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Follicular Dynamics Around the Recruitment of the First Follicular Wave in the Cow¹

Department of Farm Animal Health,⁴ Department of Biochemistry and Cell Biology,⁵ Faculty of Veterinary Medicine, Utrecht University, 3508 TD Utrecht, The Netherlands Holland Genetics,⁶ Arnhem, The Netherlands Institute for Animal Science and Health,⁷ Lelystad, The Netherlands

The present study aimed to test the generally accepted view that a follicular wave starts with follicles newly recruited from the population smaller than 3 mm, which later compete for dominance. According to this view, subordinate follicles are expected to be too atretic to join the next follicular wave. Ten cows were ovariectomized shortly prior to the LH surge, thus around the start of the first follicular wave of the cycle. Per cow, on average, 14.4 follicles of 3 mm were dissected. Follicular health was determined on the basis of four parameters: 1) judgment of the degree of atresia by stereomicroscope, 2) incidence of apoptotic nuclei among the granulosa cells, 3) estradiol and progesterone concentrations, and 4) insulin-like growth factor-I (IGF-I) binding proteins (IGFBPs)-2, -4, and -5 concentrations in the follicular fluid. In addition to the preovulatory follicle, 3.1 other follicles, mainly sized 3–4.5 mm, were found to be healthy based on the proportion of apoptotic nuclei, and concentrations of estradiol/progesterone, and IGFBPs. The ability of these follicles to respond with growth on the preovulatory and periovulatory FSH surges was supported by a comparison to the follicular population of four cows 31–68 h after the LH surge. The present results point to an alteration of the view on the follicular wave. The larger follicles during the first days of the follicular wave are, in general, derived from follicles that also joined the previous wave. A portion of these growing follicles are estradiol active and compete for dominance. Other growing follicles lack estradiol production and are probably derived from rather atretic follicles. The first newly recruited follicles do not reach the size of 3 mm before 31 h after the preovulatory FSH surge. At that time, the larger follicles are already competing for dominance.

¹ This paper is dedicated to Dr. Mart M. Bevers, who died in the autumn of 2002, in remembrance of the significant contributions he made to the research of our group in Utrecht. Supported by Holland Genetics, Arnhem, the Netherlands, and the European Union in the framework of the Eureka BTIP project Oocyte-Dev-Comp EU1569.

² Correspondence: P.J.M. Hendriksen, Department of Urology, Erasmus MC, JNI, Rm Be355a, Dr. Molewaterplein 50, 3015 GE Rotterdam, The Netherlands. FAX: 31 10 408 9386; p.hendriksen{at}erasmusmc.nl

³ Current address: Department of Urology, Erasmus MC, Rotterdam, The Netherlands

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