Incessant Bilobed Thymus in an CASE REPORT Old Male Cadaver KC Sudikshya*1, Shah Aashish Kumar1, Khaling Rai Dipa2 1 Department of Anatomy, National Medical College, Birgunj, Nepal 2 Department of Medical Surgical Nursing, National Medical College, Birgunj, Nepal ABSTRACT INTRODUCTION Thymus is bilobed primary lymphoid organ Thymus is one of the primary lymphoid organs. situated in superior and anterior mediastinum. It is a soft, capsulated, bilobed gland which The appearance of thymus varies considerably belongs to endocrine system1responsible for the with age. It reaches maximum size at puberty and provision of thymus-processed lymphocytes (T eventually undergoes “fibro fatty involution”, lymphocytes) to the entire body, and provides but it may persist actively to old age. Therefore, a unique microenvironment in which T-cell normal appearance and size of thymus have precursors (thymocytes) undergo development, been elusive. Embryologically, it develops, as differentiation and clonal expansion. It’s lobes lie two separate organs from the endoderm of third close together side by side, joined in the midline pharyngeal pouches of embryo in common by connective tissue that merges with the capsule with inferior parathyroid glands and fuse in the of the organ. The greater part of the thymus lies midline. in the superior and anterior inferior mediastinum behind the manubrium. Superiorly, it extends The study was carried on 10 embalmed cadavers commonly into the neck and may reaches the of known sex in Anatomy Department of National inferior poles of the thyroid gland or even Medical College, Birgunj, Nepal. higher. Inferiorly, the lower border of the thymus reaches the level of the fourth costal cartilages. We found a large bilobed thymus in about 65 year Anterior to the gland in the neck are sternohyoid old male cadaver. This fact is clinically important and sternothyroid muscles and fascia; in the to make differential diagnosis of radiological thorax the gland is covered anteriorly by the review in cases of mediastinal mass. A thorough manubrium, the internal thoracic vessels, the knowledge of its anatomical and embryological upper three costal cartilages and laterally by the features of the thymus, it’s normal variations, pleura. Posteriorly, it is in contact with the vessels incessant in adults is necessary before doing any of the superior mediastinum, especially the left therapeutic, diagnostic and invasive procedures. brachiocephalic vein, the thoracic trachea and the upper part of the anterior surface of the heart. Keywords: Diagnostic, Fibrofatty Involution, The thymus is supplied mainly from branches of Incessant, Invasive procedures, Lymphoid organ the internal thoracic and inferior thyroid arteries. Thymic veins drain to the left brachiocephalic, *Corresponding Author: Dr. Sudikshya KC, internal thoracic and inferior thyroid veins. The Department of Anatomy, National Medical College, thymus has no afferent lymphatics. The thymus Birgunj, Nepal, Email- [email protected]. is innervated from the sympathetic chain via the cervicothoracic (stellate) ganglion or from the ansa subclavia and the vagus.2 MED PHOENIX : An Official Journal of NMC, Birgunj, Nepal, Volume (2), Issue (1) July 2017, 67-70 67 KC Sudikshya et al. The gland develops from the epithelial cells CASE REPORT derived from the endoderm of the third pair of During routine dissection of thorax in National pharyngeal pouches and from mesenchyme Medical College, Birgunj, Nepal, a bilobed into which epithelial tubes grow. Thymus incessant thymus gland was found in about 65 development also depends on direct interaction years old male cadaver. The dissection was of mesenchymal derivatives of the neural crest done as follows- A vertical incision over the with pharyngeal epithelium. Some cells of skin of thorax was given extending from the the epithelial cords become arranged around suprasternal notch to the xiphoid process and a central point, forming small groups of cells another horizontal incision from jugular notch the thymic corpuscles (Hassall corpuscles). along the clavicle to the acromion. The skin was The mesenchyme between the epithelial cords reflected following which the pectoral muscles forms thin incomplete septa between the lobules. were exposed. The pectoralis major and minor Lymphocytes soon appear and fill the interstices muscles were reflected. The thoracic cavity was between the epithelial cells. The lymphocytes are exposed cutting at the sternoclavicular joints derived from hematopoietic stem cells. Growth and ribs at midaxillary lines on either sides and development of the thymus are not complete respectively extending from 2nd to 10th ribs and the at birth. It is a relatively large organ during the anterior thoracic wall was reflected