NORMAL DEVELOPMENT, ANATOMY, AND FUNCTION
The spleen develops from mesenchymal cells that migrate during embryogenesis into the dorsal mesogastrium. Spleen development begins during the fifth week of gestation. The mesenchymal cells of the splenic precordium differentiate to form the splenic parenchyma, the connective tissue framework, and the capsule. The mesenchymal tissue elements of the developing spleen fuse into a single structure by the third gestational month, at which time the characteristic lobulated configuration of the fetal spleen is present.
Anomalous development of the spleen can lead to abnormalities of morphology or location. The most common anomaly of splenic development is isolation of splenic tissue from the main portion of the organ, resulting in the formation of an accessory spleen. Complete lack of spleen formation is a component of the most common form of heterotaxy syndrome, that is, asplenia. Heterotaxy can also be associated with multiple foci of splenic development, that is, polysplenia. Splenic-gonadal fusion is a rare developmental lesion that relates to the close proximity of the left gonadal anlage to the developing spleen in the left dorsal mesogastrium. Wandering spleen is a developmental lesion that is caused by deficient formation of various ligaments in the left upper quadrant.1,2
The major components of the spleen are lymphoid tissue, erythrocytes, and reticuloendothelial cells. It is the largest unit of lymphatic tissue in the body. The primary functions are filtering blood and participation in the immune response. The 2 anatomic divisions of the splenic parenchyma are the white pulp and red pulp. The white pulp contains lymphocytes, plasma cells, and macrophages. Structurally, the white pulp is a sheath of lymphatic tissue that surrounds the intrasplenic arteries, with thickening of this sheath in areas to form lymphatic nodules. The red pulp is the predominant component of the spleen. The red pulp comprises a network of tortuous branching venous sinuses and cylindric partitions of connective tissue, that is, the splenic cords. The red pulp contains phagocytic cells, and is the site where senescent or damaged erythrocytes are removed from the circulation. There is exaggerated splenic red blood cell destruction in patients with abnormal erythrocyte morphology (e.g., hereditary spherocytosis) or abnormal antibody coating of erythrocytes (e.g., the immune hemolytic anemias).
The spleen serves to filter certain bacteria from the blood and to produce opsonizing antibodies. These functions are particularly important during the first 2 years of life, before specific immunity to various bacteria has fully developed. The splenic bacterial filtering mechanism is most effective for those organisms that have a polysaccharide capsule. This filtering does not require the presence of specific antibodies. Children who lack a spleen or who have deficient splenic function are prone to develop infections with organisms such as Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis, Escherichia coli, Staphylococcus aureus, and group A streptococci. The spleen also contributes to the systemic immune response by producing antibodies.