Mediastinum
Thymus
Normal



Topic Completed: 1 December 2012

Revised: 13 February 2019

Copyright: 2003-2018, PathologyOutlines.com, Inc.

PubMed Search: Normal thymus[TI]

Hanni Gulwani, M.B.B.S.
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Cite this page: Gulwani H. Thymus - normal. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/mediastinumthymusnormal.html. Accessed September 23rd, 2019.
Definition / general
  • Embryology:
    • Primordial thymus appears at week 6 from third branchial (pharyngeal) pouch
    • Descends from high in neck with inferior parathyroid glands; lymphocytes invade at 10 weeks
    • Differentiation into cortex and medulla is completed by 14 - 16 weeks
    • Thymus attains its greatest weight in relation to body weight before birth (15 g)
  • Has critical role in development of cell mediated immunity and T cell differentiation
  • Grows until puberty, then involutes and undergoes fatty replacement, although still present in adult prepericardial or retrocarinal fat
  • Early thymus programming, sexual dimorphism, efficiency of specific T cell progenitors and thymic microenvironment may determine immune activity (Aging Dis 2012;3:280)
  • At age > 60 years, thymocytes have higher levels of Ki67 and p53 (Bull Exp Biol Med 2011;151:460)
Diagrams / tables

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Fetal thymus

12 year old boy normal thymus

Gross description
  • Lobulated organ covered by a capsule, with cortical (outer and deep) and medullary compartments
  • May resemble lymph nodes (if no epithelial cells) or tumors (if no lymphocytes)
Microscopic (histologic) description
  • Composed of epithelial cells (endoderm, important for T lymphocyte differentiation), Hassall corpuscles (regressed epithelial cells), T cells of varying phenotypes; B cells in thymic medulla and thymic perivascular space (increase with age, Hum Pathol 2001;32:926); also interdigitating reticulum cells, Langerhans cells, mast cells, eosinophils, stromal cells
  • Dendritic cells modulate autoimmune regulation through tissue restricted antigens (TSA) and promote central tolerance in thymus (Am J Pathol 2010;176:1104)
  • Wnt4 regulates thymic cellularity through expansion of thymic epithelial cells and early thymic progenitors (Blood 2011;118:5163)
Microscopic (histologic) images

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Positive stains
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