downwards. prenatal period and may extend through the The fascia over the anterior mediastinum was superior thoracic aperture at the root of the neck dissected carefully and a bilobed thymus gland and plays an essential role in the development of was found at the region of manubrium below the immune system. Subsequently, it descends with thyroid gland.6 pericardium from the neck into the thorax.3 Observations were noted as follows: Ectopic thymic tissue is sometimes found. Small . An uneven length bilobed thymus gland with accessory nodules may occur in the neck. They half or more soft-tissue parenchyma along represent portions that have become detached with partial fibrofatty tissue. during their early descent.2 As both lobes develop . The gland was extending from root of the independently; each lobe has its own separate neck below the thyroid gland, to the superior blood supply, lymphatic drainage and nerve and anterior mediastinum. supply.3 . The gland was flat and two lobes were connected by fat and fibrous connective It weighs about 10-15 gm at birth and it tissue. progressively increases in size and weighs to about 20-30 gm till puberty. Then it undergoes The gland showed following relations: fibrofatty involution.1 By adulthood, it is often Superiorly – Thyroid gland scarcely recognizable because of fat infiltrating the Anteriorly – Manubrium of sternum cortex of the gland; however, it is still functional Posteriorly – Aortic arch, pericardium and heart and important for the maintenance of health. In Laterally – Cervical pleura covering right and addition to secreting thymic hormones, the adult left lobes of lung thymus primes thymocytes (T-cell precursors) before releasing them to the periphery.3 Inferiorly - Pericardium. Two thymic veins were found on posterior Acute shrinkage of thymus also occurs in surface draining into the right brachiocephalic conditions such as stress, infection, pregnancy, trunk separately. malnutrition, surgery, chemotherapy and . The gland was not of uniform length, malignancy.4 breadth and thickness and hence maximum measurements were considered. Thymic dysfunction leads to diseases such as myasthenia gravis, tumors, growth disorders, After measurements a small section was sent rheumatic disease etc.5 in pathology laboratory for histological slide 68 MED PHOENIX : An Official Journal of NMC, Birgunj, Nepal, Volume (2), Issue (1) July 2017, 67-70 Bilobed Thymus preparation. Slide revealed only fibrous tissue. of thymus during bodily stress. During recovery Probably the reason may be due to delayed period, it grows back to its original or even larger preservation of body in formalin after death. size. This phenomenon is known as thymic rebound hyperplasia. Rebound hyperplasia is 1. Right thymus commonly seen in children but also occurs in adults. These anatomic variations and dynamic 2. Left Thymus changes appears as main source of confusion. 3. Thyroid Gland These misinterpretations with pathological 4. Right Lung covered by Pleura condition lead to prolonged chemotherapy or 5. Left Lung covered by Pleura radiotherapy or unnecessary biopsy. Thus it sets a major confusion for radiologist.4 6. Pericardium covering Heart and Great Blood Vessels A study by Krishna Murthy JV, et al. shows 7. Thyroid cartilage thymus in adult life may be normal or abnormal. The prepubertal thymic glands were pyramidal in shape, pinkish grey to brown in color while post pubertal thymus were flattened, varied from greyish white to yellow.7 As described by Sushmita Bhatnagar, et al. posterior mediastinal accessory thymus was reported. Surgeons dealing with mediastinal structures should have a better knowledge about these variatons.8 Studies of Yasumasa Monden, et al. proposed that 63% (44% males and 19% females) of myasthenia gravis patients have thymomas. Scientist believed that thymus gland may give incorrect instructions to developing immune cells, ultimately resulting in autoimmunity and the production of acetylcholine receptor antibodies. Thymectomy performed in myasthenia gravis patients has great prognosis.9 Figure 1: Anterior view of Bilobed Thymus of Prevalence of large anterior mediastinal thymus Cadaver in situ. is also reported in 65 year old male cadaver.1 Table 1: Measurements of the Gland A study by Tetsuro Araki, et al. suggested Thymus Right side Left side Cigarette smoking and high BMI are associated Maximum Length (upper pole to 5 cm 3.2 cm lower pole) with advanced fat degeneration of the thymic Maximum Breadth (at superior 1.4 cm 2 cm gland. Cigarette may facilitate fatty degeneration mediastinum) of the thymus. However, the effect of smoking on Maximum Thickness 0.4 cm 0.4 cm 10 Weight including connective tissue 4.9 gm 2.6 gm the adult thymus has not been understood. DISCUSSION