Lymph node histology

Now, let's get a closer look. The lymphatic system is an essential part of the immune system and it consists of a network of lymphatic vessels, tissues and organs.

The lymphatic vessels drain interstitial fluid or lymph from peripheral tissues back into the blood lymphoid tissue. And organs contain a lot of lymphocytes and other white blood cells.

The primary lymphoid organs include the thymus and bone marrow and the secondary lymphoid organs include the tonsils, lymph nodes, spleen and mucosa associated lymphoid tissue or malt for short lymph nodes are small secondary lymphoid organs that are found along lymphatic vessels throughout the body.

They're encapsulated bean shaped structures that usually have a diameter of about one centimeter along the short axis and 2.5 centimeters along the long axis.

And they support the immune system by filtering the lymph in order to identify and fight infections. If we zoom closer, we can more easily identify the outer capsule of connective tissue as well as the three functioning regions of the lymph node just beneath the capsule is the outer cortex which contains spherical nodules or follicles of B cells.

Each with a germinal center similar to the follicles of the spleen. The germinal center is where B cells differentiate into plasma cells.

The next region is the inner cortex or pericortex, which doesn't have any nodules. And finally, the innermost region of the lymph node is the medulla.

The distinction between the inner cortex and medulla is hard to see at this magnification. But the medulla will have cords of lymphoid tissue as well as passageways for lymph called medullary sinuses.

These sinuses eventually join one another and drain into the efferent lymphatic vessels. If we take a closer look at the lymph nodes capsule, there are spaces underneath the capsule called subcapsular sinuses.

These sinuses contain lymph that has drained into the lymph node from afferent lymphatic vessels similar to the spleen. The capsule also extends into the parenchyma and forms trabecula of connective tissue.

But these trabecula can also have narrow spaces surrounding them called trabecular sinuses. The trabecular sinuses allow the lymph to flow deeper into the cortex from the subcapsular sinuses.

All right. The next region of the lymph node is the cortex.

The outer cortex has the distinct follicles of B cells, but the inner cortex is slightly more eosinophilic or pink and has a more homogenous appearance.

Overall, the inner cortex consists mostly of T cells but also has high endothelial venules or he vs which are unique to this region and can help differentiate the inner cortex from the medulla.

The image on the right is a cross section of an H EV. And the image on the left is a longitudinal section of an H ev.

The endothelial cells of venules are typically flat, but these venules have unusually tall or high endothelial cells. These endothelial cells allow lymphocytes in the venules to move out of the blood and into the surrounding lymphoid tissue.

Now, if we take a closer look at the innermost region of the lymph node, we can see that the medulla has a lighter or brighter appearance compared to the inner cortex because it's not as densely packed with cells.

That's because the medulla has many open spaces or areas that form the medullary sinuses, which is where lymph from the cortex collects before exiting the lymph node through the efferent lymphatic vessel.

The shape of the sinuses is formed by the surrounding medullary cords, which are the irregular cords that contain macrophages.

B cells and plasma cells. In this image, we can also see a few blood vessels within the medullary cord as well.