**2. Development of the thymus**

The thymus, a retrosternal lymphoid tissue, develops from the third and fourth pharyngeal pouches like the parathyroid glands at the sixth week of gestational age [7–10]. During the next weeks of development, the thymus migrates through a path, called thymopharyngeal duct, to its final destiny which is the anterior mediastinum [11]. The liver and bone marrow are the primary organs responsible for production of lymphoid cells. These cells migrate to the thymus gland which results in differentiation of the thymus into a cortex and medulla [7].

The thymus is usually located in the anterior mediastinum, although it can be found anywhere in the thymopharyngeal path. The thymus has a bilobed or quadrilateral shape; however, it can be found in other shapes. The thymus is commonly found in chest radiographs of infants and children as a large mediastinal mass. During adolescence, the thymus encounters a fibrofatty change as the age increases. Growth of the thymus continues from birth to 2–3 years of age, when it reaches its highest weight, while sex hormones make the thymus smaller during adolescence. Appropriate function of the thymus in childhood guarantees the condition of the immune system [12].
