Research articleEffect of ovarian stimulation on oocyte gene expression in cattle
Introduction
Superovulation increases the number of offspring from genetically valuable cows and decreases the cost of embryo transfer by increasing embryo collection rates. Superovulation is also critical for other types of assisted reproduction technology in cattle [1], sheep [2], rabbits [3], monkeys [4], and mice [5]. However, the yield and quality of the embryos obtained after superovulation are variable and unpredictable, because of variations in ovarian response, fertilization rate, and embryo development [6]. Other factors, such as genetic background, age [7], breed [8], and reproductive and lactation status, may contribute to this variability. Perhaps some follicles in which growth is stimulated by the gonadotrophin contained rescued atretic follicles that remained aberrant. In addition, the shorter interval between luteolysis and the LH surge in gonadotrophin-stimulated compared to unstimulated cows could also be involved [9]. Variability in the FSH: LH ratios of gonadotrophin preparations is also thought to affect the superovulatory response.
Cytologic characteristics of oocytes recovered from cattle after superovulation were reported and no adverse effect on the pattern of nuclear maturation was detected. However, in subsequent reports, one third of oocytes aspirated from superovulated cattle during the preovulatory period had abnormal nuclear maturation [10]. In other species, aberrant DNA methylation of imprinted loci was detected in oocytes collected from superovulated women and mice [11], [12]. Regardless, in many studies, superovulation had no significant effect, other than a small increase in the number of transferable embryos [13]. Moreover, treatment with FSH provided good quality embryos in sheep [2], a higher percentage of embryos in rabbits [3], as well as a higher developmental potential for monkey embryos (through a 300 UI rhFSH regimen [4]). In the present study, cDNA microarrays and QPCR were used to determine the impact of hormonal stimulation on the bovine oocyte transcriptome or mRNA reserve.
Section snippets
Cows
Twenty cyclic, clinically healthy, non-lactating Holstein Friesian cows, 15 to 20 mo of age, were selected and distributed into two groups for superovulation and non-superovulation treatments. Cows were fed 20 kg corn silage and 4 kg concentrate per day, with grass silage and water available ad libitum. To perform the micro array analysis, tissues were obtained from Prof. Dieleman in Utrecht and the superovulation protocol used was previously described [14]. In brief, cows were presynchronized
Microarray hybridization
Hybridization experiments were performed with the custom-made bovine oocyte microarray using aRNA from pools of oocytes at three stages of maturation from two treatments: −2, 6, and 22 h post-LH surge from non-superovulated or superovulated cows. Seven hybridization experiments were performed with a dye swap design (Table 1). Determination of the background hybridization signal threshold was performed by considering all the GFP negative controls and SpotReport Alien cDNA. All misshapen spots
Discussion
Messenger RNA expression profiling has emerged as a useful tool to identify factors that affect oocyte and early embryo transcription. Numerous studies have demonstrated a close relationship between oocyte or embryo quality and the mRNA abundance of panels of various genes [18], [19], [20], [21]. Their relationship with oocyte quality has been assessed by various means, including morphology of the ovaries [9], developmental timing of embryos [22], isolating oocytes from follicles of various
Acknowledgments
We thank Dieleman for his help in supplying the oocytes used for microarray and Christian Vigneault at L'Alliance Boviteq for his help in collecting the oocytes used for QPCR. Thi-Kieu-Oanh Chu is supported by a VOSP.
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