Natural History and Treatment of Malignant Thymoma

The thymus gland is a lymphatic organ involved in lymphoid cell development and maturation. However, its embryologic origins come from the endoderm as epithelial outgrowths of the lower portion of the third pharyngeal pouches. Cells of these epithelial outgrowths grow into the surrounding mesenchyma and subsequently constitute the medullary areas of the lobules of the thymus. In some areas, epithelial cells accumulate and undergo keratinization and degeneration, leading to distinct structures called Hassall’s corpuscles, which are localized in the medulla of the lobules. Lymphocytes derived from hematopoietic stem cells of the bone marrow are secondarily localized within the spaces between the epithelial cells of the cortex of the lobules. The lymphoid cells are generally concentrated in the periphery of the cortex, while medullary areas of the lobules of the mature thymus contain mostly epithelial cells and few lymphocytes.[1] These epithelial cells have a major role in the production of humoral factors necessary for lymphoid differentiation, which occurs in fetal and early postnatal life. The thymus gland reaches its maximal size in the adolescent, weighing 30 to 40 g. Lymphoid components gradually disappear after puberty and the gland involutes, leading to a fatty residue. Hassall’s corpuscles remain, and the thymus never completely disappears. Although the gland is usually situated beneath the upper part of the sternum, thymic tissue can be found in ectopic areas, such as the retrocarinal adipose tissue.