Of 222 infertile Arabian mares, unilateral ovarian
enlargement was detected in three cases. Ultrasonography suggested
the presence of ovarian tumours. Blood samples were
taken for hormonal analysis, and the mares underwent surgery
for ovarian exploration. Ovarian specimens were processed for
microscopic examination. On histopathology, an adult-type granulosa
cell tumour (GCT), a mixed germ cell tumour and a lowgrade
Sertoli cell tumour were diagnosed. The GCT (6.5 cm in
diameter) was rounded and cystic, with a central hypoechogenic
area. The ovarian tissue was entirely replaced by polygonal
neoplastic cells. The mixed germ cell tumour (15.3 cm in diameter)
was hard and non-homogeneous. The ovary contained
insular patterns of round, oval or spindle cells with myxomatous
and hyalinised stroma. The stroma also contained osteoid tissue.
The ovarian low-grade Sertoli cell tumour (19×20 cm) was a
solid, fairly round homogeneously echogenic structure. The
ovarian tissue was completely replaced by aggregated tubules
lined by Sertoli-type cells surrounded by abundant fibrous stroma.
The three types of tumours secreted relatively low levels of
estradiol 17β and progesterone, whereas the level of inhibin-B
was extremely high in the case of the Sertoli cell tumour. In
conclusion, this study described three types of ovarian tumours
causing infertility in Arabian mares. Furthermore, the Sertoli cell
tumour is described here for the first time in mares.