. These sacs undergo remodelling and endothelial and mesenchymal outgrowths form the meshwork of channels and spaces that produces the cortex-medulla structure. Lymphocytes then populate the cortex and medulla. The sub-capular sinus is a remainder of the lymphatic vessel.
Structure
Grossly the lymph nodes has an outer cortex and an inner medulla. Microscopically the nodes has a follicle area, a paracortical zone and medullary cords sinuses.
The nodes are surrounded in a fibrous capsule that extends into the node as trabeculae. Below the capsule is the sub-capsular sinus. The nodes parenchyma contain a fine network of reticular fibres and cells that provide "scaffolding" for other cells. The cortex has aggregations of B cells (follicles) in its outer region and a paracortex consisting of a rim of T cells surrounding these follicles. The medulla contains medullary cords of cells (B cells, plasma cells and some macrophages) and between these cords is the medullary sinus lined with endothelial cells and macrophages.
Lymph nodes have two types of follicles primary and secondary. Secondary follicles contain germinal centres (sites of B-cell proliferation) and have three layers. The central dark zone contains a high density of dividing centroblasts, B cells without surface Ig. These centroblasts migrate to the Basal light zone (centerocytes) where they express surface Ig and become exposed to the follicular dendritic cells. Here there is a high rate of apoptosis but surviving cells migrate to the apical light zone or mantle zone, which contains cells which are destined to become B memory (lymphoblasts) or plasma cells (plasmablasts). Follicles in the cortex of a stimulated node are larger and have a pale germinal centre .
High endothelial venules contain a large number of aquaporin-1 channels allowing for a largeuptake of water which in turn drives lymph flow through the cortex. The venules are the source of most of the nodes T and B cells and express selectins (receptors for lymphocytes primed with antigens).
Pig Lymph Node
The structure of the pig lymph node is inverted compared with that of most mammals.
- Most follicles are found deep in the paracortex
- The paracortex is surrounded by loose medullary tissue
- Afferent lymphatics enter at the hilus
- Connect with para-trabecular sinuses and exit from efferent lymphatics on the node surface.
- Blood vessels enter and leave at the hilus
Histology
- Images will be re-arranged
Gross view
©RVC 2008
Gross view
©RVC 2008
Capsule
©RVC 2008
Medullary sinus
©RVC 2008
Trabecular sinus
©RVC 2008
Follicular layers
©RVC 2008
High endothelial venule
©Nottingham Uni 2008
Functions
As the spleen removes antigens from the blood, lymph nodes remove antigens from tissue. Antigen presenting cells (B and T cells) migrate from peripheral tissue via afferent lymphatic vessels to the lymph nodes where they present their antigen to lymphocytes. B cells and T cells enter via the high endothelial venules by diapedesis and B cells migrate to the cortex while T cells to the deep cortex.
Antibodies and immunologically competent cells leave the lymph nodes via the efferent lymphatics.
In pathology
Lymph node pathology can be found here