Thymic architecture

In sections, the thymus is divided into two zones: the cortex and medulla (Figure 1). The cortex is the dominant zone in healthy young animals and accounts for about 80% of thymic tissue. This region is packed with small lymphocytes and has a characteristic dark appearance in sections stained with hematoxylin, a reflection of the high nuclear-to-cyto-plasmic ratio of cortical lymphocytes (Figure 1A). The vast majority of lymphoid cells in the cortex are immature thymocytes expressing high levels of CD4 and CD8 molecules. These 'double-positive' (DP) cells express low levels of the <*p T cell receptor (TCR). Special stains reveal that DP thymocytes reside within a rich network of epithelial cells expressing major histocompatibility complex (MHC) molecules (Figure IB). As discussed later, MHC expression on cortical epithelium plays a crucial role in positive selection, the process by which a proportion of DP cells are instructed to differentiate into mature T cells.

The medullary region of the thymus contains mature T cells. These cells comprise a mixture of 'single-positive' (SP) CD4h8- and CIM"^ cells expressing the aft TCR. SP cells are intermingled with antigen-presenting cells, such as dendritic cells and macrophages. Like DP cells in the cortex, SP cells are enmeshed in a dense network of epithelial cells (Figure IB). The typical pale appearance of the medulla in hematoxylin-stained sections probably reflects the greater density of epithelial cells in the medulla than the cortex. In humans, medullary epi-

Figure 1 Histology of the mouse thymus. (A) Frozen thymus section stained with hematoxylin, a dye which stains the nucleus. Two zones of distinct cell density are visible: the cortex (cor) is densely packed with small lymphocytes, whereas the medulla (med) contains larger lymphocytes at a lower density. (B) A serial frozen thymus section stained with an antibody specific for epithelial cells and counterstained with hematoxylin. Note that a dense network of epithelial cells is apparent throughout the thymus: the epithelial cells in the cortex are dendritic in morphology with a thin cytoplasm, whereas the epithelial cells in the medulla show a more heterogeneous morphology.

Figure 1 Histology of the mouse thymus. (A) Frozen thymus section stained with hematoxylin, a dye which stains the nucleus. Two zones of distinct cell density are visible: the cortex (cor) is densely packed with small lymphocytes, whereas the medulla (med) contains larger lymphocytes at a lower density. (B) A serial frozen thymus section stained with an antibody specific for epithelial cells and counterstained with hematoxylin. Note that a dense network of epithelial cells is apparent throughout the thymus: the epithelial cells in the cortex are dendritic in morphology with a thin cytoplasm, whereas the epithelial cells in the medulla show a more heterogeneous morphology.

thelial cells form cystic structures termed Hassal's corpuscles; these structures are inconspicuous in mice.

In addition to «(3 T cells, the thymus contains small numbers of T cells expressing y8 TCRs. These cells are a minority population (<1% of total thymocytes) and are found scattered throughout the cortex and medulla.

Thymocytes, especially DP cells, are highly sensitive to the action of glucocorticoids and other steroids such as sex hormones. Because of this sensitivity to steroids, the thymus undergoes atrophy after the onset of puberty. Thymic atrophy is usually marked by mid-life, although remnants of thymic tissue are generally apparent even in old age. Thymic atrophy is also prominent in diseases associated with stress, e.g. chronic infection. With acute stress, thymic atrophy preferentially affects the cortex.

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