A condition resembling pagetoid reticulosis in a laboratory mouse

A raised, hairless cutaneous nodule was found incidentally at necropsy of a 24-month-old CD1 mouse. Histologically there was infiltration of the epidermis by a monomorphic population of moderate to large lymphocytoid cells. Many large cells had bizarre convolutions of the nuclear membrane and resembled the so-called Sézary or mycosis cells seen in epidermotropic T-cell proliferative disorders. The pattern of cutaneous involvement and the presence of the large cells with convoluted nuclei is characteristic of pagetoid reticulosis. No previous reports of such cutaneous lymphoid neoplasms in mice were found in an extensive literature search.