Case Report: presence of granulosa cells in the uterine tissue of a spayed Labrador Retriever bitch.

A 10-year-old Labrador Retriever bitch, spayed at 2 years of age, was presented to our veterinary care facility with a history of vulvar swelling, vaginal discharge, and male attraction for over 1 month. Vaginoscopy revealed edema and reddening of the vaginal mucosa. Vaginal cytology was consistent with estrus, while serum progesterone levels remained at baseline. On abdominal ultrasound, the uterus was hyperplastic, and fluid-filled cysts were visualized at both ovarian anatomic sites. Hysterectomy and excision of tissue from both ovarian sites were performed 10 days after the initial presentation. The gross appearance of the uterus was consistent with cystic endometrial hyperplasia (CEH), confirmed by histopathology. Ovaries were not detected microscopically; however, small multifocal nests, occasionally forming pseudocysts or follicle-like structures, were observed in continuity with the uterine horns and embedded within the myometrium. These nests consisted of well-differentiated round to polygonal and occasionally spindle-shaped cells that underwent luteinization. These cells were immunopositive for inhibin-α, weakly immunopositive for progesterone receptor, and immunonegative for estrogen receptor α. Serum anti-Müllerian hormone (AMH) was elevated before hysterectomy and returned to basal levels afterward. Based on these findings, the cells were identified as granulosa cells embedded within the uterine tissue. We propose an embryological defect or iatrogenic seeding of ovarian tissue within the uterus as the most likely differential diagnosis, although neoplastic granulosa cells could not be completely ruled out, even though this is extremely unlikely. All clinical signs resolved after surgery, and the bitch did not exhibit any recurrence of estrus behavior until her death, approximately 2.5 years later.