Embryologically, the thymus gland originates from the pouch and cleft of the third pharyngeal pouches. During the eighth gestational week, the primordial thymus elongates from the brachial arch caudally, forming two epithelial bars.14 After caudal migration, these enlarge and fuse in the midline and descend to the anterior superior mediastinum. Thus, the second hypothesis is that ectopic locations of the thymus might be the result of the nondescending or overdescending process or the result of migration to other organs. During migration, small remnants of thymic tissue may persist along its course and can present in the form of ectopic thymic tissue.10 The origin of pericardial thymoma has been ascribed to this misplacement of thymic remnants during its phase of embryonic descent.
Because we could not find any intracardiac myxoma in our case, the second hypothesis is more reasonable to explain the origin of intracardiac ectopic thymoma. Embryologically, the third pharyngeal pouch lies close to cardiac bulge. Some progenitor cells of the thymus or at least the endodermal rests demonstrate a degree of regional promiscuity and could migrate into the cardiac cavity to form an ectopic thymoma.
1. Weissferdt A, Kalhor N, Petersson F, et al. Ectopic
—new insights into a challenging entity: a clinicopathologic and immunohistochemical study of 9 cases. Am J Surg Pathol
2. Moran CA, Suster S, Fishback NF, et al. Primary intrapulmonary thymoma
: a clinicopathologic and immunohistochemical study of eight cases. Am J Surg Pathol
3. Moran CA, Travis WD, Rosado-de-Christenson M, et al. Thymomas presenting as pleural tumors: report of eight cases. Am J Surg Pathol
4. Arai H, Rino Y, Fushimi K, et al. Pericardial ectopic thymoma
presenting with cardiac tamponade: report of a case. Surg Today
5. Miller DV, Tazelaar HD, Handy JR, et al. Thymoma
arising within cardiac myxoma. Am J Surg Pathol
6. Travis WD, Brambilla E, Muller-Hermelink HK, et al. Tumours of the thymus. Pathology & Genetics: Tumours of the Lung, Pleura, Thymus, and Heart
. Lyon, France: IARC Press; 2004:148–171.
7. Bailly JM, Jaucot J, Darras T. Ectopic thymoma
. J Belge Radiol
8. Lewis JE, Wick MR, Scheithauer BW, et al. Thymoma
: a clinicopathologic review. Cancer
9. Tan A, Holdener GP, Hecht A, et al. Malignant thymoma
in an ectopic
thymus: CT appearance. J Comput Assist Tomogr
10. Cordier AC, Haumont SM. Development of thymus, parathyroids, and ultimo-branchial bodies in NMRI and nude mice. Am J Anat
11. Kodama H, Hirotani T, Suzuki Y, et al. Cardiomyogenic differentiation in cardiac myxoma expressing lineage-specific transcription factors. Am J Pathol
12. Markwald RR, Fitzharris TP, Manasek FJ. Structural development of endocardial cushions. Am J Anat
13. Acebo E, Val-Bernal JF, Gomez-Román JJ. Prichard's structures of the fossa ovalis are not histogenetically related to cardiac myxoma. Histopathology
14. Shimosato Y, Mukai K, Matsuno Y. Anatomy and anatomic compartments of the mediastinum. Thymoma
. In: Tumors of the Mediastinum. AFIP Atlas of Tumor Pathology (Series 4)
. Washington, DC: American Registry of Pathology; 2010:1–114.