Thymus

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Thymus is a little organ that is in the mediastinum. It is often removed in pediatric cardiac surgery 'cause it is in the way. In adults, it is commonly removed 'cause the patient has myasthenia gravis.

Micrograph of a thymic corpusle (Hassall's corpusle). H&E stain.

Overview

General

  • Important for development of the immune system.
  • One of two primary lymphoid organs - the other one is the bone marrow.[1]
  • Thymus involutes after childhood.
    • The line between thymoma and persistent normal thymus in the adult is not well-defined in the radiologic context.[2]
  • May be absent due to genetic abnormalities, e.g. DiGeorge syndrome.

Anatomy

Location:

Anatomically in contact with:

Normal histology

General

Features:[3]

  • No germinal centres.
  • Hassall's corpusle (thymic corpusle).
    • Round eosinophilic thingy.
    • Thought to arise from medullary epithelial cells (see cell types).[4]

Note:

Cell types

Cells of the thymus (short version):

  1. Cortical epithelial cells.[4]
    • Epithelioid.
    • Abundant cytoplasm.
    • Pale nuclei with small nucleoli.
  2. Medullary epithelial cells.[4]
    • Spindle morphology.
    • Scant cytoplasm.
    • Oval dark nuclei.
  3. T lymphocytes.

Other cells:

  • Macrophages.
  • Dendritic cells.
  • Other WBCs: B lymphocytes, neutrophils, eosinophils.
  • Myoid cells.

Note:

  • Thymic tumours are derived from the epithelial component of the thymus, i.e. the cortical epithelial cells and medullary epithelial cells.

Images

  • Thymic corpusle. (WC/Nephron)

IHC and thymus

Types A, AB, B:[5]

Type C:

  • CD5 +ve.[5] (???)
  • D2-40 +ve.[6]

All types:[5]

  • CD1a +ve (immature T cells, Langerhans cells, dendritic cells[7]), CEA +ve (focal), vimentin -ve.

Others (immature T cells):

  • TdT +ve.
  • CD99 +ve.

Anterior mediastinum mass DDx

Main article: Mediastinum

4 Ts (mnemonic):

Thymus and stress

  • Stress -> increased endogenous steroid -> lymphocyte death -> increased tingible body macrophages.[8]

Specific conditions

Thymic follicular hyperplasia

  • AKA thymic follicular hyperplasia.

Features:[9]

  • Follicular centres in the thymus.

Associations:[9]

Thymoma

Main article: Thymoma

Metaplastic thymoma

  • AKA thymoma with pseudosarcomatous stroma.[10]

General

Microscopic

Features:[10]

  1. Epithelioid cells.
  2. Spindle cells.
  • Few lymphocytes.

DDx:

Images

www:

IHC

CD5 -ve.[10]

Thymic carcinoma

Main article: Thymic carcinoma

See also

References

  1. ↑ URL: http://www.life.umd.edu/classroom/bsci423/song/Lab1.html. Accessed on: 28 March 2012.
  2. ↑ Araki, T.; Nishino, M.; Gao, W.; Dupuis, J.; Hunninghake, GM.; Murakami, T.; Washko, GR.; O'Connor, GT. et al. (Jan 2016). "Normal thymus in adults: appearance on CT and associations with age, sex, BMI and smoking.". Eur Radiol 26 (1): 15-24. doi:10.1007/s00330-015-3796-y. PMID 25925358.
  3. ↑ URL: http://www.kumc.edu/instruction/medicine/anatomy/histoweb/lymphoid/lymph03.htm. Accessed on: 17 June 2010.
  4. ↑ 4.0 4.1 4.2 4.3 Cotran, Ramzi S.; Kumar, Vinay; Fausto, Nelson; Nelso Fausto; Robbins, Stanley L.; Abbas, Abul K. (2005). Robbins and Cotran pathologic basis of disease (7th ed.). St. Louis, Mo: Elsevier Saunders. pp. 706. ISBN 0-7216-0187-1.
  5. ↑ 5.0 5.1 5.2 CJS. January 2010.
  6. ↑ Yokota, K.; Tateyama, H.; Yano, M.; Moriyama, S.; Hikosaka, Y.; Okuda, K.; Shitara, M.; Okumura, M. et al. (Jan 2013). "Clinicopathological analysis of small-sized thymoma with podoplanin and Ki 67 expression analysis.". Mol Clin Oncol 1 (1): 88-92. doi:10.3892/mco.2012.2. PMID 24649128.
  7. ↑ URL: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1886385/pdf/amjpathol00102-0156.pdf. Accessed on: 26 August 2010.
  8. ↑ Toti P, De Felice C, Stumpo M, et al. (September 2000). "Acute thymic involution in fetuses and neonates with chorioamnionitis". Hum. Pathol. 31 (9): 1121–8. PMID 11014581.
  9. ↑ 9.0 9.1 Cotran, Ramzi S.; Kumar, Vinay; Fausto, Nelson; Nelso Fausto; Robbins, Stanley L.; Abbas, Abul K. (2005). Robbins and Cotran pathologic basis of disease (7th ed.). St. Louis, Mo: Elsevier Saunders. pp. 707-8. ISBN 0-7216-0187-1.
  10. ↑ 10.0 10.1 10.2 URL: http://surgpathcriteria.stanford.edu/thymus/thymoma/metaplastic_thymoma.html. Accessed on: 22 December 2011.
  11. ↑ 11.0 11.1 Lu, HS.; Gan, MF.; Zhou, T.; Wang, SZ. (Oct 2011). "Sarcomatoid thymic carcinoma arising in metaplastic thymoma: a case report.". Int J Surg Pathol 19 (5): 677-80. doi:10.1177/1066896909355458. PMID 20034984.
  12. ↑ Kang, G.; Yoon, N.; Han, J.; Kim, YE.; Kim, TS.; Kim, K. (Feb 2012). "Metaplastic thymoma: report of 4 cases.". Korean J Pathol 46 (1): 92-5. doi:10.4132/KoreanJPathol.2012.46.1.92. PMID 23109986.
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