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;
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:
- 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).