Mechanisms Affecting Oocyte Competence in the Bovine Model: Effect of Follicular and Maternal Aging

影响妊娠成败的最关键时期是胚胎发育的前7天。超过95%的卵母细胞（卵子）在授精（受孕）后受精，但不到50%的胚胎不能发育。失败率如此之高的原因尚不清楚，但与卵母细胞及其生长的卵泡的质量有关（卵泡是由颗粒和卵泡细胞组成的充满液体的结构）。我的研究计划的长期目标是阐明影响卵母细胞发育成健康胚胎的能力（即卵母细胞能力）的内分泌、细胞和分子机制。本文提出的研究是基于2002年建立的富有成果的研究，并在以前的NSERC资助下成为可能。我在萨斯喀彻温大学的研究小组已经开发了用于研究卵泡和生殖衰老的牛模型。通过延长促卵泡激素（FSH）治疗的时间，我们能够增强卵母细胞产生胚胎的能力。但是，如果没有适当的促卵泡刺激素支持，卵泡生长的寿命仅延长3-4天（卵泡老化），卵母细胞的能力就会受到损害。我们还记录了年龄≥13岁的奶牛（母亲衰老）与其年幼的女儿相比，卵母细胞能力明显下降。有趣的是，从小于4个月大的小牛（青春期前早期）获得的卵母细胞也具有较低的发育能力。卵母细胞似乎在小牛大约10个月大（青春期周围年龄）时达到完全的能力。我未来5年的研究重点是了解影响青春期前卵母细胞能力的细胞和内分泌途径。我们的工作假设是，在青春期前造成卵母细胞发育能力低下的内分泌环境和局部卵巢因素是独特的，与那些导致老年期能力丧失的因素不同。我们的目标是1)确定较长时间的促性腺激素支持对青春期前衰老过程中卵泡动力学和卵母细胞能力的影响，并检查2)参与卵泡功能的分子途径和3)青春期前发育过程中下丘脑-垂体-卵巢轴的反应性。从这些研究中获得的信息对衰老生物学具有广泛的意义，并且与开发有效的动物辅助生殖技术密切相关。我们预计我们的结果将允许直接改善牛的超排卵，体外胚胎生产和后代测试。在更长的时间尺度上，我的NSERC关于卵泡和母体衰老的发现项目将继续扩大我们对哺乳动物物种中卵母细胞发育成胚胎的能力机制的基本理解。

The most critical period influencing the success or failure of pregnancy is the first 7 days of embryo development. More than 95% of oocytes (eggs) become fertilized after insemination (conception) but >50% of the embryos fail to develop. The reason for this high rate of failure is unclear, but is associated with quality of the oocyte and the ovarian follicle in which it grows (a follicle is a fluid-filled structure composed of granulosa and theca cells). The long-term objective of my research program is to elucidate the endocrine, cellular and molecular mechanisms that affect an oocyte's ability to develop into a healthy embryo (i.e., oocyte competence). Studies proposed herein are based on a fruitful line of research established in 2002 and made possible with previous NSERC funding. My research team at the University of Saskatchewan has developed the bovine model for the study of follicular and reproductive aging. By extending the duration of treatment with follicle stimulating hormone (FSH), we are able to enhance the oocytes' ability to produce embryos. But, oocyte competence is compromised when the life-span of a growing follicle is prolonged by as little as 3-4 days (follicular aging) without proper FSH support. We have also documented that cows aged ≥13 years (maternal aging) have a marked decrease in oocyte competence compared to their young daughters. Interestingly, oocytes obtained from calves ≤4 months of age (early pre-pubertal age) also have lower developmental competence. It appears that oocytes attain full competence when the calf is about 10 months of age (peri-pubertal age). The focus of my research program for the next 5 years is to understand the cellular and endocrine pathways that influence the oocyte competence during the pre-pubertal period. Our working hypothesis is that the endocrine milieu and local ovarian factors responsible for poor developmental competence of oocytes during the pre-pubertal period are unique and distinct from those that cause the loss of competence during old age. Our objectives are to 1) determine the effects of a longer period of gonadotropin support on follicular dynamics and oocyte competence during the pre-pubertal aging, and to examine the 2) molecular pathways involved in follicular function and 3) responsiveness of hypothalamic-pituitary-ovarian axis during the pre-pubertal development. Information gained from these studies has broad implications regarding the biology of aging and is germane to the development of efficient assisted reproductive techniques for animals. We anticipate that our results will allow direct improvements of superovulation, in vitro embryo production and progeny testing in cattle. On a longer time–scale, my NSERC Discovery program on follicular and maternal aging will continue to expand our basic understanding of mechanisms that imbue competence of the oocyte to develop into an embryo in mammalian species.