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

描述（由申请人提供）：建立原始卵泡储备，能够启动生长并可能发育到排卵前阶段，是女性生殖成功的先决条件。然而，对于非啮齿动物物种，我们对性腺发育中这些关键步骤的调控一无所知。在牛、羊和其他大型哺乳动物（包括人类）中，原始卵泡在胎儿时期的数周内形成，而在小鼠和大鼠中它们在出生后的同步发育。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，原始卵泡不会离开静止池（激活），而在新生大鼠和小鼠卵巢中，一小部分新形成的原始卵泡会迅速激活。胎牛和胎羊的卵巢积极合成类固醇激素，特别是在卵泡形成之前和卵泡形成期间，雌二醇受体已经定位于体细胞和配子体细胞。我们已经开发了实验模型，在该模型中，牛原始卵泡的形成及其激活能力的发展（即，根据刺激物启动生长）发生在体外。我们的初步数据表明了R21应用的总体假设：胎牛卵巢产生的类固醇，特别是雌二醇和黄体酮，调节卵泡的形成和卵泡的激活能力。我们提出了三个具体目标来检验这一新的假设。在具体目标#1中，提出了一些实验来验证卵巢产生雌二醇和黄体酮的下降，在体内启动或允许卵泡形成的假设。我们在Specific Aim #2中提出实验来验证以下假设：卵巢类固醇（雌二醇和黄体酮）通过抑制或调节减数分裂前期I的进展来防止新形成的原始卵泡的过早激活（牛原始卵泡形成于卵母细胞在减数分裂前期I的二倍体阶段达到减数分裂停止之前）。根据我们的工作模型，卵巢分泌雌二醇和黄体酮的逐渐下降，就在原始卵泡形成之前和形成期间，首先允许卵泡形成，然后促进卵泡能力的发展。在特异性目标#3中，我们将开始确定支持类固醇对卵泡形成和激活能力影响的信号通路。总之，这些实验将提供关于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优秀的非灵长类动物模型。公共卫生相关性：在哺乳动物女性中，原始卵泡池在胎儿或新生儿时期形成，由于这些卵泡储备在女性的生殖生命中被用作排卵卵母细胞的来源，它对生殖成功至关重要，也是增强生育能力或避孕的潜在目标。在大型哺乳动物中，如牛、羊和灵长类动物（包括人类），原始卵泡是在雌性还是胎儿时形成的，实际上我们对是什么调节了这一过程一无所知。我们建议用牛作为动物模型来验证胎儿卵巢产生的类固醇在调节卵巢卵泡发育的早期阶段发挥作用的假设。

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.