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.
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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.
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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.
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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-lymphocytes
are 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.
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Title:
Function of the Thymus
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at
2013-02-15T03:26:00+07:00
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Function of the Thymus