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Thymus

Thymus

Thymus

  • The site of T cell development; a primary organ of the immune system.
  • After puberty, thymus involutes, adipose replaces thymic tissues, and thymus is no longer necessary for immune functioning.

Features

Capsule

  • Gives rise to trabecuale, which divide tissues into lobules; provide space for blood vessels.
  • Lined by epithelia.

Subcapsular space

  • Nurse cells envelope the developing T-cells (thymocytes).
  • Double negative thymocytes
    • When they exit the thymus, T cells will display either CD8 or CD4 glycoprotein co-receptors;
      However, at this early stage in development, the thymocytes express neither, so they are referred to as "double negative" (DN).

Cortex

  • Double positive thymocytes
    • Express both CD4 and CD8 glycoproteins – they are referred to as "double positive" (DP).
    • Within the thymus, these cells undergo a process of positive selection:
      T-cells that can recognize self-MHC are retained, and travel to the medulla (those that cannot recognize self-MHC molecules are removed).
  • Cortical epithelial cells and collagen fibers comprise the supporting stroma/
  • Capillaries are surrounded by a dual basal lamina and thymic epithelial cells.
    • The dual lamina is produced by the capillary endothelial and thymic epithelial cells; the dual lamina forms the blood-thymus barrier, which, with the help of nearby macrophages, protects against autoimmune reactions.

Medulla

  • Dendritic cells direct thymocyte maturation by presenting class I and II MHC molecules and self-antigens to DP cells.
  • Single Positive
    • In response to dendritic cells, thymocytes commit to expression of CD4 or CD8 co-receptors; thus, they are referred to as "single positive".
  • Macrophages
  • Medullary epithelial cells
    • Give rise to Hassall's corpuscles; each corpuscle comprises epithelial cells arranged in layers, which look like onions or rosettes; they become more prominent as the thymus regresses in age.
    • Hassall's corpuscles are believed to produce thymic stromal lymphopoietin to stimulate the dendritic cells (not all authors are in agreement on this point).

Additional Resources:

Goldstein G, Mackay IR. The Human Thymus. Elsevier. 2013.
https://www.hindawi.com/journals/jir/2012/925135/
Janeway CR. The Co-Receptor Function of CD4. Seminars in Immunology. May 3(3): 153-160. 1991.
Wantanabe N, Wang YH, Lee HK, Wang YH, Cao W, Liu YJ. Hassall's Corpuscles Instruct Dendritic Cells to Induce CD4+CD25+ Regulatory T Cells in Human Thymus. 436(7054):1181-5. 2005.

Images:
Thymus (Mark Braun, MD http://medsci.indiana.edu/c602web/602/c602web/virtual_nrml/nrml_lst_pad.htm)