The thymus is a lymphoid gland comprised of two identically sized lobes, located behind the sternum (breastbone) but in front of the heart. It derives its name from a resemblance it bears to the bud of the thyme plant (thymus in Latin). At puberty, the thymus reaches the height of its use, becoming its largest. After this age, the size of the thymus declines as the lymphoid tissue disappears and fat and fibrous tissue appears. T-cells have derived their name from the thymus, because this is where they are produced in the human body. Lymphoid stem cells are delivered to the outer cortex, or layer, of the thymus in blood. After multiplying within the outer cortex, they then move to the inner cortex where they develop T-cell surface markers. The maturation of T-cells is guided by thymopoietin, thymosin, and other hormones created in the thymus. In the center of the thymus (the medulla) T-cells complete their process of maturing and are then released into the bloodstream.