The thymus is larger in infants and young children because it grows through the pre-adolescent years to support rapid growth then shrinks in size through adulthood as growth rates slow. The thymus gland has a unique shape that resembles a thyme leaf. This gland belongs to the lymphoid system and is situated below the breastbone in humans.
The thymus, along with several other structures in the lymphoid system, plays a key role in human growth and development. Therefore, the thymus and other organs in the lymphoid system are generally largest during the childhood and pre-adolescent years then shrink as humans pass through puberty and enter adulthood. In addition to its distinct outer appearance, the thymus has a unique inner structure. Within the thymus are two lobes which are located on either side of the breastbone. Within the two lobes of the thymus are several smaller units, which are called lobules. These structures are surrounded by tough protective shells, called capsules, which form from dense connective tissues. These tissues in turn form the walls of the cortex, the outermost layer and the medulla. Thymus organs also have two types of cells, which are lymphocytes and reticular cells.