Submitted to: Endocrinology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/22/1995
Publication Date: N/A
Citation: Interpretive Summary: In cattle, normal growth and selection of a single ovulatory follicle is a prerequisite for maturation and release of a viable female gamete (oocyte or egg). In addition, hormones from the ovulatory follicle, and the resulting corpus luteum, prepare the uterus for gamete transport, fertilization and embryonic development. Fertilization rate for the female egametes is above 95%; however, only 70 to 75% of the fertilized gametes result in birth of a calf. An evaluation of steroid, insulin-like growth factors-I and -II (IGF-I and -II) and IGF binding protein concentrations in follicular fluid indicated that androstenedione and estradiol (aromatase enzyme activity) production increased early in the selection of the ovulatory follicle. The wall of the follicle is composed of granulosa and theca cells which contain the steroidogenic enzymes. Theca cells of the ovulatory follicles had increased binding of luteinizing hormone (LH) which hin turn increased androstenedione production, the precursor for estradiol production. The majority of the IGF-I and -II in body tissues and fluids is bound to IGFBPs which sequester the biological activity of the IGFs. Although concentrations of IGF-I and -II did not change with follicle status, IGFBP concentrations were significantly reduced in ovulatory follicles which would increase the biological activity of the IGFs and enable them to stimulate the follicular cells to replicate and to produce steroids. These results indicate that selection of the ovulatory follicle results from an increase in LH binding by theca cells, and a subsequent increase in androgen and estrogen production, and reduction in IGFBP.
Technical Abstract: Associations among concentrations of androstenedione, estradiol, insulin-like growth factors-I and -II (IGF-I and -II) and IGF binding proteins (IGFBP) in follicular fluid, levels of LH and IGF-I binding in follicular cells and development of the dominant follicle were evaluated during the first follicular wave of the estrous cycle in lactating Holstein ncows. Ovarian follicular development was monitored daily by ultrasound an dominant follicles were collected by ovariectomy either early in dominance (first detectable, Day 4 to 6) or late in dominance (stopped growing, Day 8 to 12). Follicles were classified as dominant (DF), large (LG, subordinate >/= 6 mm) or small (SM, < 6 mm). In addition to the larger diameter, the second characteristic of a dominant follicle was the elevation in follicular fluid estradiol concentration. Although the DF continued to grow between early and late period, estradiol was significantly (P<.05) reduced in the late DF but higher (P<.01) than in LG and SM follicles. Levels of IGFBP-2 in follicular fluid were significantly (P<.05) lower in DF than in LG or SM follicles; whereas IGFBP-3, progesterone and IGF-I were unaffected by follicle size. Levels of specific 125I-hCG binding were severalfold greater in granulosa than in theca cells. The 125I-hCG binding was elevated in theca cells from both early and late DF whereas, only granulosa cells of late DF had elevated 125I-hCG binding. Concentrations of IGF-II were greater (P<.1) in SM than in DF or LG follicles and unaffected by Day. Results suggest that a reduction in IGFBP-2 and an increase in thecal binding of LH/hCG are associated with establishment of the dominant follicle of the first follicular wave of the bovine estrous cycle.