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Mechanisms regulating follicle selection in ruminants: lessons learned from multiple ovulation models

机译：调节反刍动物卵泡选择的机制：从多种排卵模型中学到的经验教训

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Selection of a single dominant follicle from a cohort of growing follicles is a unique biological process, a key step in female reproductive function in monovular species, and lies at the core of reproductive technologies in cattle. Follicle growth and the number of follicles that ovulate are regulated by precise endocrine, paracrine, and autocrine mechanisms. Most of our current understanding about follicle selection focuses on the role of FSH, LH, and the IGF family in follicle growth and selection of the dominant follicle. However, more recently the role of members of the TGF-? family has been highlighted, particularly in high fecundity genotypes in sheep. Intercellular signaling between the oocyte and granulosa cells (GC) regulates proliferation and differentiation due to actions of bone morphogenetic protein 15 (BMP15) and growth and differentiation factor 9 (GDF9) within the follicle. Mutations that either knockout or reduce the activity of BMP15 or GDF9 have been found to increase ovulation rate in heterozygotes and generally cause severe follicle abnormalities in homozygotes. A mutation in the intracellular kinase domain of the BMPR1B receptor (Booroola fecundity gene) increases ovulation rate in heterozygotes with further increases in ovulation in homozygotes. The physiological mechanisms linking these mutations to increased ovulation rates are still not well defined. A recently identified high fecundity bovine genotype, Trio, causes increased expression of SMAD6, an intracellular inhibitor of the BMP15/GDF9 signalling pathways. This bovine model has provided insights into the mechanisms associated with selection of multiple dominant follicles and multiple ovulations in carriers of fecundity alleles. The present review focuses on the mechanisms involved in follicle selection in ruminants with a special emphasis on the contribution made by multiple ovulation models in both cattle and sheep. The evaluation of multiple ovulation models in ruminants has allowed us to construct a new physiological model that relates changes in the BMP15/GDF9 signalling pathways to the physiological changes that result in selection of multiple dominant follicles. This model is characterized by acquisition of dominance at a smaller follicle size but at a similar time in the follicular wave with multiple follicles acquiring dominance in a hierarchal sequence, delaying FSH suppression and, thus allowing additional follicles to continue to grow and acquire dominance.
机译：从生长的卵泡队列中选择单个优势卵泡是一个独特的生物学过程，是单孔物种雌性生殖功能的关键步骤，并且是牛生殖技术的核心。卵泡的生长和排卵的卵泡数受精确的内分泌，旁分泌和自分泌机制调控。我们目前对卵泡选择的大多数了解都集中在FSH，LH和IGF家族在卵泡生长和优势卵泡选择中的作用。但是，最近TGF-？成员的作用家族已被强调，特别是在绵羊的高繁殖力基因型中。卵母细胞和颗粒细胞（GC）之间的细胞间信号传导调节由于卵泡内骨形态发生蛋白15（BMP15）和生长分化因子9（GDF9）的作用而引起的增殖和分化。已发现敲除或降低BMP15或GDF9活性的突变会增加杂合子的排卵率，并通常在纯合子中引起严重的卵泡异常。 BMPR1B受体（Booroola繁殖力基因）的胞内激酶结构域中的突变可增加杂合子的排卵率，并进一步增加纯合子的排卵。将这些突变与排卵率增加联系起来的生理机制仍不清楚。最近鉴定出的高繁殖力牛基因型Trio导致SMAD6表达增加，SMAD6是BMP15 / GDF9信号通路的细胞内抑制剂。这种牛模型提供了与生育力等位基因携带者中多个优势卵泡和多个排卵的选择相关的机制的见解。本综述着重于反刍动物卵泡选择的机制，特别着重于牛和羊的多种排卵模型所做出的贡献。反刍动物中多个排卵模型的评估使我们能够构建新的生理模型，该模型将BMP15 / GDF9信号通路的变化与导致选择多个优势卵泡的生理变化相关联。该模型的特征是在较小的卵泡大小但在卵泡波中的相同时间获得了支配地位，其中多个卵泡按层级顺序获得了支配地位，从而延迟了FSH抑制，从而使其他卵泡继续生长并获得了支配地位。

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