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Mantle zone of germinal center

Figure 2 Spleen section showing the periarteriolar lymphoid sheath (PALS). This view shows the lymphoid tissue arranged around an arteriole. The T cells are found close to the central arteriole and the B cell area has a germinal center. In the unstimulated' state the B cell area consists of a primary follicle. The lymphoid tissue is separated from the red pulp by the marginal zone. This contains blood vessels and is the site of entry of bloodborne lymphocytes into the splenic lymphoid areas. H & E stain, x125. (Reproduced with permission from Roitt IM, Brostoff J and Male D (eds) (1998) Immunology, 5th edn. London: Mosby.)

Red pulp T cell area

Mantle zone of germinal center

Figure 2 Spleen section showing the periarteriolar lymphoid sheath (PALS). This view shows the lymphoid tissue arranged around an arteriole. The T cells are found close to the central arteriole and the B cell area has a germinal center. In the unstimulated' state the B cell area consists of a primary follicle. The lymphoid tissue is separated from the red pulp by the marginal zone. This contains blood vessels and is the site of entry of bloodborne lymphocytes into the splenic lymphoid areas. H & E stain, x125. (Reproduced with permission from Roitt IM, Brostoff J and Male D (eds) (1998) Immunology, 5th edn. London: Mosby.)

called penicilli which may terminate as such or become finer arterial capillaries. These terminal arterial capillaries may be surrounded by a sheath of closely packed macrophages (sheathed capillaries). A few arterial capillaries may connect with the venous sinuses, but many of them open directly in the reticular meshwork of the cellular cords. The venous sinuses are long vascular channels with a unique endothelium and basement membrane. Endothelial cells are elongated and lie parallel to the long axis of the vessel. They lie side by side with slit-like spaces separating them. The endothelium is perforated by large, regularly arranged, polygonal fenestrae. This arrangement is traditionally likened to a barrel in which the wooden staves correspond to the endothelium and the hoops to the reticular fibers that support the endothelial wall without a basement membrane. Sinuses are tributaries of the veins of the pulp, which in turn drain into the trabecular veins (Figure 1).

The cellular cords are supported by a reticular meshwork. The interstices of this cordal meshwork arc typically crowded with erythrocytes, macrophages, platelets, plasma cells and granulocytes.

Macrophages, which differentiate from circulating monocytes, are often the predominant cells.

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