[SPLEEN HISTOLOGY]. Spleen is a large lymphoid organ that performs two main functions: immune reaction to blood borne foreign antigens and filtration of blood of particulate substances/debris. Each of these two functions is performed by specific compartments known as the "White pulp" and the "Red Pulp" respectively. The white pulp, which appears white on gross examination, is essentially a collection of both B and T-lymphocytes (top arrow), whereas the red pulp, which appears red on gross examination, is the intervening splenic tissue that consists of sinuses and cords (lower arrowhead).
[SPLEEN HISTOLOGY]. Spleen is covered with a fibrous capsule from which trabeculae enters into the parenchyma. The fibrous trabeculae ramify throughout the spleen and form supportive sheathing around blood vessels. The main splenic artery branches as it enters the hilum and ramifies successively into smaller arteries along trabecular collagenous sheaths. The smaller arteries give rise to arterioles (arrow) which are covered by lymphoid tissue, called 'periarteriolar sheath' (arrowhead), instead of trabecular collagen sheath. The periarteriolar lymphoid tissue is composed of both T and B-cells. The T-cells form a rather uniform concentric sheath of small lymphocytes around arterioles, whereas the B-cells form lymphoid follicles which are seen as eccentric bulging along the arteriole.
[SPLEEN HISTOLOGY]. The proximal arterioles give rise to distal branches at right angles called penicillar arterioles, which eventually give rise to terminal capillaries that either open into the splenic cords (open splenic circulation system) or directly join the venules bypassing the cords (closed splenic circulation system). This image shows an arteriole with its eccentric periarteriolar lymphoid tissue in the form of a B-lymphoid follicle. Unstimulated lymphoid follicles do not contain germinal centers (primary follicles), whereas lymphoid follicles stimulated by blood borne antigens form germinal canters (secondary follicles).
[SPLEEN HISTOLOGY]. A B-lymphoid follicle around an eccentrically placed arteriole (big arrow) is shown. The denser inner collection of conentric lymphoid tissue is the mantle zone whereas the paler outer collection of concentric lymphoid tissue is the marginal zone. The terminal/penicillar branch of an arteriole (shown with an arrowhead on the right) gives rise to capillaries in or around the marginal zone of the follicle. This 'perifollicular zone' is the boundary between white and red pulp and serves as the area where macrophages are abundant supported by a dense reticulin meshwork. The macrophages serve as the 'custom officers' for the newly entered red and white blood cells and particulate material.
[SPLEEN HISTOLOGY]. Each B-lymphoid follicle contains a distinct marginal zone of lymphocytes (outer rim of lymphocytes shown with an arrowhead) around the mantle zone of the follicle (inner rim of lymphocytes shown with a thin long arrow). The marginal zone consists of loosely arranged lymphocytes and where capillaries release blood to be delivered into the red pulp. Blood from capillaries (which enters the open circulation system) is delivered to splenic cords where blood comes into direct contact with resident macrophages.
[SPLEEN HISTOLOGY]. Marginal zone of the spleen consists of loosely packed small B-lymphocytes (long arrow) with embedded resident macrophages (arrowheads). The macrophages are the first point of contact for the delivered red blood cells, white blood cells, and any particulate material. Abnormal red blood cells, cellular inclusions, bacteria, and other 'unwanted' particulate material is removed by macrophages.
[SPLEEN HISTOLOGY]. The red pulp is the area of spleen in between white pulp and consists of open sinuses and cellular cords. The red pulp can be expanded markedly by venous congestion, extramedullary hematopoiesis, or by involvement by leukemias.
[SPLEEN HISTOLOGY]. Splenic sinuses are open vascular spaces lined by a discontinuous layer of endothelial cells and supported by a fenestrated basal lamina and reticular fibers. The surrounding cellular splenic cords provide a tissue framework maintaining the network of sinuses. This image shows four sinuses joining to form a larger sinus, which will ultimately opens up into a venule and thus the venous system.
[SPLEEN HISTOLOGY]. The microanatomy of a sinus is shown in a longitudinal section. The sinus is lined by a discontinuous population of endothelial cells (curved arrows) with slit-like spaces (long arrow). However, larger gaps in endothelial coverage are often present that serve as the conduit for blood cells to pass from surrounding splenic cords into the sinuses (three tailed arrow marking the oblique gaps in endothelial coverage). The splenic cords contain passing red blood cells, lymphocytes, monocytes, granulocytes, in addition to resident reticular fibroblasts, plasma cells, and macrophages. The sinus contains all the blood elements.
[SPLEEN HISTOLOGY]. The microanatomy of a sinus is shown in a transverse section. The curved arrow indicates an endothelial cell. A red blood cell is moving from the cord into the sinus (long arrow). Deformed or less pliable red blood cells cannot squeeze effectively from the cord into the sinus and upon their mechanical fragmentation are removed by resident macrophages. Note the wide gaps between endothelial cells which allow for movement of entire cells from cords into sinuses.
[SPLEEN HISTOLOGY]. An immunohistochemical staining for the B-cell marker CD20 shows large B-lymphoid follicles and a paucity of B-cells in the intervening red pulp. In response to soluble antigens the B-cells proliferate and give to memory B-cells and immunoglobulin-producing plasma cells. The circulating immunoglobulins serve to neutralize soluble antigens in the blood. The CD4+ T-cells in the periarteriolar sheath serve a helper function to B-cells.
[SPLEEN HISTOLOGY]. A large cuff of CD3+ T-cells is present around arterioles (arrow) forming the T-cell compartment of the periarteriolar sheath. The T-cells in the spleen provide the same function as they do in a lymph node.
[SPLEEN HISTOLOGY]. A longitudinal section of an arteriole (arrow) with its T-cell component (arrowhead) of the lymphoid sheath. The B-lymphoid follicles are present eccentrically at some point along the length of the periarteriolar sheath and merge imperceptibly with the T-cell component.
[SPLEEN HISTOLOGY]. Immunohistochemical staining shows that most of the T-lymphocytes are CD4+ helper T-cells.
[SPLEEN HISTOLOGY]. Immunohistochemical staining shows that only a very small population of of the T-lymphocytes are CD8+ cytotoxic T-cells.
[SPLEEN HISTOLOGY]. The sinus endothelial cells also show positive immunohistochemical staining for CD8 marker.