Figure 1328

Schematic drawing of the major steps in thymic education. The process of multipotential lymphatic stem cell (CFU-L) maturation and differentiation into immunocompetent T cells is accomplished by the expression and deletion of specific surface CD antigens. The CFU-L stem cells enter the medulla of the thymus via a postcapillary venule and then migrate to the periphery of the thymic lobule. The presence of CD2 and CD7 molecules on the cell surface indicates an early stage of differentiation. This is followed by expression of the CD1 molecule, indicating the mid-stage of T cell differentiation. As maturation progresses, the cells express TCRs, CD3, CD4, and CD8 molecules. These cells are then presented with self and foreign antigens by type II and III epithe-lioreticular (ere) cells. If the lymphocyte recognizes self MHC and self or foreign antigen, It will survive the selection (positive selection); if not, death of the cell will occur. Cells that pass the positive selection test leave the cortex and enter the medulla. Here they undergo another selection process in which cells directed to self-antigen displayed by self MHC are eliminated (negative selection). Cells that survive that selection then become either cytotoxic CD8+ T lymphocytes or helper CD4+ T lymphocytes. These cells are now ready for the immune response; they leave the thymus from the medulla and enter the blood circulation. Hormonal substances secreted by type VI epithelioreticular cells within the thymic (Hassall's) corpuscle promote the process of thymic cell education. Note the distribution of all six types of epithelioreticular cells.

many mammals the spleen holds large volumes of red blood cells in reserve. In these species, contraction in the capsule and trabeculae helps discharge stored red blood cells into the systemic circulation. The human spleen normally retains relatively little blood, but it has the capacity for contraction by means of the contractile cells in the capsule and trabeculae.

The hilum, located on the medial surface of the spleen, is the site for the passage of the splenic artery and vein, nerves, and lymphatic vessels. The lymphatic vessels originate in the white pulp near the trabeculae and constitute a route for lymphocytes leaving the spleen.

White pulp consists of a thick accumulation of lymphocytes surrounding an artery

The white pulp consists of lymphatic tissue, mostly lymphocytes. In H&E-stained sections, white pulp ap pears basophilic because of the dense heterochromatin in the nuclei of the numerous lymphocytes. Branches of the splenic artery course through the capsule and trabeculae of the spleen and then enter the white pulp. Within the white pulp, the branch of the splenic artery is called the central artery. Lymphocytes that aggregate around the central artery constitute the periarterial lymphatic sheath (PALS). The PALS has a roughly cylindrical configuration that conforms to the course of the central artery. In cross sections, the PALS appears circular and may resemble a lymphatic nodule. The presence of the central artery, however, distinguishes the PALS from typical lymphatic nodules found in other sites. Nodules appear as localized expansions of the PALS and displace the central artery, so that it occupies an eccentric rather than a central position.

The nodules are the territory of B lymphocytes; other lymphocytes of the PALS are chiefly T lymphocytes that

splenic germinal nodulel center (white I marginal pulp) area trabecular artery and vein central artery

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