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.
The spleen is the largest lymphoid organ.
It receives blood from the splenic artery and is the only lymphoid organ that primarily filters blood instead of lymph.
It’s an encapsulated organ that’s typically about 12 cm in length, 7 cm wide, and 3 cm deep.
The spleen’s functional tissue or parenchyma consists of red pulp, with small white nodules of lymphatic tissue scattered throughout called the white pulp.
Although, when looking at the spleen histologically after it’s been stained with H&E, the red pulp is actually stained a combination of pink and purple; and the white pulp is stained dark purple because it contains a large number of basophilic nuclei.
If we take a closer look at the outer edge of the spleen, the capsule that surrounds the spleen is seen as a dense layer of pink connective tissue.
The connective tissue also forms short extensions into the spleen called trabeculae.
The trabeculae will also occasionally surround arteries as they enter the parenchyma of the spleen.
These trabecular arteries are branches of the splenic artery.
The remainder of the tissue seen in this image is the highly vascular red pulp.
Now, if we move to a different region of the spleen, we can see there are areas that are more basophilic and stain mainly purple.
The trabecular arteries from the previous image branch even more and become central arterioles.
Each central arteriole is surrounded by a periarteriolar lymphoid sheath or PALS for short, which is a cylindrical mass or sheath of mostly mature T cells that surround the central arteriole.