Cell atlas of the human female reproductive system across the lifespan

有性繁殖依赖于受精过程中配子(精子和卵子)的融合，然后将产生的胚胎植入子宫内层(子宫内膜)。配子在一种被称为减数分裂的特殊细胞分裂期间将其基因组减半，为受精做准备。雌体减数分裂与配子形成协调的过程知之甚少，主要是因为减数分裂始于子宫，直到几十年后成熟卵子排卵和受精时才完成。受精后3-5周，原始生殖细胞(PGC)开始填充卵巢。对小鼠的研究表明，PGCs形成卵原细胞，经历多轮有丝分裂细胞分裂，由于细胞分裂不完全，仍然通过细胞质桥梁联系在一起。在第一个三个月的末期，这些相互关联的细胞巢开始在整个卵巢内从有丝分裂异步过渡到减数分裂。减数分裂前一轮DNA复制后进入减数分裂前期，减数分裂前期以减数分裂重组为标志，随后两对同源染色体在DNA相互交换的位置保持连接。这与细胞质桥梁的破裂相协调，形成单个卵母细胞，这些卵母细胞又被少数哺育细胞(颗粒细胞)包围，形成原始卵泡。我们对发育中的人类卵巢的这些事件的了解是零散的，而且老鼠和人类之间似乎有关键的区别。出生时存在的原始卵泡池包含一生所需的卵母细胞供应。人们对卵母细胞在数十年的非生长停滞期间如何维持细胞内环境平衡知之甚少。特别令人感兴趣的是用来维持它们的核和线粒体基因组处于原始状态以便传递给下一代的机制。不断地从池中招募原始卵泡队列。卵母细胞的开始生长伴随着周围颗粒细胞的形态变化和重组，目前尚不清楚这是否是卵母细胞开始生长的原因或结果，或者是否其他周围的细胞，即间质细胞，在启动卵母细胞生长方面起到了作用。青春期开始后，卵巢卵泡可以在荷尔蒙刺激下发育到促排卵阶段。卵母细胞周围颗粒细胞的分化使成熟的卵母细胞能够退出减数分裂前期，并在排卵前不久经历第一次减数分裂。此外，颗粒细胞产生的激素为子宫内膜的植入做好了准备。虽然我们对排卵和子宫内膜启动的内分泌控制有了广泛的了解，但对这些转变在卵泡和子宫内膜细胞水平上的分子调控知之甚少。为了解决我们对人类女性生殖系和生殖系统发育的许多认识上的空白，我们建议制作一份关于从胎儿发育到高龄生殖年龄的人卵巢和子宫内膜的全面细胞图谱。我们将结合单细胞水平的基因表达谱与直接在组织上检测基因表达的新方法，以前所未有的分辨率和覆盖率研究细胞环境。关于人类卵巢和子宫内膜中无数细胞和细胞状态表达的信息将极大地促进我们对女性生殖系统的发育以及人类女性生殖系的出现和维持的了解。这项工作将为不孕不育的原因提供新的见解，并将为如何在体外培养卵母细胞提供策略。

Sexual reproduction depends on the fusion of gametes (sperm and eggs) during fertilisation followed by implantation of the resulting embryo in the lining of the womb (the endometrium). Gametes halve their genome in preparation for fertilisation during a specialized cell division known as meiosis. The process by which meiosis is co-ordinated with gamete formation in females is poorly understood, principally because it commences in utero and is not completed until decades later when the mature egg is ovulated and fertilised. From 3-5 weeks after fertilisation primordial germ cells (PGCs) begin to populate the ovary. Work on mice indicates that the PGCs form oogonia which undergo multiple rounds of mitotic cell division, remaining linked by cytoplasmic bridges as a consequence of incomplete cell division. Towards the end of the first trimester, these interlinked nests of cells begin to transition from mitosis to meiosis asynchronously across the ovary. The pre-meiotic round of DNA replication is followed by entry into meiotic prophase, which is marked by meiotic recombination following which pairs of homologous chromosomes remain linked at the sites of reciprocal exchange of DNA. This is co-ordinated with breakdown of cytoplasmic bridges to form individual oocytes, which in turn become surrounded by a small number of nurse cells (granulosa cells) to form a primordial follicle. Our knowledge of these events in the developing human ovary is fragmentary, and there appears be key differences between mouse and human. The pool of primordial follicles present at birth contain the life-time supply of oocytes. Little is known about how oocytes maintain cellular homeostasis during decades of arrest in the non-growing phase. Of particular interest are the mechanisms employed to maintain their nuclear and mitochondrial genomes in pristine condition for transmission to the next generation. Cohorts of primordial follicles are recruited from the pool on an ongoing basis. Initiation of oocyte growth is accompanied by morphological changes and reorganisation of the surrounding granulosa cells and it is not known whether this is a cause or a consequence of the initiation of oocyte growth, or indeed whether other surrounding cells, known as stromal cells, have a role in initiating oocyte growth. Following the onset of puberty, ovarian follicles can develop to the pro-ovulatory stage in response to hormonal triggers. Differentiation of the granulosa cells surrounding the oocyte enables fully grown oocytes to exit meiotic prophase and undergo the first meiotic division shortly before ovulation. In addition, hormones produced by the granulosa cells prepare the endometrium for implantation. While we have a broad understanding of the endocrine control of ovulation and endometrial priming, less is known about the molecular regulation of these transitions at the cellular level in the follicle and in the endometrium. To address the many gaps in our knowledge of the development of the female germline and reproductive system in humans, we propose to generate a comprehensive cell atlas of the human ovary and endometrium from fetal development until advanced reproductive age. We will combine gene-expression profiling at the single-cell level with new methods of gene-expression detection directly on the tissue to study the cellular environment at an unprecedented resolution and coverage. Information about the expression of the myriad of cells and cell states in the human ovary and endometrium will greatly advance our understanding of the development of the female reproductive system and the emergence and maintenance of the female germline in humans. The work will provide new insights into the causes of infertility and will inform strategies on how to grow oocytes in vitro.