Figure 1329

Schematic diagram and photomicrograph of splenic structure, a. The substance of the spleen is divided into white pulp and red pulp. White pulp consists of a cylindrical mass of lymphocytes arranged around a central artery that constitutes the periarterial lymphatic sheath (PALS). Splenic nodules occur along the length of the PALS. When observed in cross section through part of the sheath that contains a nodule, the central artery appears eccentrically located with respect to the lymphatic mass. The red pulp consists of splenic sinuses surrounded by splenic cords (cords of Billroth). A capsule surrounds the spleen and from it trabeculae project into the substance of the spleen. Both capsule and trabeculae give the appearance of dense connective tissue infiltrated by numerous myofibroblasts. Blood vessels traverse the capsule and trabeculae before and after passage within the substance of the spleen. Lymphatic vessels originate in the white pulp near the trabeculae. b. This low-magnification photomicrograph of the spleen reveals the same components shown in the previous drawing. Note the capsule with several trabeculae projecting into the substance of the spleen. In the center, there is a trabecula containing a trabecular vein through which blood leaves the organ. The red pulp constitutes the greater bulk of the splenic tissue. The white pulp contains lymphatic tissue that follows and ensheathes the central artery. Expansion of the white pulp creates the splenic nodules. x45.

splenic germinal nodulel center (white I marginal pulp) area splenic cords (red pulp) lymph vessel trabecular artery and vein central artery venous sinuses red pulp)

surround the nodules. Thus, the PALS may be considered a thymus-dependent zone similar to the deep cortex of a lymph node. The nodules usually contain germinal centers, which, as in other lymphatic tissues, develop as B cells proliferate following their activation. In humans, germinal centers develop within 24 hours after antigen exposure and may become extremely large and visible with the naked eye. These enlarged nodules are called splenic nodules or Malpighian corpuscles (not to be confused with the renal corpuscles that have the same name).

Red pulp contains large numbers of red blood cells that it filters and degrades

Red pulp has a red appearance in the fresh state as well as in histologic sections because it contains large numbers of red blood cells. Essentially, red pulp consists of splenic sinuses separated by splenic cords (cords of Billroth). Splenic cords consist of the now-familiar loose meshwork of reticular cells and reticular fibers that contain large numbers of erythrocytes, macrophages, lymphocytes, plasma cells, and granulocytes. Splenic macrophages phagocytose damaged red blood cells. The iron from destroyed red blood cells is used in the formation of new red blood cells; splenic macrophages begin the process of hemoglobin breakdown and iron reclamation. Megakaryocytes are also present in certain species, such as rodents and the cat, but not in humans except during fetal life.

The splenic or venous sinuses are special sinusoidal vessels lined by rod-shaped endothelial cells

The endothelial cells that line the splenic sinuses are extremely long. Their longitudinal axis runs parallel to the direction of the vessel (Fig. 13.30). There are few contact points between adjacent cells, thus producing prominent intercellular spaces. These spaces allow blood cells to pass readily into and out of the sinuses. Processes of macrophages extend between the endothelial cells and into the lumen of the sinuses to monitor the passing blood for foreign antigens.

The sinuses do not possess a continuous basal lamina. Strands of basal lamina loop around the outside of the sinus much like the hoops that loop around the staves of a barrel. These strands are at right angles to the long axes of the endothelial cells. This material stains with silver-containing reagents or with the PAS reaction. Neither smooth muscle nor pericytes are present in the wall of splenic sinuses. Reticular cell processes may extend to the basal side of the endothelial cells and are probably associated with the reticular fibers that appear to merge with the perisinusoidal loops of basal lamina. Blood fills both the sinuses and cords of the red pulp, often obscuring the underlying structures and making it difficult to distinguish between the cords and the sinuses in histologic sections.

Circulation within red pulp allows macrophages to screen antigens in the blood

Branches of the splenic artery enter the white pulp from the trabeculae. The central artery sends branches to the white pulp itself and to the sinuses at the perimeter of the white pulp, called marginal sinuses (see Fig. 13.29). The central artery continues into the red pulp, where it branches into several relatively straight arterioles called penicillar arterioles. The penicillar arterioles then continue as arterial capillaries. Some arterial capillaries are surrounded by aggregations of macrophages and are thus called sheathed capillaries. Sheathed capillaries then empty directly into the reticular meshwork of the splenic cords rather than connecting to the endothelium-lined splenic sinuses. Blood entering the red pulp in this manner percolates through the cords and is exposed to the macrophages of the cords before returning to the circulation by squeezing through the walls of the splenic sinuses (Fig. 13.31). This type of circulation is referred to as open circulatioît, and it is the only route by which blood returns to the venous circulation in humans. In other species such as the rat and dog, some of the blood from the sheathed capillaries passes directly to the splenic sinuses of the red pulp. This type of circulation is referred to as closed circulation.

Open circulation exposes the blood more efficiently to the macrophages of the red pulp. Both transmission and scanning electron micrographs often show blood cells in transit across the endothelium of the sinus, presumably reentering the vascular system from the red pulp cords. The blood collected in the sinuses drains to tributaries of the trabecular veins that converge into larger veins and eventually leaves the spleen by the splenic vein. The splenic vein in turn joins the drainage from the intestine in the hepatic portal vein (see page 535).

The spleen performs both immune and hemopoietic functions

Because the spleen filters blood as the lymph nodes filter lymph, it functions in both the immune and the hemopoietic systems.

Immune system functions of the spleen include

• Antigen presentation by APCs and initiation of immune response

• Activation and proliferation of B and T lymphocytes

• Production of antibodies against antigen present in circulating blood

• Removal of macromolecular antigens from the blood

• Proliferation of lymphocytes and differentiation of B cells and plasma cells, as well as secretion of antibodies, occur in the white pulp of the spleen; in this regard, the white pulp is the equivalent of other lymphatic organs

Hemopoietic functions of the spleen include

• Removal and destruction of senescent, damaged, and abnormal erythrocytes and platelets

• Retrieval of iron from erythrocyte hemoglobin

• Formation of erythrocytes during early fetal life

• Storage of blood, especially red blood cells, in some species

The role of the red pulp is primarily blood filtration, i.e., removal of particulate material; macromolecular antigens; and aged, abnormal, or damaged blood cells and platelets from the circulating blood. These functions are accomplished by the macrophages embedded in the reticular meshwork of the red pulp. Senescent, damaged, or abnormal red cells are broken down by the lysosomes of the macrophages; the iron of the hemoglobin is retrieved and stored as ferritin or hemosiderin for future recycling. The heme portion of the molecule is broken down to bilirubin, which is transported to the liver via the portal system and there conjugated to glucuronic acid. Conjugated bilirubin is secreted into the bile, giving it a characteristic color.

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