|Roberts, Andrew - Andy|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/16/2003
Publication Date: 2/20/2004
Citation: ECHTERNKAMP, S.E., ROBERTS, A.J., LUNSTRA, D.D., WISE, T.H., SPICER, L.J. 2004. OVARIAN FOLLICULAR DEVELOPMENT IN CATTLE SELECTED FOR TWIN OVULATIONS AND BIRTHS. JOURNAL OF ANIMAL SCIENCE. v. 82(2). p. 459-471. Interpretive Summary: Long-term selection of cattle for ovulation rate and twinning rate has modified the regulatory system of ovarian folliculogenesis to enhance follicular development and to induce selection of two ovulatory follicles within a follicular wave; consequently, frequency of twin births has increased to over 50%. The increased follicular development may result from greater (100%) blood and follicular concentrations of IGF-I in Twinners vs Control cows; gonadotropin and steroid secretion did not differ between cattle populations. Absence of IGFBP-2, -4, and -5 in developing preovulatory follicles suggests roles for these binding proteins in the recruitment and selection of the dominant follicle(s), possibly by regulating IGF-I and -II availability to the follicular cells. Disappearance of IGFBP-2 binding activity occurred only in the advanced maturational stages of development of the preovulatory follicle(s) further suggests a role for IGFBP-2 in the selection of ovulatory follicles.
Technical Abstract: Comparisons of numbers of antral ovarian follicles and CL, of blood hormone concentrations, and of follicular fluid steroid concentrations and IGFBP activity were conducted between cows selected (Twinner) and unselected (Control) for twin births to illucidate genetic differences in the regulation of ovarian follicular development. Ovarian follicular development was synchronized among cows by a single i.m. injection of PGF2alpha on d 18 of the estrous cycle; six cows per population were slaughtered at 0, 24, 48, and 72 h after PGF2alpha. Jugular vein blood (20 mL) was collected from each animal at PGF2alpha injection and at 24-h intervals until slaughter. Ovaries of Twinner cows contained more small (</= 5 mm in diameter, P < 0.05), medium (5.1 - 9.9 mm, P < 0.05), and large (> 10.0 mm, P < 0.01) follicles and more (P < 0.01) CL than ovaries of Controls. Follicular fluid concentrations of estradiol (E), androstenedione, testosterone, and progesterone (P) in medium and large follicles reflected stage of follicular development and were similar for Twinner and Control follicles at the same stage. Earlier initiation of follicular development and(or) selection of twin dominant follicles in some Twinner cows resulted in greater concentrations of E in plasma at 0, 24, and 48 h and of E, androstenedione, and testosterone in follicular fluid of large follicles at 0 h after PGF2alpha for Twinner vs Control cows (follicular status x time x population, P < 0.01). Binding activities of IGFBP-5 and -4 were absent or reduced (P < 0.01) in follicular fluid of developing medium and large E-active (E:P ratio > 1) follicles but increased with intensity of atresia. Preovulatory Graafian follicles lacked IGFBP-2 binding activity suggesting a possible role for IGFBP-2 in selection of the dominant follicle(s). IGF-I concentrations were two-fold greater (P < 0.01), but GH (P = 0.1) and cholesterol (P < 0.05) were less, in blood of Twinners. Three generations of selection of cattle for twin ovulations and births has enhanced ovarian follicular development as manifested by increased numbers of follicles within an emerging follicular wave and subsequent selection of twin (multiple) dominant follicles. Because gonadotropin secretion and ovarian steroidogenesis were similar for Control and Twinner cattle, enhanced follicular development in Twinners may result from decreased inhibition by the dominant follicle(s), increased ovarian sensitivity to gonadotropins, and(or) increased intra-gonadal stimulation, possibly by increased IGF-I.