Role of Fetal Steroids in Formation and Activation of Bovine Ovarian Follicles

描述（由申请人提供）：建立能够启动生长并可能发育至排卵前阶段的原始卵泡储备是女性生殖成功的先决条件。然而，对于非啮齿动物物种，对性腺发育中这些关键步骤的调节一无所知。在牛、羊和其他大型哺乳动物（包括人类）中，原始卵泡在胎儿生命期间的数周内形成，与小鼠和大鼠的出生后同步发育相反。此外，在反刍动物和灵长类动物中，在卵泡形成开始后的几周内，没有原始卵泡离开静止池（激活），这与新生大鼠和小鼠卵巢中新形成的原始卵泡子集的快速激活形成对比。胎牛和羊的卵巢积极合成类固醇激素，特别是在卵泡形成之前的时期，在较小程度上，在卵泡形成期间，雌二醇受体定位于体细胞和配子发生细胞。我们已经开发了实验模型，其中牛原始卵泡的形成和它们激活（即，以响应刺激物而启动生长）在体外发生。我们的初步数据表明，这种R21应用的总体假设：即胎牛卵巢产生的类固醇，特别是雌二醇和孕酮，调节卵泡形成和卵泡的能力被激活。我们提出了三个具体的目标来测试这个新的假设。在具体目标#1中，提出了一种实验来检验卵巢雌二醇（可能还有孕酮）产生的下降启动或允许体内卵泡形成的假设。我们在具体目标#2中提出了实验来检验以下假设：卵巢类固醇（雌二醇和孕酮）防止新形成的原始卵泡的过早激活，并且它们通过抑制或调节减数分裂前期I的进展来实现这一点（牛原始卵泡在卵母细胞在前期I的双线期达到减数分裂停滞之前形成）。根据我们的工作模型，在原始卵泡形成之前和期间，卵巢雌二醇和孕酮的产生逐渐下降，首先允许卵泡形成，然后促进卵泡能力的发展以激活。在具体目标#3中，我们将开始确定有助于类固醇对卵泡形成和激活能力的影响的信号通路。总之，这些实验将提供有关卵巢发育关键时期调控的新信息，为人类卵巢发育提供了一个极好的非灵长类动物模型。公共卫生相关性：在哺乳动物雌性中，原始卵泡池在胎儿或新生儿生命期间形成，并且由于该卵泡储备在雌性的生殖生命中用作排卵的卵母细胞来源，因此它对于生殖成功至关重要，并且也是生育力增强或避孕的潜在目标。在大型哺乳动物中，如牛、羊和灵长类动物（包括人类），原始卵泡在女性胎儿时形成，几乎没有人知道是什么调节了这一过程。我们建议使用牛作为动物模型来检验胎儿卵巢产生的类固醇在调节卵巢卵泡发育的这些早期阶段中发挥作用的假设。

DESCRIPTION (provided by applicant): Establishment of a stockpile of primordial follicles, capable of initiating growth and potentially developing to the preovulatory stage, is a prerequisite for female reproductive success. However for non-rodent species, nothing is known about the regulation of these critical steps in gonadal development. In cattle, sheep, and other large mammals, including humans, primordial follicles form over a number of weeks during fetal life, in contrast to their synchronous post-natal development in mice and rats. In addition, in ruminant and primate species no primordial follicles leave the resting pool (activate) for several weeks after follicle formation begins, in contrast to the rapid activation of a subset of newly formed primordial follicles in neonatal rat and mouse ovaries. The ovaries of fetal cattle and sheep actively synthesize steroid hormones, especially in the period before and, to a lesser extent, during follicle formation and estradiol receptors have been localized to both somatic and gametogenic cells. We have developed experimental models in which formation of bovine primordial follicles and development of their capacity to activate (i.e., to initiate growth in response to a stimulator) occur in vitro. Our preliminary data suggest the overall hypothesis of this R21 application: that steroids produced by the fetal bovine ovary, particularly estradiol and progesterone, regulate both follicle formation and the competence of follicles to be activated. We propose three specific aims to test this novel hypothesis. In Specific Aim #1, experiments to test the hypothesis that a decline in ovarian production of estradiol, and perhaps progesterone, initiates or allows follicle formation in vivo are proposed. We propose experiments in Specific Aim #2 to test the hypotheses that ovarian steroids (estradiol and progesterone) prevent the premature activation of newly formed primordial follicles and that they do this by inhibiting or regulating the progression of prophase I of meiosis (bovine primordial follicles form before the oocyte has reached meiotic arrest in the diplotene stage of prophase I). According to our working model, the gradual decline in ovarian production of estradiol and progesterone, just before and during the time when primordial follicles are forming, first allows follicle formation to occur and then promotes the development of follicular competence to activate. In Specific Aim #3, we will begin to determine signaling pathways that subserve the effects of steroids on follicular formation and competence to activate. Together these experiments will provide new information about the regulation of a critical period in ovarian development, in a species that provides an excellent, non-primate model for human ovarian development. PUBLIC HEALTH RELEVANCE: In mammalian females, a pool of primordial ovarian follicles is formed during fetal or neonatal life and since this stockpile of follicles is used through the female's reproductive life as a source of oocytes for ovulation, it is critical for reproductive success and also a potential target for fertility enhancement or contraception. In large mammals, such as cattle, sheep and primates (including humans) the primordial follicles form when the female is a fetus and virtually nothing is known about what regulates this process. We propose to use cattle as an animal model to test the hypothesis that steroids made by the fetal ovary play a role in regulating these earliest stages of ovarian follicular development.