What is Thymus?

In mammals thymus is a flattened, grayish, bilobed organ with lymphoepithelial tissue. It is located in the thorax immediately below to the sternum and above the heart with its upper narrower part extended into the neck.

In the early stages of development, it is entirely an epithelial structure derived from the third and fourth pharyngeal pouches. This epithelial structure gradually grows and descends to thorax losing its connection with the pharyngeal pouches.

Progenitor lymphocytes of hemophilic stem cells migrate from bone marrow to the thymus through blood stream for differentiation and maturation into immuno competent lymphocytes. The fully developed thymus is encapsulated by connective tissue and becomes a bilobed structure.

The number of lobes in thymus varies in different animals. For instance it is seven lobed in chick; ten lobed in pig and bilobed in man. The connective tissue of capsule penetrates deep into the lobes, dividing each lobe incompletely into many lobules. The strands of connective tissue in between the lobules are known as “trabeculae”.

Each lobule in thymus is organized into an outer cortex and inner medulla. The cortex or peripheral part of each lobule is densely populated with differentiated but immature “T cells or Thymocytes” and the medulla or central portion of the lobule is packed with loosely arranged mature lymphocytes.

Since mature T cells are noticed in both cortex and medulla, the sequence of maturation of T cells within the thymus is not completely understood.

The lymphocytes in thymus multiply rapidly, but majority of the cells (about 90-95%) die within 3 to 4 days leaving only a small fraction (about 5%) of cells for maturation. The hormone thymosin secreted by thymus epithelial cells might be helping in the differentiation and maturation of T lymphocytes.

The matured T lymphocytes with longer life span leave the thymus to circulate in peripheral blood and lymph, and colonise in the thymus dependent areas of secondary / peripheral lymphoid organs. On interaction with antigens the T cells in secondary lymphoid organs become active and offer cell mediated immunity.

The reason for death of large number of developing T cells in thymus might be a selection process during the maturation of T cells. T cells that fail to react with antigens and the cells that can react with self antigens might be identified for elimination in the selection process to avoid any irregularity in the function of immune system.

A three dimensional network composed of inter digitating dendritic cells, stromal cells, epithelial cells and macrophages are present in thymus as a structural frame work of the organ. Some of the epithelial cells in the cortex show long membrane extensions that surround as many as 50 thymocytes.

The membrane extensions of epithelial cells surround the blood vessels and lymph spaces of the cortex and medulla also. The possible function of this membrane is to prevent the macro molecules (including the antigens) of the blood to contact lymphocytes of the thymus.

Hence they are referred as nurse cells. Their membrane may be responsible for nonexistence of immune reactions in thymus. Some concentric layers of degenerating epithelial cells referred as “Hassall’s corpuscles” are found in the medulla of thymus. The function of Hassall’s corpuscles is not clearly understood.

The thymus activity reaches peak in childhood but attains its largest size by puberty, later it shows atrophy (decrease in size) and becomes extremely small in old age. Hence aging is accompanied by a decline in thymus function and immunity. Significant decrease in cortical and medulla cells, increase in the total fat content of the organ, results in atrophy of the organ.

T cells are essential for cell mediated immunity, removal or defective formation or function of thymus affects immunity mediated by T cells. Experimental study of neonatal thymectomy (surgical removal of thymus) in mice has revealed deficiency of T cells and cell mediated immune response. Aplasia of thymus (Failure of development of thymus) also results in defects in cell mediated immune reactions.

As T cells are involved in immunity to cancer, bacterial, viral, and fungal attacks, a person with T cells deficiency is vulnerable to many diseases and cancer. A baby born with Aplasia of thymus may die due to infections.

Replacement of thymus or supplement of thymes hormone can correct the problems developed in the absence of thymus. Thymectomy in adolescence may not be fatal, but lead to gradual reduction in cell mediated immune response and affects resistance to infections. This is due to long life of lymphocytes and memory cells that are developed earlier than thymectamy.

Since death of established lymphocytes is inevitable, cell mediated protection may not continue for longer time in the absence of T cells replacement, and there is a gradual reduction in cell mediated immune response.