Immune System

Thymus - Cyst

Narrative
Comment:
Thymic cysts (Figure 1, arrows) in the rodent represent either a dilatation of thymic tubular structures or remnants of the thymopharyngeal duct. They are common findings in the involuted and/or atrophied thymus glands of rats and mice. Thymic cysts are typically lined by cuboidal to squamous epithelial cells (Figure 2, arrow), which may be ciliated and contain variable amounts of homogeneous eosinophilic material (Figure 2, arrowhead). Thymic cyst formation becomes more prominent with age and is associated with involution. Formation of cysts may occur more commonly in 2-year-old female rats than in age-matched males.
Recommendations:
For routine studies, cysts should be diagnosed only when indicative of a treatment-related change. The preferred diagnostic term is "Thymus - Cyst." Cysts are not routinely graded in NTP studies, but they may be graded if the pathologist feels it is necessary to fully characterize the response to the chemical. If more than one cyst is present, the modifier "multiple" may be added to the diagnosis. Thymic cysts should also be described in the pathology narrative (e.g., appearance of lining endothelium, location, number). Cysts associated with another lesion (e.g., secondary to thymic atrophy) should not be diagnosed separately, but should be described in the pathology narrative.
References:

Dooley J, Erickson M, Farr AG. 2005. An organized medullary epithelial structure in the normal thymus expresses molecules of respiratory epithelium and resembles the epithelial thymic rudiment of nude mice. J Immunol 175:4331-4337.

Abstract: https://www.ncbi.nlm.nih.gov/pubmed/16177073

Khosla S, Ovalle EK. 1986. Morphology and distribution of cystic cavities in the normal murine thymus. Cell Tissue Res 246:531-542.

Abstract: https://www.ncbi.nlm.nih.gov/pubmed/3791381

National Toxicology Program. 2001. NTP TR-497. Toxicology and Carcinogenesis Studies of Methacrylonitrile (CAS No. 126-98-7) in F344 Rats and B6C3F1 Mice (Gavage Studies). NTP, Research Triangle Park, NC.

Abstract: https://ntp.niehs.nih.gov/go/14872

Pearse G. 2006. Histopathology of the thymus. Toxicol Pathol 34:515-547.

Full Text: http://tpx.sagepub.com/content/34/5/515.long

Pearse G. 2006. Normal structure, function and histology of the thymus. Toxicol Pathol 34:34:504-514.

Full Text: http://tpx.sagepub.com/content/34/5/504.full

Stefanski SA, Elwell MR, Stromberg PC. 1990. Spleen, lymph nodes, and thymus. In: Pathology of the Fischer Rat: Reference and Atlas (Boorman GA, Eustis SL, Elwell MR, Montgomery CA, MacKenzie WF, eds). Academic Press, San Diego, 369-394.

Ward JM, Mann PC, Morishima H, Frith CH. 1999. Thymus, spleen, and lymph nodes. In: Pathology of the Mouse (Maronpot RR, ed). Cache River Press, Vienna, IL, 333-360.

Wijnands MVW, Kuper CF, Schuurman HJ, Woutersen RA. 1996. Non-neoplastic lesions of the hematopoietic system. In: Pathobiology of the Aging Mouse (Mohr U, Dungworth DL, Capen CC, Carlton VW, Sundberg JP, Ward JM, eds). ILSI Press, Washington, DC, 205-235.

  • Image of cyst in the thymus from a female F344/N rat in a chronic study

    Thymus - Cyst, Multiple in a female F344/N rat from a chronic study. Multiple cysts (arrows) are present within the thymus.

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Thymus - Cyst, Multiple in a female F344/N rat from a chronic study. Multiple cysts (arrows) are present within the thymus.
Thymus - Cyst, Multiple in a female F344/N rat from a chronic study. Multiple cysts (arrows) are present within the thymus.
Thymus - Cyst, Multiple in a female F344/N rat from a chronic study (higher magnification of Figure 1). Thymic cyst is lined by cuboidal to squamous epithelium (arrow) and contains homogeneous eosinophilic material (arrowhead).
Thymus - Cyst, Multiple in a female F344/N rat from a chronic study (higher magnification of Figure 1). Thymic cyst is lined by cuboidal to squamous epithelium (arrow) and contains homogeneous eosinophilic material (arrowhead).

Authors

Kristen Hobbie, DVM, PhD
Principal Pathologist
Huntingdon Life Sciences
Peterborough, UK

Susan A. Elmore, MS, DVM, DACVP, DABT, FIATP
Staff Scientist, NTP Pathologist
NTP Pathology Group
National Toxicology Program
National Institute of Environmental Health Sciences
Research Triangle Park, NC

Holly M. Kolenda-Roberts, DVM, PhD, DACVP
Veterinary Pathologist
SNBL USA
Everett, WA

Reviewers

Jerrold M. Ward, DVM, PhD, DACVP, FIATP
Veterinary Pathologist
Global VetPathology
Montgomery Village, MD

Robert R. Maronpot, DVM, MS, MPH, DACVP, DABT, FIATP
Senior Pathologist
Experimental Pathology Laboratories, Inc.
Research Triangle Park, NC