Thymus

The origin of the organ can thus be dated to about 500 million years ago. "However, the first precursors of the Foxn1 gene already appear in phylogenetically older chordates such as lancelet fish and lampreys. Gene products can be detected in the pharyngeal tissues of these animals - an explanation as to why the thymus gland evolved from the 3rd-4th pharyngeal pocket. Fully differentiated T lymphocytes are only found in this zone in higher vertebrates.

The thymus (Latinized from the ancient Greek "thymos" = life force) or sweetbread, is a gland of the lymphatic system of vertebrates and thus part of the immune system. The thymus was first identified in vertebrates that lived about 500 million years ago. It plays an important role in the body's immune defence. T-lymphocytes play a central role in the body and take on a variety of tasks in the immune defense. Their maturation and differentiation takes place in the thymus gland.

In humans, the two-lobed thymus, which is surrounded by a collagenous connective tissue sheath, is located above the heart and is divided into small lobules by septa. Numerous T-lymphocytes (thymus dependent) or their immature precursor cells (thymocytes) are embedded in the epithelial stroma. Their density decreases from the cortex (cortex) to the interior (medulla). In the medulla, the epithelial cells are arranged in a reticular pattern, rarely in short strands. The so-called Hassal's corpuscles are formed by agglutination with an onion-skin-like arrangement. In addition, the medulla also contains a relatively large number of eosinophilic leukocytes, some lymphocytes, as well as abundant blood vessels and nerves.

With the onset of puberty, the thymus regresses (involution), so that in adults only a thymic remnant remains - in humans also called retrosternal fat body - which consists mainly of fatty tissue. The complete regression of the thymus is an essential factor for immunosenescence.

Almost all vertebrates living today have an adaptive immune system that protects the body against unwanted invaders such as bacteria, viruses or even the body's own degenerated cells. T lymphocytes (T cells) play a central role in this process. They recognise foreign molecules (antigens) and initiate a targeted immune defence. The precursors of the T cells originate from the bone marrow. From there, the still immature lymphocytes migrate to the thymus, a central organ of the immune system. There, the precursor cells "learn" to differentiate between the body's own and foreign structures. Potentially self-reactive cells die by apoptosis (negative selection). Non-self-reactive lymphocytes differentiate into mature T cells and are released into the bloodstream as naive T lymphocytes. Only after antigen contact in the secondary lymphoid organs do they differentiate into effector cells and perform a variety of different tasks in the organism. This complex interaction is controlled by various genes.