EM 379 Thymus

Thymus Medulla

Transmission electron microscopy (TEM) of the edge of a Hassall’s corpuscle in the medulla of the thymus.

The thymus is where thymus lymphocytes or T cells mature. It comprises an outer cortex rich with cells and an inner medulla that is less dense. Two major categories of cells within the thymus:

• Thymocytes (green) - maturing T-cell lymphocytes
  • Small, rounded cells with minimal cytoplasm
  • Highly heterochromatic nuclei
• Thymic Epithelial Cells (#1, #2, and#3; tan) - stellate cells that form the cytoreticulum that is analogous to the collagenous reticular network in other lymphoid tissues
  • Stellate shape with elongated slender cytoplasmic projections
  • Pale, ovoid nuclei
  • Keratin filaments - cytoplasm contains numerous intermediate filaments

Dendritic cells (yellow) are also found in the medulla. They are antigen-presenting cells that help with the thymocyte’s maturation.

• Irregularly shaped nuclei with a thin marginal layer of heterochromatin
• Surround lymphoid cells with long interdigitating cytoplasmic processes

Unique to the thymic medulla are concentric aggregations known as Hassall’s corpuscles (cyan). They are composed of layered whorls of medullary epithelial cells. Vary in size, shape, and number during life.

• Periphery composed of flattened epithelial cells that become heavily keratinized
• Lose their nuclei toward their keratinized centers
• Cellular debris (black) found at the center of larger whorls

Though their exact function is still under investigation, they are believed to be involved in the regulation of the maturation and selection of T-cells.

Courtesy of Brita von Gaudecker, Anatomical Institute, Christian Albrechts Universitat, Kiel, Germany.