Ovarian follicular development in mink (Mustela vison)

Abstract

Ovarian follicular dynamics were studied during the breeding season, before and after ovulation in mink. Nulliparous female mink were stimulated to ovulate with an injection of 4 micrograms GnRH. Ovaries from three animals were collected on days 0, 2, 3, 4, 5, 6 and 7 after hormone treatment. A second dose of GnRH was administered on day 8 and ovaries were collected from three animals on day 9. Corpora lutea and follicles were identified in histological sections and follicles were classified by stage of development, healthy versus atretic, and by diameter. Preovulatory follicles (diameter 0.7-1.0 mm) were present in the ovaries of all animals on day 0 and these responded to GnRH treatment by ovulating. A synchronized wave of follicular development occurred following ovulation. Changes in follicle populations indicated that follicles are recruited from the small antral follicle class (0.2-0.4 mm) into the 0.4-0.6 mm class, with the first defined changes occurring between days 2 and 4. From the recruited group, a smaller cohort of follicles is selected to become the dominant follicles between days 4 and 6, and these acquire the ability to respond to a stimulus which induces ovulation at diameters of > 0.7 mm. The ovaries of unmated mink also contained substantial numbers of large, degenerating, luteinized, unruptured follicles. These degenerating, luteinized follicles are considered to represent the demise of large follicles that failed to receive an ovulatory stimulus.