Spontaneous fibroadenoma in a twenty-one-week-old female Wistar rat - a case report

We describe the characteristic macro-and microscopic findings of a new case of spontaneous fibroadenoma in a 21-week-old, non-treated female Wistar rat. A mobile and non-ulcerated mass was detected affecting the caudal inguinal mammary gland. At necropsy, the mass was 5.5 cm in size, spherical in shape, and well demarcated with a smooth surface. The cut surface was pink, with a multilobulated pattern. Microscopically, fibroadenoma was composed of dense connective tissue surrounding luminal epithelial tissue, organized in tubular structures. The luminal epithelial cells were typical and, in some areas, presented lipid vacuoles in the cytoplasm and proteinaceous material in the lumen of the tubules. The interlobular and intralobular stroma showed variable amounts of mast cells. Immunohistochemically, epithelial cells expressed both oestrogen and progesterone receptors in the nucleus. Fibroadenoma is a frequent tumour, but it is uncommon at an early age.