Spleen has a capsule of connective tissue rich in elastic tissue and smooth muscle, over the capsule is serosa. Trabeculae from the capsule extend into the interior and form complicated framework by anastomoses.
The spaces within the connective tissue framework are filled with a soft sponge like tissue known as splenic pulp. On the basis of colour difference seen in the fresh preparation different regions have been named as white pulp and red Pulp. Both types consists of lymphoid tissue, (i.e) reticular tissue with lymphocytes but with different arrangement and different relations to blood vessels.
The white pulp is composed of lymph nodules distributed diffusely throughout the substance of the spleen and passing through each nodule often eccentrically is a branch of an artery. These spherical or ovoid lymph nodules with an eccentrically placed artery passing through them are referred to as Splenic or Malpighian Corpuscle.
The Red pulp consists of anastomosing lymph cords or cords of Billroth between which are numerous venous sinuses filled with blood. The lymphoid tissue itself (i.e. spaces between reticular network and lymph cords) is infiltrated with erythrocytes, the number varying under different conditions. The presence of venous sinuses and infiltration with erythrocytes are responsible for the colour of red pulp.
The framework of the splenic pulp is made up of reticular tissue. The cells of the reticulum like those in the lymph node are of two types – Phagocytic cells belonging to the reticulo endothelial system and non-phagocytic undifferentiated cells.
Lymphocytes, large, medium and small are present forming the nodules in White pulp and lymph cords in red pulp. The Red pulp also contains monocytes, granular leucocytes, megakaryocytes or giant cells.
The structures of spleen are best understood by following the course of blood vessels, in their passage through the organ.
The Splenic artery enters the hilum and divides into a number of branches, which enter the trabeculae as trabecular arteries. After a short course in the trabeculae, the trabecular artery leaves the septa and enters the splenic pulp.
The tunica adventitia of the artery is replaced by reticular tissue, which is infiltrated with lymphocytes. The artery becomes enclosed within a sheath of dense lymphatic tissue or a nodule forming the splenic or Malpighian corpuscle. This artery is called the central artery or artery of white pulp, but this is actually located eccentrically within nodule. From this artery numerous capillaries are given off which supply the lymphoid tissue and then enter the red pulp.
The central artery after passing through the splenic corpuscle, becomes reduced in size, looses the sheath of lymphoid tissue and enters the red pulp. As it enters the red pulp the artery divides into a number of straight branches, which lie close together at the origin and later diverge like the bristles of a brush. These are called penicilliform artery.
Each penicillus pursues a course through the red pulp, which can be divided, into three successive parts. The pulp artery, sheathed artery and terminal arterial capillary.
The pulp artery is the longest of the three segments and has the thin coat of plain muscle surrounding the endothelium and external to the muscle in the tissue of red pulp. The pulp artery becomes smaller and divides into sheathed arteries.
The sheathed artery has no muscular layer and the endothelium is surrounded by a compact mass of concentrically arranged reticular cells. These become continuous peripherally with the reticulum of red pulp. This sheath is known as the Schweiger-Seidel sheath and the entire arrangements are referred to as an ellipsoid.
The sheathed artery divides into two or more terminal arterial capillaries. The terminal arterial capillary may have conical shaped enlargement or ampulla at its termination.
The exact manner in which these capillaries terminate has been a controversial subject. Some suggest that the empty into intercellular space of the red pulp reticulum and that blood finds its way from the pulp spaces into the venous sinuses through perforation in the walls of the sinuses. This is called the theory of open circulation. Other workers believe that capillaries empty directly into venous sinuses. This is called the theory of closed circulation. An intermediate view that some capillaries connect directly with the venuous sinuses and that others open into pulp spaces has also been put forward.
The venous sinuses form an anastomosing plexus through out red pulp. The cells lining the sinuses are elongated and are arranged longitudinally along the course of the sinus. The cell bodies are enlarged at the level of the nuclei and bulge at these sites into the lumen. “Surrounding these elongated cells are circularly disposed reticular fibres, which resemble the “staples” and “hoops” arrangement of a barrel.
Some believe that clefts are present between the lining cells through which blood may escape and other hold the view that a thin homogenous membrane exists between the cells. These lining cells are believed to be phagocytic and considered to be not a true endothelium but belonging to the reticuloendothelial system.
The venous sinuses unite to form large pulp veins or collecting venules line by endothelium. These venules reach the trabeculae to become trabecular veins which reaches the hilum to form the splenic vein.
Recent studies on circulation in spleen have shown that there is a cyclical activity in the venous sinuses, regulated by sphincters at various levels including the terminal capillaries.
The activity is in different phases and consists of ;
Conducting phase with all sphincters open all the blood enters the sinus at the afferent and leaves it at efferent and
Filtration phase –The sphincter at the efferent end closes, blood enters the sinus through the afferent end, the fluid passes rapidly through the sinus wall and the sinus gets gradually distended and packed with erythrocytes
The afferent sphincter also closes and the sinus remains distended with both sphincters closed for some time. Then the storage phase terminates with the opening of the efferent sphincter and the movement of erythrocytes into the collecting venules. The afferent sphincter also opens and the conducting phase begins.
Species differences
Mammalian spleens, are of different types: The storage spleen of horse, dog and cat is relatively large, rich in tarbeculae and muscle and poor in white pulp. The abundant red pulp serves as storage space of blood elements and the muscular framework serves to empty the spleen.
The defense spleen is small and has few trabeculae and smooth muscle fibres but abundant lymphatic tissue. The red pulp, the storage space of the spleen is poorly developed in the rabbit, only moderately so in man. The ruminant and swine belong to an intermediate type. The Ellipsoids are large in the pig and are will developed in the dog and cat.
