The spleen is the largest lymphatic organ; it is surrounded by a capsule and located in the path of the bloodstream (splenic artery and vein). The spleen filters the blood and reacts immunologically to blood-borne antigens. It has both morphologic and immunologic filtering functions. The substance of the spleen, the splenic pulp, consists of red pulp and white pulp, so named because of their appearance in fresh tissue. The white pulp is rich in lymphocytes that form a periarterial lymphatic sheath (PALS) around branches of the splenic artery that penetrate the white pulp. The red pulp contains large numbers of red blood cells that it filters and degrades. Aged, damaged, or abnormal red blood cells are trapped by macrophages associated with unusual vascular sinuses in the red pulp. These macrophages break down the red cells, begin the metabolic breakdown of hemoglobin, and retrieve and store the iron from the heme for reutilization in the formation of new red blood cells in the bone marrow.
Figure 1, spleen, human, H&E x65.
This low-magnification micrograph of the spleen reveals its two major components, the red pulp (RP) and white pulp (WP). In the center of the figure, there is a trabecule containing a blood vessel, a trabecular vein (TV) through which blood leaves the organ. The red pulp constitutes the greater bulk of the splenic tissue. In life, the red pulp has pulp-like texture; it is red as a result of the natural coloration of the numerous red blood cells present, hence its name.
The white pulp, on the other hand, is so named because its content of lymphocytes appears in life as whitish areas. In tissue sections, however, the nuclei of the closely packed lymphocytes impart an overall blue-staining response. The lymphatic tissue that constitutes the white pulp differs from nodules seen elsewhere in that it follows and ensheathes a blood vessel, the central artery. The lymphatic tissue surrounding the artery exhibits periodic expansion, thus forming the nodules. When this occurs, the central artery (CA) is displaced peripherally within the nodule.
In those regions where the lymphatic tissue is not in nodular form, it is present as a thin cuff around the central artery and is referred to as the periarterial lymphatic sheath. If the plane of section does not include the artery, the sheath may appear only as a localized and irregular aggregation of lymphocytes.
Figure 2, spleen, human, H&E x160.
This figure reveals, at a higher magnification, the red pulp and a portion of the trabecular vein from the area enclosed in the uppermost rectangle in Figure 1. The red pulp is composed of two elements: venous sinuses (VS) and the splenic cords (of Billroth), the tissue that lies between the sinuses. In this specimen, the venous sinuses can be seen to advantage because the red blood cells in the sinuses have lysed and appear as unstained "ghosts"; only the nuclei of the white cells are readily seen. (This is better shown in Plate 36.) The paler, unstained areas thus represent the sinus lumina.
Figure 3, spleen, human, H&E x240.
This figure reveals, at higher magnification, the splenic nodule in the rectangle in the right portion of Figure I. Present are a germinal center (GC) and a cross section through the thick-walled central artery (CA). As noted above, the central artery is eccentrically placed in the nodule. The marginal zone (MZ) is the area that separates white pulp and red pulp (RP). Small arterial vessels and capillaries, branches of the central artery, supply the white pulp, and
Near the top of the micrograph, two venous sinuses (arrows) empty into the trabecular vein (TV), thus showing the continuity between venous sinuses and the trabecular veins. The wall of the vein is thin, but the trabecula (T) containing the vessel gives the appearance of being part of the vessel wall. In humans as well as in other mammals, the capsule and the trabeculae that extend from the capsule contain myofibroblasts. Under conditions of increasing physical stress, contraction of these cells will occur and cause rapid expulsion of blood from the venous sinuses into the trabecular veins and, thus, into the general circulation.
some pass into the reticular network of the marginal zone, terminating in a funnel-shaped orifice. Venous sinuses are also found in the marginal zone, and occasionally, arterial vessels may open into the sinuses. The details of the vascular supply are, at best, difficult to resolve in typical H&E preparations. The penicillar arterioles, the terminal branches of the central artery, supply the red pulp but are likewise difficult to resolve.
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