Spleen:
is the largest accumulation of lymphoid tissue in the body, lies in the left hypochondrium beneath the diaphragm extending in the area between 9th & 11th ribs.Histological structure:
The spleen is surrounded by connective capsule containing few smooth muscle fibers from which connective tissue trabeculae extend even to reach the red pulp containing nerves & "trabecular vessels".
The spleen is formed of 2 areas: white pulp & red pulp.
The white pulp also called "splenic pulp" or "Malpighian corpuscle" is formed of aggregations of lymphocytes which are mainly B-lymphocytes arranged as multiple lymphatic nodules differ from that of the lymph node in that they contain "central artery" which on sectioning under light microscope appears in the center of the nodule surrounded by a sheath of T-lymphocytes, it could be eccentric, sometimes two sections of the central artery is seen in one lymphatic nodule, or may be the nodule shows no central artery.
The red pulp is formed of two elements:
splenic cords: which are formed of lymphocytes (mainly B-lymphocytes) with all types of blood cells (RBCs, granular leukocytes, platelets, plasma cells & macrophages) arranged in the form of thin cords.
venous sinusoids: are irregular dilated blood spaces containing blood (which differ from the medullary sinuses of the lymph nodes that contain lymph) lie between the splenic cords, they are lined with cubical epithelium with gaps in between the cells lie on an incomplete basal lamina & they are surrounded by reticular fibers& macrophages.
The "marginal zone" lies between the white & red pulp containing many blood sinuses, lymphocytes & macrophage, it encircle the nodules of the white pulp & play a role in filtering the blood & initiating an immune response because the macrophage remove the antigen debries.
Circulation of the spleen:
The hilus is an indentation in the medial surface of the spleen through which the splenic artery enters then it branches into trabecular arteries that follow the course of the connective tissue trabeculae to the inside of the spleen, the arteries run side by side with the veins called trabecular vessels.
then the arteries leave the trabeculae & enter the parenchyma, immediately enveloped by a sheath of T-lymphocytes these vessels called "central arteries" the lymphocytic sheath thickened gradually around the artery as far it passes in the red pulp till they form spherical nodules (white pulp) in which the arteries may occupy eccentric position.
The central artery extends to the periphery of the white pulp &subdivides into many straight, symmetrical branches which are similar to the branches of a teethbrush, these are called pencilli or "pencillar arteries, each of which terminates as 3 divisions:
Pulp arteries: true arteries that are the wall is thick formed of circularly arranged smooth muscle fibers.
Sheathed artery or (elipsoial arteries): near termination some of the penicillar arteries become surrounded by a sheath of reticular fibers with macrophages (so they are not true arteries because their walls contain no smooth muscle fibers).
Arterial capillaries: are normal capillaries that carry the blood to the venous sinusoids.
The manner in which blood flows from the arterial capillaries to the venous sinusoids has not been completely explained but 3 suggested theories:
Closed theory: the arterial capillaries open directly into the venous sinusoids.
Open theory: the blood from the arterial capillaries is rushed into the spaces between the red pulp cords then moves on to be collected by the venous sinusoid.
Combined theory: that the connection between the arterial & venous side is closed when the spleen is contracted,& it is open when the spleen is relaxed.
From the sinusoids the blood proceeds to the red pulp veins that joined together & enter the trabeculae forming the trabecular vein which drain into splenic vein which emerge from the hilum of the spleen.
Function of Spleen:
formation of lymphocyte.
immunity for the body.
development of new RBCs which are stored in the spleen thus it act as a reservoir for the blood. It can give blood whenever it is needed such as after car accident.
filtration of blood.
destruction of old RBCs. which are destroyed in the spleen & the iron which is left is used for the formation of new RBCs.