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Mid-semester essay exam notes - Thymus

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Monash University

B5.1 Thymus The thymus is a specialized organ of the immune system. The thymus is derived from the 3 pharyngeal pouches. The development of the thymus is a series of epithelial/mesenchymal inductive interactions between neural crest-derived arch mesenchyme and pouch endoderm. Unlike most other lymphoid structures, the thymus grows rapidly and reaches its greatest size relative to the rest of the body during fetal life and the first years after birth. Thereafter, it continues to grow, but more slowly than the other organs. It can be said that the thymus enlarges during childhood, and atrophies at puberty. The thymus of older people is barely distinguishable from surrounding fatty tissue. As one ages, the thymus slowly shrinks, eventually degenerating into tiny islands of fatty tissue. The thymus is divided into two lobes, where the two lobes are divided into lobules by connective tissue septa that are continuous with the outer fibrous capsule. Each lobule is subdivided into an outer zone, the cortex, and the inner zone, the medulla, where they are distinguishable. The organ is composed principally of two types of cells, lymphocytes and reticular cells. The reticular cells form a loose meshwork, while the spaces between them are packed with lymphocytes. The cortex, characterized by its heavy lymphocyte concentration, is the site of lymphocytic proliferation. Proliferation of lymphocytes in the thymus is distributed evenly throughout the cortex. T cells that are produced in the cortex migrate to the medulla, where they enter the bloodstream through the m
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