The Lost Organ

There is an organ in the chest, which tends to be ignored or neglected by the medical community, not least due to the fact that we usually lose it anyway: the thymus. Often of impressive size after birth and a constant nuisance for the pediatric cardiac surgeon, it keeps growing during childhood, then undergoes a fatty degeneration called involution and is virtually gone by the time we reach adulthood. One apparently has to be equipped with a jaw to be eligible for a thymus, because it is found in jawed vertebrates only. Embryologically, it arises from the third pharyngeal pouch and in humans moves downward into the direction of the chest to be, most of the time trailing the two horns behind, often wrongly described as cervical extensions. (They do not extend into the neck, they come from it.) In birds and some mammals, it simply stays in the neck. As its tissue is eventually derived from all three germinal sheets the thymus is classified as a lymphepitheloid organ. Its function is T-cell maturation by various complicated processes involving antigen exposure. The thymus is also considered to be essential for the development of self-tolerance. Genetic aberrations with thymic hypoplasia, such as the DiGeorge syndrome, can lead to severe immunodeficiency. As the development of the immune system with its various answer mechanisms is a learning process, the thymus is apparently no longer necessary at a certain age and therefore involutes into fatty tissue in all species.

Mysteriously, it sometimes comes back later in life, rebounds, which is not a good thing. It took a long time to find out about the potential relation of thymic tissue to myasthenia and the various acetylcholine receptor antibodies involved. To determine if a thymoma is benign or malign an extremely thorough histological examination is needed, looking for a potential invasion of neighboring structures. The numerous potential cell types alone are not good enough. Many mechanisms related to the thymus are still poorly understood.

So it is virtually a lost organ for most of us, both physically and mentally. I am therefore very grateful that our Editorial Board member Henning Gaissert volunteered to subedit an issue on thymectomy. He invited three experts to review the various techniques currently employed and supplemented this with regularly submitted articles on the outcome of surgically treated thymoma and thymic cancer, taking us deep into the confusing world of this erratic organ.

For completeness' sake and to end on a light note, it should be mentioned that there is also a culinary use for the thymus of young animals, usually calves or lambs. It is best prepared fried briefly for only a few minutes, but can also be gently simmered in broth. A mild mushroom risotto is an ideal accompaniment. On an English menu it is usually called “sweetbread(s).” Any pediatric cardiac surgeon who has dissected a thymus using electrocautery can easily relate to this term because the sweet and delicate smell is so unique and unforgettable.