Thymus - Atypical Hyperplasia Lymphocyte

Image of atypical hyperplasia, lymphocyte in the thymus from a female p53+/- (C57Bl/6) mouse in a subchronic study
Thymus - Atypical hyperplasia, Lymphocyte in a female p53+/- (C57Bl/6) mouse from a subchronic study. When atypical lymphocyte hyperplasia is unilateral, the affected lobe (arrow) is typically smaller than the unaffected lobe.
Figure 1 of 2
Image of atypical hyperplasia, lymphocyte in the thymus from a female p53+/- (C57Bl/6) mouse in a subchronic study
Thymus - Atypical hyperplasia, Lymphocyte in a female p53+/- (C57Bl/6) mouse from a subchronic study (higher magnification of Figure 1). Normal thymic architecture is replaced by a sheet of large, atypical lymphocytes, including lymphoblasts and fewer, admixed small lymphocytes.
Figure 2 of 2
next arrow

comment:

Atypical hyperplasia of the thymic lymphocytes has been described in chemically treated B6C3F1 and p53-deficient mice. This lesion also occurs in other mouse strains that develop thymic lymphomas and possibly in rats with chemically induced thymic lymphomas. It may be unilateral or bilateral; when unilateral, the unaffected lobe is typically larger than the abnormal lobe ( Figure 1image opens in a pop-up window , arrow), and atrophy of the other lobe may occur. This lesion is characterized by depletion of small lymphocytes and a diffuse change with loss of the normal thymic corticomedullary junction. Normal thymic architecture is replaced by a sheet of large, atypical lymphocytes, including lymphoblasts and fewer, admixed small lymphocytes ( Figure 2image opens in a pop-up window ); these cells do not extend beyond the capsule of the thymus. Atypical hyperplasia of the thymic lymphocytes is regarded as a proliferative change that may progress to lymphoma, although it may occur without progression. This putative preneoplastic lesion can be differentiated from lymphoma by the heterogeneous population of lymphocytes, mitotic rate, and lack of capsular invasion.

recommendation:

Whenever present, atypical hyperplasia of the thymic lymphocytes should be diagnosed and assigned a severity grade. It should be noted in the pathology narrative whether the lesion is present in one or both thymic lobes.

references:

Dunnick JK, Hardisty JF, Herbert RA, Seely JC, Furedi-Machacek EM, Foley JF, Lacks GD, Stasiewicz S, French JE. 1997. Phenolphthalein induces thymic lymphomas accompanied by loss of the p53 wild type allele in heterozygous p53-deficient (+/-) mice. Toxicol Pathol 25:533-540.
Abstract: http://www.ncbi.nlm.nih.gov/pubmed/9437796

National Toxicology Program. 2007. NTP GMM-12. Toxicology and Carcinogenesis Study of Phenolphthalein (CAS No. 77-09-8) in Genetically Modified Haploinsufficient p16Ink4a/p19Arf Mice (Feed Study). NTP, Research Triangle Park, NC.
Abstract: http://ntp.niehs.nih.gov/go/28495

Pearse G. 2006. Histopathology of the thymus. Toxicol Pathol 34:515-547.
Full Text: http://tpx.sagepub.com/content/34/5/515.long

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.

Ward JM, Tadesse-Heath L, Perkins SN, Chattopadhyay SK, Hursting SD, Morse HC 3rd. 1999. Splenic marginal zone B-cell and thymic T-cell lymphomas in p53-deficient mice. Lab Invest 79:3-14.
Abstract: http://www.ncbi.nlm.nih.gov/pubmed/9952106