Immune System

Thymus - Hyperplasia, Epithelial

Narrative
Comment:
Epithelial hyperplasia is often associated with involution and/or atrophy of the thymus and may occur at relatively high incidences in some rodent strains. Thymic epithelial hyperplasia is found in the medulla and may be focal to diffuse. Epithelial cells are cuboidal to columnar, have paler-staining abundant cytoplasm, and may be ciliated. They often form tubules (Figure 1, arrow), ribbons, or cords (Figure 1, arrowhead). Tubules can contain variable amounts of eosinophilic secretory material (Figure 2, arrow). Epithelial changes are more prominent in females than in males and in rats more so than in mice. Pleomorphic variations of thymic epithelial hyperplasia are similar to the spectrum of cellular forms of thymoma. Consequently, thymic epithelial hyperplasia may be difficult to distinguish from early, benign thymomas. However, thymomas are nodular and compressing, whereas hyperplasias are not.
Recommendations:
Epithelial hyperplasia in the thymus should be diagnosed and graded when not associated with involution.
References:

Elmore SA. 2006. Enhanced histopathology of the thymus. Toxicol Pathol 34:656-665.

Full Text: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1800589/

Frith CH, Ward JM, Brown RH, Tyler RD, Chandra M, Stromberg PC. 1996. Proliferative lesions of the hematopoietic and lymphatic systems in rats. HL-1. In: Guides for Toxicologic Pathology. STP/ARP/AFIP, Washington, DC.

National Toxicology Program. 1998. NTP TR-477. Toxicology and Carcinogenesis Studies of 1-Chloro-2-Propanol (Technical Grade) (CAS No. 127-00-4) in F344/N Rats and B6C3F1 Mice (Drinking Water Studies). NTP, Research Triangle Park, NC.

Abstract: https://ntp.niehs.nih.gov/go/9756

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

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

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.

  • Image of hyperplasia, epithelial in the thymus from a male B6C3F1/N mouse in a chronic study

    Thymus - Hyperplasia, Epithelial in a male B6C3F1 mouse from a chronic study. Hyperplastic epithelial cells form tubules (arrow) and cords (arrowhead) within the thymus medulla.

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Thymus - Hyperplasia, Epithelial in a male B6C3F1 mouse from a chronic study. Hyperplastic epithelial cells form tubules (arrow) and cords (arrowhead) within the thymus medulla.
Thymus - Hyperplasia, Epithelial in a male B6C3F1 mouse from a chronic study. Hyperplastic epithelial cells form tubules (arrow) and cords (arrowhead) within the thymus medulla.
Thymus - Hyperplasia, Epithelial (higher magnification of Figure 1). Tubules formed by hyperplastic epithelial cells contain eosinophilic secretory material (arrow).
Thymus - Hyperplasia, Epithelial (higher magnification of Figure 1). Tubules formed by hyperplastic epithelial cells contain eosinophilic secretory material (arrow).

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