Thymus
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Thymus is an annoying little organ that is in the mediastinum.
Overview
- Thymus involutes after childhood.
- Location: anterior mediastinum.
- Important for development of the immune system.
- May contain within it parathyroid.[1]
Normal 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):
- Cortical epithelial cells.[1]
- Epithelioid.
- Abundant cytoplasm.
- Pale nuclei with small nucleoli.
- Medullary epithelial cells.[1]
- Spindle morphology.
- Scant cytoplasm.
- Oval dark nuclei.
- 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:
IHC and thymus
Types A, AB, B:[3]
- CK7-, CK20-, CAM5.2+, CK5/6+, p63+, CD5-.
Type C:[4]
- CD5+.
All types:[5]
- CD1a (immature T cells, Langerhans cells, dendritic cells[6]), CEA +ve (focal), vimentin -ve.
Others (immature T cells):
- TdT.
- CD99.
Anterior mediastinum mass DDx
Main article: Mediastinum
4 Ts (mnemonic):
Absence of the thymus
Di George syndrome
- Things go wrong with the thymus... very wrong.
Thymus and stress
- Stress -> increased endogenous steroid -> lymphocyte death -> increased tingible body macrophages.[7]
Specific conditions
Thymic follicular hyperplasia
- AKA thymic follicular hyperplasia.
Features:[8]
- Follicular centres in the thymus.
Associations:[8]
- Myasthenia gravis.
- Graves' diseases.
- Systemic lupus erythematosus (SLE).
- Rheumatoid arthritis.
- Other autoimmune diseases.
Thymoma
General
- Strong association with autoimmune disease, esp. myasthenia gravis.
Classification
The WHO published a widely used system - WHO classification:[9]
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.
Note:
- Neoplastic cells derived from the thymus with cytologic features of malignancy are thymic carcinomas.
Images:
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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:[10]
- Cytologically malignant - variable morphology.
- +/-Squamous differentiation.
DDx:
Images:
- Thymic carcinoma - low mag. (webpathology.com).
- Thymic carcinoma - high mag. (webpathology.com).
- Thymic carcinoma - lymphoepithelioma-like - high mag. (webpathology.com).
IHC
Features:[10]
- CD5 +ve.[11]
- CD7+ve.
- CD117 +ve.
- TTF-1 -ve.
Staging
There is a system by Masaoka et al..[12]
See also
References
- ↑ 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.
- ↑ URL: http://www.kumc.edu/instruction/medicine/anatomy/histoweb/lymphoid/lymph03.htm. Accessed on: 17 June 2010.
- ↑ CJS. January 2010.
- ↑ CJS. January 2010.
- ↑ CJS. January 2010.
- ↑ URL: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1886385/pdf/amjpathol00102-0156.pdf. Accessed on: 26 August 2010.
- ↑ 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.
- ↑ 8.0 8.1 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.
- ↑ 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.
- ↑ 10.0 10.1 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.
- ↑ 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.
- ↑ 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.