Function of the Thymus

Function of the Thymus

The thymus is necessary for the development of the recirculating pool of small, long-lived (in humans many years) lymphocytes, the T-lymphocytes. These cells are mainly responsible for the cell-mediated part of an immune response. Stem cells invade the cortical regions of the thymus, where they divide to form lymphocytes. Only a small fraction (estimates range from 10-30%) of the cells generated in the cortex leave the thymus. They migrate via the medulla into the blood stream to populate the T-lymphocyte areas of other lymphoid tissues and organs. Cells which do not express the necessary receptors to recognize antigens presented to them or which react incorrectly towards "self-antigens" die and are removed by cortical macrophages.
Since the function of the thymus is to produce lymphocytes for the other lymphoid tissues it is a primary lymphoid organ.

Involution of the thymus

After puberty much of the parenchyma of the thymus, in particular cortical lymphoid tissue, is replaced by adipose tissue. The process, which is called involution, initially proceeds rapidly but slows down in adulthood. Involution is under the control of steroid hormones (both sexual hormones and stress hormones). Although most pronounced in the thymus, involution is a common feature of all lymphoid tissues.

Another age-related phenomenon is the increase in size of the thymic (or Hassall's) corpuscles. Thymic corpuscles are rounded eosinophilic structures, which consist of concentrically arranged, flattened cells. Thymic corpuscles are likely to be formed by reticular cells. Similar structures occur also in the tonsils. The size of these structures varies from 20 µm to more than 100 µm in diameter. Thymic corpuscles  may calcify, and their core may "dissolve" leading to the formation of a cyst.

Thymus, adult human - H&E Draw a part of the tissue, which illustrates the presence of adipose tissue, and the decrease in the amount of cortical and medullary thymic tissue. Include thymic corpuscles in your drawing if you can find them.

Lymph Nodes

Lymph nodes are small, flattened, oval or bean shaped organs, which are situated in the course of the collecting lymph vessels. Their size is variable (from a few mm to more than 2 cm). The capsule and trabeculae of lymph nodes are formed by connective tissue. Afferent lymph vessels penetrate the capsule and empty into the subcapsular space. The lymph continues thereafter through cortical and medullary sinuses towards the efferent lymph vessels, which emerge from the hilus of the lymph node. The walls of the sinuses can be traversed freely by all components of the lymph, which allows lymphocytes to enter/leave the lymphoid tissue (as part of their constant circulation) or to get in contact with antigens/antigen-presenting cells that may arrive with the lymph.

In lymph nodes we find B- and T-lymphocytes, macrophages and reticular cells.

Reticular cells (and reticular fibres) form a delicate network between the capsule and trabeculae. Only their large and light nuclei are easily visible in the microscope. The cytoplasm of reticular cells is only weakly eosinophilic. Lymphocytes and macrophages are housed in the network of reticular cells and the reticular fibres formed by them. The processes of reticular cells and reticular fibres extend into and criss-cross within the sinuses.

Lymphocytes

which are located in the outer cortex of the lymph node are likely to represent B-lymphocytes. They are organised into spherical masses - lymphoid nodules or follicles. Sites within the cortex at which B-lymphocytes have been stimulated to proliferate (by contact with an antigen) appear lighter than the surrounding tissue and allow you to identify the centres of lymphoid nodules. The lighter stained parts of the nodules are called germinal centres. Mature B-lymphocytes (plasma cells) are located in cord-like extensions of the lymphoid tissue into the medulla, the medullary cords. T-lymphocytesare located in the more diffuse tissue between the nodules and in the paracortex, i.e. the deep part of the cortex.

Macrophages

are found scattered within the lymphoid tissue. In many preparations they are difficult to distinguish from the reticular cells, but if an H&E stain turns out nice, macrophages can be distinguished from the reticular cells in the sinus system of the lymph node.