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Diagnosis | Thymoma

Lab Animal volume 38, pages 116–117 (2009)Cite this article

In rats, the thymus is a pinkish-gray, soft organ located along the ventral aspect of the trachea, dorsal to the sternum at the thoracic inlet. It is composed of two distinct encapsulated lobes. In contrast to most mammals, in rats, the thymus does not completely involute and atrophy with age, although thymic weight does decrease by a factor of 7 between 3 months and 18 months of age1. The thymus is a primary lymphoid organ that serves the immune system by providing an optimal microenvironment for developing T cells. The microenvironment of the organ is peculiar among lymphoid organs, as the supporting stroma consists of reticular epithelial cells2. Lymphocyte precursors from the bone marrow travel to the thymus and develop into thymocytes (immature lymphocytes) and, subsequently, T lymphocytes (or T cells). Mature T lymphocytes exit the thymus and comprise the peripheral T-cell population responsible for directing many components of the adaptive immune system. Congenital absence of a thymus (as in nude mice and rats) or surgical removal at an early age results in severe immunodeficiency and a high susceptibility to infection3.

Thymomas are composed of a mixture of lymphocytes and epithelial cells. The epithelial cells are considered the tumor cell component in thymomas, whereas lymphocytes are the neoplastic component in thymic lymphomas. In thymomas, the relative composition ranges from almost entirely epithelial cells to almost entirely lymphocytes with various proportions between4,5. A definitive diagnosis may be challenging in lymphocyte-rich thymomas because the histological sections may resemble a lymphoma more than a thymoma. The presence of small groups of epithelial cells in enlarged thymuses and the absence of involvement of other organs are generally considered diagnostic features of thymomas6,7.

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Figure 2: Photomicrograph of a section of a rat thymoma stained with hematoxylin and eosin.
Figure 3: Photomicrograph of a rat thymoma immunostained with wide-spectrum screening cytokeratin antibody.

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Acknowledgements

We thank Matthew Davis (Georgia State University) for veterinary technical support and for taking the gross photographs.

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Diagnosis | Thymoma. Lab Anim 38, 116–117 (2009). https://doi.org/10.1038/laban0409-116

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