Thymus is a primary lymphoid organ and is an initial site for T-cell development. Sharing its origin with thyroid and parathyroid gland, Thymus develops in the 6th week of gestation from the third pharyngeal pouch.
It is located in the anterior-superior mediastinum, anterior to the major vessels, base of heart and aortic arch behind the sternum in the pericardial fat. It is a butterfly shaped gland which has 2 identical lobes, surrounded by thick fibrous tissue, which may extend into the cervical region. Each lobe is made up of an external cortex and central medulla surrounded by a capsule.
Both cortex and medulla consist of reticular epithelial cells and lymphocytes, with medullary portion having fewer lymphocytes and presence of Hassall’s corpuscles. The gland gets its blood supply from the Internal Thoracic arteries and drain into the left Brachiocephalic vein and internal thoracic veins. The size of thymus depends upon the age as it reduces gradually after puberty and is replaced by fatty tissue as it involutes.
Major function of thymus is maturation of prothymocytes into immunocompetent mature T-cells. Thymocytes enter into thymus from bone marrow and undergo four stages of maturation, proliferation and differentiation. It also helps in evolution of naive T-cells into T helper cells (CD4) and cytotoxic suppressor cells (CD8), and migration of mature T cells into the circulating lymphocyte pool and peripheral tissues.
Thymus also has a role in specificity of T-cells by positive and negative selection, which may contribute to MHC compatibility. Thymus is one of the least understood organs of the body, apart from its known role in diseases like Ulcerative colitis (UC) and Myasthenia gravis (MG), in which altered thymic structure is seen due to thymic growth factors.
It is involved in nearly 15 histological variants of neoplasms despite having just 2 major cell types, approx. 20 different parathymic syndromes are known associated with these neoplasms, the 3 major being Pure Red-cell aplasia (PRCA), MG and hypogammaglobulinemia in the order of prevalence. It is postulated that like normal thymus, a “sick” thymus produces T-cells, difference being the later produces defected T-cell which may cause cellular autoreactivity which is relevant to the auto-immune phenomenon occurring in various thymic abnormalities.
Thymoma is the most prevalent mediastinal tumor in adults and MG the most common disease associated with it. A complete surgical resection of the thymus i.e. Thymectomy is usually the answer for the treatment of these ailments. Various approaches have been used for the procedure including median sternotomy, which has been long considered the best approach to Video-assisted Thoracoscopic Surgery (VATS) and Robotic Video-assisted Thoracoscopic Surgery (R-VATS). Approaches differ even within the VATS and R-VATS with the classical lateral or bilateral approach and the upcoming sub-xiphoid approach as well as uniportal approach, even trans-cervical approach as an option.
The side of approach is also up for debate while some surgeons preferring the left side with a more suitable dissection of all the soft fatty tissue around the peri-cardiophrenic angle because the superior vena cava lies outside the surgical field. Other surgeons prefer a right sided approach because they clearly identify and use the superior vena cava as a landmark to dissect around the innominate veins.
Median Sternotomy was long considered to be standard of care but as the expertise in minimal invasive approach is increasing and various patient related advantages being identified VATS, R-VATS, u-VATS methods are becoming popular regardless of the approach. A subxiphoid approach is another method which takes away much of the pain associated with thoracic approach.