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Reviewer: Hanni Gulwani, M.D. (see Reviewers page)
Revised: 10 November 2014, last major update December 2012
Copyright: (c) 2003-2014, PathologyOutlines.com, Inc.


  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 (15g)
● 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 Ki-67 and p53 (Bull Exp Biol Med 2011;151:460)


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)

Micro description

● Composed of epithelial cells (endoderm, important for T lymphocyte differentiation), Hassall’s 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)

Micro images

Low power

High power

Virtual slides

Normal thymus gland (bottom slide)

Positive stains

● Epithelial cells: keratin, HLA-DR

End of Mediastinum > Thymus > Normal

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