The spleen is derived from condensation of the mesoderm in the dorsal mesogastrium of the embryo. It plays a modest haematopoietic role in the middle part of fetal life, but in the adult haematopoiesis is usually only seen in pathological states. An average adult spleen weighs about 150 g and it has to become enlarged to at least three times its normal size before becoming palpable on clinical examination (p. 17).
The splenic artery penetrates the thick capsule which invests the organ (Fig 5.1). Branches of the splenic artery are surrounded by a highly organised aggregate of lymphoid tissue which is termed the ‘white pulp’ (Fig 5.2). Intimate to the central arteriole is the ‘periarteriolar lymphatic sheath’ – an area mainly populated by T-lymphocytes. Among these T-lymphocytes are non-phagocytic, antigen-presenting cells known as ‘interdigitating cells’. Spaced at intervals in the periarteriolar lymphatic sheath are lymphoid follicles (‘Malpighian bodies’). In an inactive state these follicles are composed of recirculating B-lymphocytes intertwined with cytoplasmic processes of follicular dendritic cells. The latter cells may play a role in long-term antibody production. When contact with antigen stimulates B-cell activation, a germinal centre of rapidly dividing cells forms in the follicle. This is a key area in the normal B-lymphocyte proliferative response and development of B-cell memory (see p. 8 for discussion of lymphocytes).
The periarteriolar lymphatic sheath and B-lymphocyte follicles are separated from the red pulp by a ‘marginal zone’ constituted mainly of non-circulating B-cells. The marginal zone also contains specialised macrophages able to take up carbohydrate antigens. The red pulp is composed of two alternating structures: the splenic sinuses and the splenic cords (the ‘cords of Billroth’). The cords are a reticular meshwork packed with macrophages and antibody-secreting plasma cells. The sinuses are broad channels lined with fusiform endothelial cells.
Most of the central arterioles open into the marginal zone. As alluded to already, circulating T-lymphocytes move into the periarteriolar lymphatic sheath and B-lymphocytes migrate to the follicles. Other blood cells move slowly through the complex meshwork of the red pulp, and cells which are sufficiently deformable and compliant squeeze between the endothelial cells in the sinus wall into the lumen of the sinus and back into the circulation. The organisation of the spleen into the different compartments is under the control of various cytokines and adhesion molecules.
