A 9-month-old primiparous mouse, a retired breeder from a colony of transgenic adenocarcinoma of the mouse prostate (TRAMP) mice, was examined for perianal swelling (Fig. 1). The mouse had never undergone any experimental manipulations.

Figure 1
figure 1

A euthanized 9-month-old, retired breeder female TRAMP mouse with a firm, raised perianal mass.

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All animals in the TRAMP colony were group-housed in microisolator cages on flat-shelf racks and maintained in specific pathogen-free conditions. The mice had ad libitum access to a standard autoclaved laboratory mouse diet and autoclaved reverse osmosis-treated water. The microbiological status of the colony, as determined by quarterly sentinel testing, was negative for 12 viral pathogens and for endoparasites and ectoparasites.

The mouse was euthanized via CO2 narcosis. On necropsy, we observed a firm, raised perianal mass (Fig. 1). The mass was excised and submitted for histopathologic examination; we observed no other gross abnormalities.

What is the most likely histologic diagnosis of the perianal mass? What is the most likely tissue of origin of the perianal mass? Do you think the perianal mass is a tumor? If it is a tumor, could tumorigenesis be associated with expression of the transgene? How would you then prove or disprove your hypothesis?

What's your diagnosis?