The structures of the Spleen includes:
- Trabeculae, which extends from the capsule and brings blood and nerves.
- Splenic white pulp, which include tiny islands designed to facilitate immune responses. Appears blue in H&E. Branches of the splenic artery exit the capsule and trabeculae to enter the white pulp, becoming the central artery. Lymphocytes aggregate around the central artery, constituting the periarterial lymphatic sheath (PALS). PALS has a roughly cylindrical configuration and conforms to the course of the central artery. It is populated with T and B lymphocytes. PALS may be associated with lymphatic nodules (aka splenic nodules or malpighian corpuscles). These nodules appear when B lymphocytes encounter blood-borne antigens. A germinal center appears due to proliferation of B lymphocytes. Expansion of the nodules displaces the central artery to an eccentric position. In humans, germinal centers develop within 24 hrs after exposure to an antigen.
- Splenic red pulp, which is designed for the removal of defective blood cells. It contains RBCs and appears red in H&E. It consists of splenic sinuses and is separated by splenic cords (aka cords of Billroth). Splenic cords are a meshwork of reticular cells and fibers that contain larger numbers of red cells, macrophages, lymphocytes, plasma cells, dendritic cells, and granulocytes. Splenic venous sinuses are special sinusoidal vessels, lined by oddly shaped endothelial cells. There are few contact points between the cells (i.e. many gaps).
In terms of circulation, branches of the splenic artery enter the white pulp from the trabeculae, become central arteries. They then continue to the red pulp, where they branch into several straight arterioles called penicillar arterioles. Some are surrounded by macrophages, which are called sheathed capillaries. They empty directly into the meshwork of the red pulp (open circulation). In the other model, sheathed capillaries open directly into the splenic sinus (closed circulation). Experimental and morphologic evidence supported the open circulation model as it has more efficient exposure of red cells to the macrophages of the red pulp.