Primary Lymphoid Organ:Thymus:
The thymus builds up at about the sixth week of gestation. By eighth week, it nurtures into a compact epithelial structure. Mesenchymal stem cells (i.e., precursors of lymphocytes) from the yolk sac, foetal liver and bone marrow reach the thymus and distinguish into the thymic lymphoid cells (i.e., thymocytes).
The thymus obtains its characteristic lymphoid look by the third month of gestation. It is therefore the first organ in all animal species to become principally lymphoid. In human beings, the thymus reaches its maximal size just previous to birth. Thymus continues to grow till around the 12th year. After puberty, it endures spontaneous progressive involution, signifying that it functions best only in early life.
The thymus is situated just behind the upper portion of the heart. It has two lobes enclosed by a fibrous capsule. Septa taking place from the capsule divide the gland into lobules that are distinguished into an outer cortex and an inner medulla. The cortex is crowded with actively proliferating small lymphocytes. The medulla consists mostly epithelial cells and matures lymphocytes amidst that are the hassall’s corpuscles that are whorl-like aggregations of the epithelial cells.The thymus was believed as an organ without any familiar function. Though its role in the development of cell mediated immunity has been originated newly. The primary function of the thymus is the manufacture of thymic lymphocytes (i.e., T cells). It is the main site for T lymphocyte proliferation in the body. Though, of the lymphocytes produced, only around one per cent leaves the thymus. The rest are destroyed nearby by programmed cell death or apoptosis. In thymus, the lymphocytes get new surface antigens (i.e., Thy antigens). Lymphocytes generated in the thymus are termed as ‘thymus (T) dependent lymphocytes’ or ‘T cells’. Dissimilar, lymphocytes proliferation in the peripheral organs, the function of thymus is not dependent of antigenic stimulation.The thymus confers immunological capability on the lymphocytes during their reside in the organ. Prethymic lymphocytes are not immune competent. In thymus they are ‘educated’ therefore they become capable of mounting cell mediated immune response against suitable antigens. This is affected by hormone-like factors generated by the thymus like thymulin, thymosin and thymopoietin. The significance of thymus in lymphocyte proliferation and development of CMI is obvious from the effects of lymphopenia, and in neonatally thymectomised mice.
T lymphocytes are selectively seeded into many sites in the peripheral lymphatic tissues. These are found in the white pulp of the spleen, about the periarteriole area, and in the paracortical regions of lymph nodes.
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