Atlas of plant and animal histology

Spleen is a lymphoid organ located in the upper part of the abdominal cavity, close to diaphragm. It shows an elongated shape and is triangular in transverse sections. It is the largest lymphatic organ of the body, about 0,2 % of the body weight. Together with the liver, spleen shows the most intense red color, which is related to its functions: storing and cleansing blood cells. Thus, it is involved in removing erythrocytes (haemolysis) and platelets, store platelets, monocytes and other blood cells. Spleen also produces lymphocytes and other blood cells (haematopoiesis), although spleen haematopoiesis only occurs during the late embryonic stages (fetal period).

The capsule is a layer of fibrous dense connective tissue, elastic fibers, and smooth muscle that covers the outer surface. A layer of mesothelium externally covers the capsule. Expansions from the capsule enter the organ and form the walls of compartments. Each of these compartments has an outer part or red pulp, and a central part or white pulp. Altogether, these compartments form the so-called parenchyma of the organ, while the remaining tissue is known as stroma, which is made up of connective tissue, reticular fibers and fibroblasts.

White pulp is composed of lymphatic tissue, mainly lymphocytes and macrophages, found around the arteries of the spleen, the so-called splenic arteries or central arteries. Three components are distinguished in the white pulp: periarteriolar lymphatic sheaths (PALS), follicles, and marginal zone. The main role of white pulp is produce and differentiate lymphocytes. Moreover, it is a place to store lymphocytes and may contain 1/4 of the body lymphocytes.

Red pulp filters the blood. It is a dense net of vein sinuses filled with blood. That is why the tissue shows a intense red color. Red pulp contains cords of tissue known as splenic cords (pulp cords or cords of Billroth). Splenic cords are composed of reticular fibers and cells, and macrophages. Reticular cells are like myofibroblast that help during the contraction of the organ. In the splenic cords there are red blood cells, granulocytes and monocytes, as well as lymphocytes and hemtopoietic cells. In the red pulp, macrophages remove damaged red blood cells and platelets, and also material recognized as foreign. Macrophages usually have cytoplasmic pigment depots. Red pulp is also a place for storing iron, red blood cells and platelets.

Splenic artery brings the blood to irrigate the spleen. This artery enters the spleen at point known as hilium, and then branches inside the organ to give rise to the central arteries of the white pulp. After that, central arteries branch off several times to become capillaries. Capillaries become vein sinuses of the red pulp. These veins join together to form the splenic vein that leaves the organ through the hilium. Efferent lymphatic vessels leave the spleen through the hilium. However, there is no afferent lymphatic vessels entering the spleen, i.e., there is no lymph coming to the organ. The thymus also has this feature.