Primary lymphoid  Organs(introduction) and Thymus

Primary Lymphoid Organs

In humans the largest part of the lymphocyte development ccurs in specialized tissue of the primary lymphatic organs: bone marrow (liver in the fetal period) andthymus. There a large number of immunocompetent lymphocytes are produced that colonize the secondary lymphatic tissue. [1]
One distinguishes two types of immunocompetent cells:

1.     T lymphocytes that are responsible for the cellular immune response and mature in the thymus

2.     B lymphocytes that are responsible for the humoral immune reaction and mature in the liver and in the bone marrow.[1]

Source: 17.2 Lymphatic tissue, Human Embryology, Organogenesis from www.embryology.ch

Source: 17.2 Lymphatic tissue, Human Embryology, Organogenesis from www.embryology.ch

Up to the sixth week, the thymus is purely epithelial. In the next weeks it becomes dented by an in-growing mesenchymal septal. In this way arise pseudo-lobules that communicate with each other in the center. The mesenchymal anlage material further forms the numerous, partially widened, perivascular spaces that are important for the cell transport into and out of the thymus. The fully developed organ consists of the thymus-epithelial tissue, responsible for the maturation of the T lymphocytes, and the perivascular spaces. [1]
Despite being tightly woven together they are separated from one another by a layer of flat epithelial cells with a basal lamina, the thymus-blood barrier. The anlage material for the non-epithelial stromal cells (precursors of the macrophages) and the free cells of the thymus (prethymic stem cells of the T lymphocytes) derive fromembryonic blood formation centers (liver, spleen, bone marrow). They appear in the thymus after the 9th week and initially colonize it relatively uniformly. The thymus (histological picture) acquires its functioning ability only when the cortex andmedulla have differentiated. The first indications of this differentiation in a fetus occurafter ca. 12 weeks and it is completed at ca. 4 months. With this differentiation the young thymocytes settle into the cortex region.[1]

Source: 17.2 Lymphatic tissue, Human Embryology, Organogenesis from www.embryology.ch

THE THYMUS AND IMMUNE FUNCTION

The role of the thymus in immune function can be studied in mice by examining the effects of neonatal thymectomy, a procedure in which the thymus is surgically removed from newborn mice. These thymectomized mice show a dramatic decrease in circulating lymphocytes of the T-cell lineage and an absence of

cell-mediated immunity. Other evidence of the importance of the thymus comes from studies of a congenital birth defect in humans (DiGeorge’s syndrome) and in certain mice (nude mice) in which the thymus fails to develop. In both cases, there is an absence of circulating T cells and of cell-mediated immunity and an increase in infectious disease. Aging is accompanied by a decline in thymic function.

This decline may play some role in the decline in immune function during aging in humans and mice. The thymus reaches its maximal size at puberty and then atrophies, with a significant decrease in both cortical and medullary cells and an increase in the total fat content of the organ.[2]

References:

[1]  17.2 Lymphatic tissue, Human Embryology, Organogenesis from www.embryology.ch

[2] Kuby, 5^th edition, Unit 2: Cells and organs of immune system, pg 43-45, Immunology.