Thymus

Anatomy

Histology

Thymus is covered by a connective tissue capsule, the septa of which penetrate into the tissue and divides it into incomplete lobules. Each lobule has a peripheral dark zone called cortex and middle lighter zone called medulla. The capsule is made up of inner and outer layers of collagen and reticular fibers, the lymphocytes are found in between.

Function

The thymus is the location where the haematopoietic precursor cells mature into T cells. Pro-thymocytes will migrate from the bone marrow, and enter the thymus gland at the corticomedullary junction. Once the maturation process has been completed, these T cells enter the circulation, and form the basis for the adaptive immune system. The T cells have receptors that are generated from the random shuffling of gene segment.s Initially, a process known as positive selection (which occurs in the cortex) functions, which checks if the developing T cell is able to recognise self MHC proteins (major histocompatibility complex; a set of proteins on the own cell surfaces of a person (that are essential for the adaptive immune system to recognise pathogens), and hence able to recognise the person’s own cells from foreign cells. Cells that do not perform this reaction, or perform it too weakly, are prevented from entering further development.

The next stage is negative selection, where the T cells undergo the process of interaction with thymic dendritic cells. Those cells with a high level of autoreactivity are eliminated, to reduce the likelihood of autoimmune reactions. As a result of this process, there is a large amount of T cells built up over early life, so that by adulthood the organ is largely obsolete and is degraded. The gland does continue to have endocrine function.

T cells are divided into T-helper cells and T-killer cells (cytotoxic cells). Cytotoxic T cells have CD8+ proteins on their surface, and attach to infected cells and destroy them directly. Helper T cells receive information about the pathogen from the antigen presenting cells (B cells and macrophages), and therefore coordinate the immune response (release cytokines which cause further white blood cell division, drive the production of memory B cells for future infections from that pathogen etc.).

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Development

The thymus gland has dual embryonic origin. Thymic epithelium develops during the sixth week of gestation, from the ventral diverticular epithelium of the third pharyngeal pouch along with the thyroid and parathyroid gland. It extends posterolaterally into the surrounding mesoderm as two flask-like structures. The cells that line these flask-like structures give rise to further proliferation, and are eventually surrounded and invaded by the mesoderm. It shares its origin with the thyroid and parathyroid glands. During the 8th week of gestation the thymus descends and takes up its final position in the anterosuperior mediastinum. It fuses with its counterpart from the opposite side. Late in development, the hematopoietic bone marrow precursor cells (mesenchymal origin) migrate into the thymus, and this is how the thymocytes come into contact with the thymus gland, and the lymphoid tissue becomes unified with the epithelial cell framework of the thymus. The growth and development of thymus continues until puberty. There are two distinct cell types within the thymus i.e. the lymphoid cells (thymocytes) and the reticular epithelial cells.

In children, T cells densely populate the cortex of the thymus. As the T cells develop they pass to the medulla, before they are released into the circulation. By the age of adolescence, the thymus gland begins to atrophy. Many factors play their role in the involution process, the level of circulating hormones in the blood i.e. sex hormones also cause the gland to atrophy and is replaced by fat.

As a last step in your learning, why not quiz yourself on thymus and the other organs of the endocrine system!