Cell atlas of the human female reproductive system across the lifespan

有性生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膜（子宫内膜）。在被称为减数分裂的特殊细胞分裂过程中，配子将其基因组减半，为受精做准备。在女性中，减数分裂与配子形成协调的过程尚不清楚，主要是因为减数分裂从子宫内开始，直到几十年后成熟的卵子排卵受精才完成。从受精后3-5周，原始生殖细胞（PGCs）开始填充卵巢。对小鼠的研究表明，PGCs形成的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，细胞质桥仍然连接在一起。在怀孕的前三个月末期，这些相互连接的细胞巢开始在卵巢内异步地从有丝分裂过渡到减数分裂。DNA复制的减数分裂前阶段之后进入减数分裂前期，减数分裂前阶段的标志是减数分裂重组，随后同源染色体对在DNA相互交换的位点保持连接。这与细胞质桥的破裂相协调，形成单个卵母细胞，卵母细胞又被少量乳细胞（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碎的，在小鼠和人类之间似乎存在关键差异。出生时存在的原始卵泡池包含终生的卵母细胞供应。对于卵母细胞如何在非生长阶段的数十年停滞中维持细胞稳态知之甚少。特别令人感兴趣的是用于将其核和线粒体基因组保持在原始状态以遗传给下一代的机制。在持续的基础上从池中招募原始卵泡群。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，但目前尚不清楚这是卵母细胞开始生长的原因还是结果，也不清楚周围的其他细胞，即基质细胞是否在卵母细胞的开始生长中起作用。青春期开始后，卵泡会在激素的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在排卵前不久退出减数分裂前期并进行第一次减数分裂。此外，颗粒细胞产生的激素为着床准备子宫内膜。虽然我们对排卵和子宫内膜启动的内分泌控制有广泛的了解，但对卵泡和子宫内膜细胞水平上这些转变的分子调控知之甚少。为了解决我们在人类女性生殖系和生殖系统发育方面的许多知识空白，我们建议生成一个从胎儿发育到生育年龄晚期的人类卵巢和子宫内膜的全面细胞图谱。我们将把单细胞水平的基因表达谱与直接在组织上进行基因表达检测的新方法结合起来，以前所未有的分辨率和覆盖范围研究细胞环境。关于人类卵巢和子宫内膜中无数细胞的表达和细胞状态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理解。这项工作将为不孕症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.