The enigmatic thymic myoid cells – their 130 years of history, embryonic origin, function and clinical significance

Thymic myoid cells form a part of the cellular microenvironment of the thymic medulla in fish, amphibians, reptiles, birds, and mammals, including humans. In lower vertebrates they resemble skeletal muscle fibers at the level of light and electron microscopy, in humans they are less noticable even though they contain proteins typically found in the muscle tissue including desmin, actin, acetylcholine receptor, myosin, troponin or myogenic transcription factor. Despite the discovery of thymic myoid cells by German histologist Sigmund Mayer employed at the Charles-Ferdinand University (now Charles University) in Prague, Czech Republic, occurred exactly 130 years ago, as of today, these cells have been mentioned neither in most of the histology textbooks, nor in the histological nomenclature Terminologia Histologica. Thus, we find it appropriate to call these cells “enigmatic.” In this narrative review, we focus on the historical background of the discovery of thymic myoid cells, and current opinions on their embryonic origin and functional significance during the maturation of T-lymphocytes within the thymus. Finally, we present the overview of the clinical importance of this cell population, including the notion of rare mediastinal rhabdomyomas and rhabdomyosarcomas probably originating from these cells. We also ponder over the possible connection between thymic myoid cells and the pathogenesis of autoimmune disease – myasthenia gravis.