Lymphoid System Histology II Flashcards
The cortex of the thymus has more
Immature T-cells
Enter near the thymic corticomedullary junction and migrate into the cortex where “education” into mature, immunocompetent T-lymphocytes begins
Immature T-lymphocytes
In the outermost cortex, immature T-lymphocytes proliferate and undergo gene rearrangement to form pre-TCR with co-receptors
CD4 and CD8
T-lymphocytes that recognize MHC but not selfantigen continue
Maturation
ERCs that take up many developing T-cells into their cytoplasm and eliminate potentially harmful T-cells
Thymic Nurse cells
Immunocompetent T-lymphocytes that survive the selection process reach the medulla for maturation completion and exit the thymus via postcapillary venules at the
Corticomedullary junction
Developing thymocytes in the cortex require protection from exposure to
Antigens
Consists of desmosome-linked ERCs plus basement membrane, perivascular CT, continuous capillary endothelium plus basement membrane
Blood/thymus barrier
Additionally, nearby macrophages ensure escaping antigens from capillaries into thymus do not react with developing
T-cells
Like the lymph node, the thymus has a cortex-medulla. Unlike the lymph node, the thymus does not have
Nodules
Also, the thymus exhibits a key distinguishing feature within the medulla, the whorlshaped clusters of degenerating, eosinophilic, keratinized ERCs called
Hassall’s thymic corpuscles
After puberty, the thymus slowly and progressively involutes, with loss of parenchymal T-cells and replacement by
Fatty connective tissue
In elderly, the involuted thymus is mostly fat and has been linked to some loss of immune function with susceptibility to infection and
Cancer
An inherited immunodeficiency in which thymus ERCs do not develop, which results in failed T-lymphocyte maturation
DiGeorge syndrome
The equivalent of DiGeorge syndrome is the nude, athymic mouse strain that lacks transcription factor
Foxn1