Most of them are benign.
Malignant tumours invade the ovary and ovarian bursa and tend to spread over the peritoneum.
There may be oestrogen, progesterone or androgen production. Histologically the cystic appearance evident grossly is reflected in the microscopic appearance. Numerous clusters of small round cells reflect the tumour s origin from granulosa cells of the ovarian follicle. . . Teratomas These are tumours with, usually-well differentiated, tissue types from each of the three embryonic germ cell layers. . They are occasionally found incidentally at ovariohysterectomy in dogs.