Mediastinum
General
Histology-thymus
Author: Hanni Gulwani, M.B.B.S.
Last author update: 1 December 2012
Last staff update: 1 July 2020
Copyright: 2003-2023, PathologyOutlines.com, Inc.
PubMed Search: Normal thymus[TI]
Table of Contents
Definition / general | Diagrams / tables | Gross description | Microscopic (histologic) description | Microscopic (histologic) images | Positive stainsCite this page: Gulwani H. Histology-thymus. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/mediastinumthymusnormal.html. Accessed May 31st, 2023.
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
Images hosted on other servers:
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
Images hosted on other servers:
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High power
