Histology

Histologically, the adult thymus may be noted for its concentration of homogeneous fibrofatty tissue. Before fat is deposited - from late puberty - it is noted for:

• connective tissue capsule:
  • extends into the body of the thymus to partition it as discrete trabeculae
  • contains a diverse range of tissues:
    • cells e.g. fibroblasts, lymphocytes, mast cells
    • collagen
    • blood vessels
    • efferent lymphatics
• an outer cortex:
  • densely packed with small lymphocytes
  • network of epithelial cells which are interconnected
  • large numbers of macrophages involved in destruction of senescent lymphocytes
• an inner medulla:
  • sparser, larger lymphocytes
  • a denser population of epithelial cells, some of which form thymic corpuscles

The blood vessels within the trabeculae are separated from the lymphocytes of the cortex and outer medulla by a complex arrangement of connective tissue, epithelial and other cells. Functionally, this arrangement seems to hinder the passage of large molecules from vessels to thymus and vice versa. It is termed the blood-thymic barrier.