**1. Where, when, and why of the thymus**

The pharynx is regarded as a part of the digestive system. It is, however, attached to the respiratory system and, thus, is referred to as the conducting zone of the respiratory system as well. The pharyngeal apparatus groups together five pharyngeal arches, four pharyngeal pouches, four pharyngeal clefts, and pharyngeal membranes that occur during 4–5 weeks of human development [1]. Within the developmental context, the thymus is closely related to parathyroid glands [2] since both are derived from the endodermal-lined pharyngeal pouches—the thymus develops from the ventral portion of pouch 3, and the inferior and superior parathyroid glands arise from the dorsal portions of pouches 3 and 4, respectively.

Late in the 4th week, the primordia of the thymus begin to form but remain attached to that of the inferior parathyroid glands for a time [3]. By the 7th week, they migrate to the position where they should reside and be functional—the thymus is located midline in the upper chest just behind the sternum between the lungs and above the heart, and the inferior parathyroid glands descend along the inferior border of the thyroid. However, it takes about 12 weeks to form the definitive structure of the thymus, a bilobed organ that is covered by a mesenchymal capsule and is composed of two internal layers: the cortex and the medulla. The cortex is the outer layer of the thymus in which cortical thymic epithelial cells (cTECs) are found. The medulla is located in the center of the thymus. It is where medullary thymic epithelial cells (mTECs) and Hassall's (thymic) corpuscles are present.

The thymus is seen as of high value, due to its functions as a part of the neuroendocrine system in addition to its actions to provide a primary lymphoid microenvironment that efficiently carries T cell differentiation and selection. The first evidence that the thymus can have immune and endocrine functions was provided from athymic nude animals and/or animals undergoing thymectomy showing impairments in their cell-mediated immunity, growth pattern, and puberty and developing organ-specific autoimmune diseases. These impairments were consistent with their relative reduction in levels of thyroxin, testosterone, progesterone, and 17-β-estradiol, whereas corticosterone levels were elevated. Of particular note was that the passive transfer of lymphocytes and thymus implantation at birth were effective in transferring cell-mediated immunity to nude mice [4]. The endocrine defects could be, in part, healed by thymus implantation at birth as well but remain stable following passive transfer of lymphoid cells [4].
