Thymus: Structure, Function & Diseases

As the primary organ of the lymphatic system, the thymus plays an important role in the human immune system. Within the thymus, the T lymphocytes responsible for acquired immune defense mature.

What is the thymus?

The thymus is the name given to an organ consisting of two asymmetrically shaped lobes located in the anterior mediastinum (middle pleura) behind the sternum (breastbone). The organ emerges from the endoderm (epithelium of the second and third pharyngeal pouches) at the end of the first embryonic month and grows to a size of about 35 to 50 g, especially in infancy, until the onset of sexual maturity. Subsequently, regression and transformation of the thymus cells into functionless adipose tissue takes place (so-called thymic involution), so that the thymic tissue can no longer be macroscopically delineated in the majority of adults. Because the thymus, unlike the other lymphoid organs (including Peyer’s plaques, spleen), does not arise exclusively from the mesoderm (middle cotyledon) but from all three cotyledons, it is also called a lymphoepithelial organ.

Anatomy and structure

The thymus is located in the anterior mediastinum behind the sternum and is surrounded by an organ capsule formed of collagenous connective tissue. The lymphoepithelial organ is divided into two asymmetric lobules, which are traversed by a central medullary cord and have a cortical zone. The basic framework of the thymus is a network consisting of radially (stellate) branched epithelial cells interconnected by cytoplasmic processes. The epithelial cells in turn form cell strands in the medullary zone as well as spherical cell clusters, the so-called Hassall bodies, and assemble epithelially on the surface of the lobules. While countless lymphocytes are embedded in the cortex zone, where they develop and differentiate, the medullary zone contains primarily macrophages and epithelial cells in addition to mature T lymphocytes. The arterial supply to the organ is provided primarily by the rami thymici, which arises from the internal thoracic artery, while the venae thymicae provide venous drainage.

Function and Tasks

As the primary organ of the lymphatic system, the primary function of the thymus is to develop and differentiate the T lymphocytes responsible for adaptive (acquired) and cell-mediated immunity. Already during the fetal period or fetogenesis, lymphocytes from the bone marrow deposit into the thymus, where they receive their immunological imprinting. For this purpose, the reticular or epithelial cells of the thymus endocrinally produce so-called thymic factors or hormones. These polypeptides (including thymopoietin I and II, thymosin) stimulate the differentiation of thymocytes (pluripotent stem cells derived from the bone marrow and stored in the thymus) into mature T lymphocytes. During maturation into T lymphocytes, the blood-thymus barrier blocks contact with endogenous antigens. The mature T lymphocytes subsequently migrate via the bloodstream to secondary lymphoid organs. In addition, the thymus influences body growth as well as bone metabolism. Following puberty, the thymus gradually loses its function as part of involution, with the parenchyma (organ-specific tissue) gradually being replaced by adipose tissue. Differentiation between the cortical and medullary zones, as well as delineation of lobules, is then generally no longer possible.

Diseases and complaints

The thymus can be affected by various impairments, especially pathological changes. For example, in thymic aplasia, the thymus may have a predisposition to develop but fail to form. This lack of thymus development can lead to pronounced immunodeficiencies and can be observed in the context of DiGeorge syndrome and other chromopathies as well as retinoid embryopathy, ataxia teleangiectatica (Louis Bar syndrome), and Wiskott-Aldrich syndrome. Especially in early infancy, a spontaneously regressing hyperplastic thymus enlargement can often be observed, which may be accompanied by mechanical displacement phenomena of the adjacent organs, especially the trachea (windpipe) and bronchi, and correspondingly lead to respiratory distress.In addition, retarded development with formation of a reduced thymus (thymic hypoplasia) due to a lack of development and maturation of T lymphocytes can cause severe immunodeficiencies with pronounced infections as well as an increased susceptibility to infections. In addition, a tumor disease (thymoma or thymic carcinoma) can originate from the thymus, which usually affects women more frequently and is accompanied by inspiratory stridor as well as dyspnea and dysphagia due to compression of the intrathoracic organs. Approximately one-fifth of these tumoral diseases of the thymus may also be associated with myasthenia gravis pseudoparalytica (severe autoimmunologic disease of skeletal muscle).