Thymus

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Thymus is an annoying little organ that is in the mediastinum.

General

  • Involutes after childhood.
  • Location: anterior mediastinum.
  • Important for development of the immune system.
  • May contain within it parathyroid.[1]

Histology

General

Features:[2]

  • No germinal centres.
  • Hassall's corpusle (thymic corpusle).
    • Round eosinophilic thingy.
    • Thought to arise from medullary epithelial cells (see cell types).[1]

Cell types

Cells of the thymus (short version):

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

Other cells:

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

Images:

Di George syndrome

  • Things go wrong with the thymus... very wrong.

Thymus and stress

  • Stress -> increased endogenous steroid -> lymphocyte death -> increased tingible body macrophages.[3]

Thymic follicular hyperplasia

  • AKA thymic follicular hyperplasia.

Features:[4]

  • Follicular centres in the thymus.

Associations:[4]

Tumours of the thymus (overview)

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

The WHO published a widely used system - WHO classification:[5]

Type A

  • AKA Spindle cell or medullary.
  • Arise from medullary epithelial cells.
  • Good prognosis.

IHC:

  • Usu. keratin+.

Type AB

  • Like Type A... but with foci of lymphocytes.

Type B1

  • Near normal, expanded cortex.

Lesion consists of:

  • >2/3 lymphocytes, <1/3 cortical epithelial cells.

Type B2

  • Neoplastic cells with some resemblance to cortical epithelial cells.
    • Epithelioid cells with distinct nucleoli.
    • May be perivascular.
  • Large population of lymphocytes.

Lesion consists of:

  • <2/3 but >1/3 lymphocytes, >1/3 but <2/3 cortical epithelial cells.

Notes:

  • Most common B type.

Type B3

  • Neoplastic cells with some resemblance to cortical epithelial cells.
    • Polygonal/round shape.
    • Form sheets (of cells) - key feature.
  • Lymphocytes - less than in Type B2.
  • AKA well-differentiated thymic carcinoma.

Lesion consists of:

  • <1/3 lymphocytes, >2/3 cortical epithelial cells.

Type C

  • AKA thymic carcinoma.
  • Neoplastic cells with some resemblance to cortical epithelial cells - with cytologic features of malignancy.
    • Any nuclear atypia of epithelial thymocytes... puts a tumour into this group.

Images:

Thymoma

General

  • Strong association with autoimmune disease, esp. myasthenia gravis.

Microscopic

Features:

  • Lymphocytes.
  • Spindle cells.

DDx:

Images:

Thymic carcinoma

  • AKA Thymic tumour type C.

General

  • Rare.
  • Usually arise de novo, i.e. thymoma is not generally a precursor.

Microscopic

Features:[6]

  • Cytologically malignant - variable morphology.
  • +/-Squamous differentiation.

Images:

IHC

Features:[6]

  • CD5 +ve.[7]
  • CD117 +ve.
  • TTF-1 -ve.

Staging

There is a system by Masaoka et al..[8]

IHC and thymus

Types A, AB, B:[9]

  • CK7-, CK20-, CAM5.2+, CK5/6+, p63+, CD5-.

Type C:[10]

  • CD5+.

All types:[11]

  • CD1a (immature T cells, Langerhans cells, dendritic cells[12]), CEA +ve (focal), vimentin -ve.

Others (immature T cells):

  • TdT.
  • CD99.

Anterior mediastinum mass DDx

4 Ts (mnemonic):

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 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.
  2. ↑ URL: http://www.kumc.edu/instruction/medicine/anatomy/histoweb/lymphoid/lymph03.htm. Accessed on: 17 June 2010.
  3. ↑ 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.
  4. ↑ 4.0 4.1 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.
  5. ↑ Mills, Stacey E; Carter, Darryl; Greenson, Joel K; Oberman, Harold A; Reuter, Victor E (2004). Sternberg's Diagnostic Surgical Pathology (4th ed.). Lippincott Williams & Wilkins. pp. 1264. ISBN 978-0781740517.
  6. ↑ 6.0 6.1 Humphrey, Peter A; Dehner, Louis P; Pfeifer, John D (2008). The Washington Manual of Surgical Pathology (1st ed.). Lippincott Williams & Wilkins. pp. 147. ISBN 978-0781765275.
  7. ↑ 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. 708. ISBN 0-7216-0187-1.
  8. ↑ Masaoka, A.; Monden, Y.; Nakahara, K.; Tanioka, T. (Dec 1981). "Follow-up study of thymomas with special reference to their clinical stages.". Cancer 48 (11): 2485-92. PMID 7296496.
  9. ↑ CJS. January 2010.
  10. ↑ CJS. January 2010.
  11. ↑ CJS. January 2010.
  12. ↑ URL: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1886385/pdf/amjpathol00102-0156.pdf. Accessed on: 26 August 2010.
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