The spleen is a brown ovoid organ that forms part of the reticuloendothelial system. It is approximately 12cm x 7cm, roughly the size of a clenched fist. The spleen is mobile with respiration. It is an intraperitoneal structure apart from at the hilum where blood vessels enter and leave.
The spleen starts to develop at the 4-5th week of foetal life. It develops between the layers of dorsal mesogastrium as lobules. As the stomach rotates the mesogastrium fuses with the peritoneum covering the left kidney. This is the origin of the splenorenal ligament. The differentiation of mesenchymal cells forms the splenic capsule and parenchyma.
The spleen is a relatively superficial organ and lies from the 9-11th rib in the left hypochondrium. It is not palpable except in pathological states.
Macro-Anatomy (e.g. lobes, surfaces, impressions etc)
The spleen is an ovoid shaped organ covered by a loose capsule that allows expansion. The spleen has a smooth diaphragmatic and an irregular visceral surface with impressions for the stomach, left kidney, left colic flexure and pancreas. The hilum is the site of entry and exit of the splenic vessels and the splenorenal and gastrosplenic ligaments attach here. The spleen is attached to the greater curve of the stomach via the gastrosplenic ligament, and to the left kidney via the splenorenal ligament.
The internal structure of the spleen is very important for its function and has a number of specialised systems. A supporting network of fibroelastic tissue forms the capsule and trabeculae. Trabeculae are fibrous tissue bands that extend into the parenchyma to provide internal structure. The major histological subtypes of splenic tissue are:
- White pulp: contains lymphocytes and macrophages, arranged around the arteries. The white pulp is responsible for the immune response.
- Red pulp: highly vascular parenchyma, sinusoids (highly permeable specialised capillary system). It is the site of erythrocyte filtration and storage of iron, erythrocytes and platelets.
- Marginal zone: this is the area between the white and red pulp, it is involved in immunity.
The arterial network of the spleen is uniquely adapted to its function. As the splenic artery enters the hilum it divides into smaller branches (trabecular arteries) that penetrate the parenchyma. Arterioles branch from the trabecular arteries and enter the red pulp to form central arterioles lined by lymphoid cells.
The splenic artery, the largest branch of the coeliac trunk, passes along the posterior surface of the pancreas, between the layers of the splenorenal ligament to reach the hilum of the spleen. At the hilum it divides into five or more branches. The venous drainage of the spleen is via the splenic vein which unites with the superior mesenteric vein to form the portal vein.
The spleen only has efferent lymph vessels which leave at the hilum.
Nerve Supply (link to dermatome and referred pain as appropriate)
The coeliac plexus provides the sympathetic nerve supply.
Physiology (e.g. functions, relationships to other body systems)
The major functions of the spleen are:
- Erythropoiesis: secondary site of red blood cell production during foetal life, this function ceases at birth.
- Erythrocyte removal: defective red blood cells are removed from the circulation.
- Storage: neutrophils and one third of all platelets are stored in the spleen, ready for release when required.
- Immune defence: synthesis of antibodies, removal of encapsulated bacteria, trapping and processing of antigens
Splenic rupture is largely secondary to trauma, although neoplasia, infection and splenic infiltration can cause atraumatic rupture. Both penetrating and blunt trauma can cause splenic rupture. The spleen is vulnerable to rupture due to the thin capsule and high vascularity. In a hypotensive patient with left upper quadrant pain, splenic rupture should be high on the differential list. Treatment is largely by partial or total splenectomy. Post-splenectomy, patients are more susceptible to certain infections but otherwise splenectomy has few long term side effects due to the other reticuloendothelial organs compensating.
This is a benign embryological variant that occurs in approximately 10% of the population. It is caused by failure of the splenic lobules to fuse during development. It is an incidental finding and requires no specific treatment.
Development: dorsal mesoderm in dorsal mesogastrium
Artery: splenic artery, origin: coeliac trunk
Vein: splenic vein into portal vein
Lymph: efferent lymph vessels only
Nerve: coeliac plexus
The spleen is the largest lymphoid organ in the body. It is only palpable in pathological states and due to it’s superficial anatomical location is vulnerable to traumatic injury.
Essential Clinical Anatomy. 3rd edition. Moore and Agur.