MH 079 Thymus

Neonatal Thymus

Thymus is an encapsulated organ where T-cells proliferate and mature behind the blood-thymus barrier.

The neonatal thymus is well-developed and fully functional.

• Capsule - thin connective tissue layer surrounding the thymus that extends inwards to form incomplete lobules.
• Cortex - outer darker, region of small lymphocytes.
  • T Lymphocytes - small nuclei of condensed chromatin.
  • Epithelial Reticular Cells - large cells with oval nuclei and lightly stained cytoplasm with processes.
  • Macrophages (#1,#2) - large cells that phagocytize T cells marked for removal.
• Medulla - inner, lighter region of larger lymphocytes.
  • Hassall's Corpuscle (#1,#2,#3) - closely packed, concentrically arranged epithelial reticular cells.

Hassall's corpuscles are the most readily identifiable feature of the thymus.

Adult Thymus

The thymus continues to enlarge up to puberty, but then undergoes slow involution being replaced by adipose cells.

• Capsule - thin connective tissue surrounding the thymus that extends inward as trabeculae to form incomplete lobules.
• Cortex - outer darker, region of small lymphocytes.
  • T Lymphocytes - small nuclei of condensed chromatin.
  • Epithelial Reticular Cells - large cells with oval nuclei and lightly stained cytoplasm with processes.
• Medulla - inner, lighter region of larger lymphocytes.
  • Hassall's Corpuscle (#1,#2) - closely packed, concentrically arranged epithelial reticular cells.

Hassall's corpuscles are the most readily identifiable feature of the thymus.