Reproductive cycles in sheep

Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, 50 Stone Road, Guelph, Ontario, Canada.
Animal reproduction science (Impact Factor: 1.51). 02/2011; 124(3-4):259-68. DOI: 10.1016/j.anireprosci.2011.02.024
Source: PubMed


During the last three decades, there has been remarkable progress in many aspects of ovarian biology due to advances in real-time ultrasonography, which permits non-invasive, repeated monitoring of ovarian structures in conscious and non-anaesthetised animals. This review is primarily concerned with ovarian activity, as determined by transrectal ultrasonography, and measurements of circulating concentrations of gonadotrophins and ovarian steroids during reproductive cycles in sheep. The growth of antral follicles reaching ostensibly ovulatory sizes occurs in a wave-like pattern throughout the breeding season in both prolific and non-prolific breeds of sheep. There are typically 3 or 4 waves of follicle development during the interovulatory interval. Follicular wave emergence is primarily controlled by changes in circulating concentrations of follicle-stimulating hormone (FSH) but diminished ovarian responsiveness to gonadotrophic signals may result in reduced numbers of follicular waves. In cyclic ewes, the largest ovarian follicles acquire the ability to secrete oestradiol from the day of emergence with peak oestradiol secretion occurring about the time they reach maximum diameter. The high ovulation rate in some prolific breeds may be achieved by the ovulation of follicles from the last two waves of the interovulatory interval. Prolific ewes tend to produce more but smaller corpora lutea (CL) and have lower serum concentrations of progesterone during the luteal phase of the oestrous cycle as compared to less prolific genotypes. Lastly, recent studies of the endocrine influences on ovarian function have brought into question the existence of strong follicular dominance, as seen in cattle, and provided new insights into the effects of luteal progesterone on antral follicular development in ewes.

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Available from: Pawel Mieczyslaw Bartlewski
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    • "In contrast to these observations, more recent investigations into the growth dynamics of the two largest follicles detected at the start of superovulation in seasonally anovular ewes showed no difference in superovulatory outcomes regardless of follicular size or stage of the life span [6]. Follicular dominance in sheep remains controversial due mainly to the inconsistent evidence present in the available literature [18]. Progestin–estradiol combination treatment synchronizes follicular wave emergence in cattle and significantly reduces the variability associated with superovulatory treatments initiated at random stages of follicular wave development [26] [27]. "
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    ABSTRACT: Follicular wave status at the beginning of exogenous FSH administration is an important contributor to variability in superovulatory responses in ruminants. Studies in ewes have shown a decrease in the number of ovulations when superovulation is initiated in the presence of ostensibly ovulatory-sized ovarian follicles. Hormonal ablation of large antral follicles with the progestin-estradiol (E2-17β) treatment significantly reduces this variability in superovulated anestrous ewes, but the effects of the treatment in cycling ewes have not yet been assessed. Sixteen Rideau Arcott × Polled Dorset ewes (November-December) received either medroxyprogesterone acetate (MAP)-releasing intravaginal sponges (60 mg) or controlled internal drug release (CIDR) devices (containing 300 mg of natural progesterone) for 14 days (Days 0-14), with a single intramuscular injection of 350 μg of E2-17β on Day 6. The superovulatory treatment consisted of six injections of porcine FSH (Folltropin-V) given twice daily, followed by a bolus GnRH injection (50 μg intramuscular) on Day 15. There were no differences (P < 0.05) in the ovulatory responses and embryo yields between the two groups of ewes. In both subsets of animals, the next follicular wave emerged ∼2.5 days after an E2-17β injection (P > 0.05). A decline in maximum follicle size after an E2-17β injection was more abrupt in CIDR- compared with MAP-treated animals, and the ewes pretreated with exogenous progesterone had significantly more 3-mm follicles at the start of the superovulatory treatment. The metabolic clearance rate of exogenous E2-17β appeared to be greater in MAP-treated ewes, but circulating concentrations of porcine FSH failed to increase significantly after each Folltropin-V injection in CIDR-treated animals. The CIDR-treated ewes exceeded (P < 0.05) their MAP-treated counterparts in serum E2-17β concentrations during superovulation. In spite of differences in antral follicle numbers and endocrine profiles between MAP- and CIDR-treated cyclic ewes receiving E2-17β before ovarian superstimulation, there were no differences in superovulatory responses. Copyright © 2015 Elsevier Inc. All rights reserved.
    Full-text · Article · Sep 2015 · Theriogenology
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    • "These are considerably shorter periods than those possible for ovaries of smaller species as described above, particularly if the length of the ovarian cycle is considered. For instance, the average ewe oestrous cycle length is 17 days and is characterised by the occurrence of three or four follicular waves (Bartlewski et al. 2011). Hence, if ovary perfusion is to be considered as a possible option for promoting follicle development, it must be able to sustain ovarian function for at least 3 to 4 days. "
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    ABSTRACT: We describe an original perfusion system for the culture of whole ovine ovaries for up to 4 days. A total of 33 ovaries were divided into six groups: control (n = 6), not perfused and fixed; Groups SM72 and SM72-FSH (n = 6 each), perfused with a simple medium for 72 h with or without FSH; Groups CM96 and CM96-FSH (n = 6 each), perfused with a complex medium for 96 h with or without FSH; Group CM96-FSH-cryo, (n = 3) cryopreserved and perfused for 96 h with Group CM96-FSH medium. Depending on the medium used, morphological parameters of cultured ovaries differed from fresh organs after 72 (SM72, SM72-FSH) or 96 (CM96, CM96-FSH) h of perfusion. Oestradiol and progesterone were secreted in all groups but FSH had an effect only on Group CM96-FSH, stimulating continued oestradiol secretion 10 times higher than in all other groups. Morphological parameters and hormone secretion of cryopreserved ovaries were not different from fresh controls. This method enables the culture of whole ovaries for up to 4 days, the time required in vivo for 0.5-mm follicles to grow to 2.2 mm and then for these follicles to reach the ovulatory size of 4 mm or more. It could be used as a research tool or to complement current techniques for preserving female fertility.
    Full-text · Article · Jun 2015 · Reproduction Fertility and Development
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    • "In response to the preovulatory gonadotropin surge occurring in each reproductive cycle, the Graafian follicle ovulates to release the mature oocyte ready to be potentially fertilized. However, throughout antral development, the majority of follicles undergo degeneration or atresia -the process responsible for the depletion of most of the ovarian follicles present in the ovary (Mcgee & Hsueh, 2000; Balasch et al., 2010; Bartlewski et al., 2011). "

    Full-text · Article · Jan 2015
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