TBL14 - Thymus Flashcards
Where does the thymus reside? When is it fully developed?
1) The bilobed thymus resides in the superior mediastinum
2) It is fully developed before birth
What occurs in the thymus before puberty? How does this change during adulthood?
1) Significant production of immunocompetent T cells occurs in the thymus before puberty
2) During adulthood, maturation of T cells in the thymus progressively decreases
When are most progenies of T cells established? Why does maturation of T cells in the thymus therefore reduce in adulthood?
1) Progenies of T cells are established by the mid-twenties
2) Thus, immune responses can be sustained in adulthood without substantial T cell maturation in the thymus
What are the two lobes of the thymus surrounded by? What projects from this structure? How are the lobes of the thymus subdivided? What two parts does each lobule contain?
1) The two lobes of the thymus are surrounded by a connective tissue capsule
2) Short trabeculae project from the capsule into outer portions of the lobes
3) The lobes are subdivided by the trabeculae into multiple lobules
4) Each lobule has an outer dark stained cortex and a central paler stained medulla
What extends between the trabeculae and what does this create? Define the hormonal functions of ERC.
1) A meshwork of epithelial reticular cells (ERC) extends between the trabeculae thereby creating a scaffolding to support billions of T cells within the thymus
2) ERCs secrete the hormones thymosin and thymopoietin, which induce T cell maturation and maintain cell-mediated immunity
What does lymphocytopoiesis do? What occurs to immature T cells once they traverse capillary endothelium in the bloodstream? What do the maturing T cells account for?
1) Lymphocytopoiesis generates mature B cells and immature T cells in the bone marrow; thus, the immature T cells enter the bloodstream and circulate to capillaries within the thymic medulla
2) After traversing the capillary endothelium, the immature cells migrate into the cortex where they become immunocompetent
4) This vast population of maturing T cells accounts for the dark staining of the cortex
Why do recurrent opportunist infections characterize DiGeorge syndrome?
1) DiGeorge syndrome, also known as thymic aplasia, is a rare congenital disorder involving failure of the thymus to develop properly
2) The syndrome is due to a defect on chromosome 22 produced by a recombination error at meiosis. Its selective T cell deficiency leads to immunodeficiency with recurrent opportunistic infections
What are cortical capillaries invested by? What function does this serve?
1) Cortical capillaries are invested by processes of the ERC
2) Tight junctions between the cortical capillary endothelial cells, thick basement membranes, and macrophages along with the ERC processes form the blood-thymus barrier
What is the function of the blood-thymus barrier? What does premature antigen exposure result in?
1) The barrier blocks premature exposure of nonself and self-antigens to the maturing T cells
2) Premature antigen exposure by circumvention of the barrier drives those T cells that react to the antigens into apoptosis thereby preventing immune reactions in the thymus
What do medullary capillaries lack? What does this result in? Relate the transitory presence of immature and mature T cells in the medulla to its weak staining intensity.
1) Medullary capillaries lack a blood-thymus barrier; thus after achieving maturation, T cells return to the medulla to enter the bloodstream
2) In the medulla, immature T cells enter the thymus, go to the cortex, mature, and leave through the medulla. Therefore, the medulla has a combination of incoming immature T cells and outgoing mature T cells that lead to a lighter/weaker staining intensity
What characterizes the thymus to be unique? What forms these unique structures?
1) Thymic (aka Hassall’s) corpuscles in the medulla uniquely characterize the thymus
2) The corpuscles are formed by clusters of ERC and many of the cells are undergoing degeneration
What do viable ERC in the thymic corpuscles do?
Viable ERC in the thymic corpuscles produce cytokines that induce development of regulatory T cells, a subclass that contributes to the termination of humoral and cell-mediated immune responses outside of the thymus
What occurs to the thymus after puberty?
After puberty, the thymus progressively involutes and the parenchyma is largely replaced by white fat
