THYMUS

The thymus is embryologically derived from the third and, inconstantly, the fourth pair of pharyngeal pouches. At birth it weighs 10 to 35 gm. It grows until puberty, when it achieves a maximum weight of 20 to 50 gm, and thereafter undergoes progressive involution to little more than 5 to 15 gm in the elderly. The thymus can also involute in children and young adults in response to severe illness and HIV infection.


Macrophages, dendritic cells, a minor population of B lymphocytes, rare neutrophils and eosinophils, and scattered myoid (muscle-like) cells are also found within the thymus. The myoid cells are of particular interest because of the suspicion that they play some role in the development of myasthenia gravis, a musculoskeletal disorder of immune origin.