Developmental Genetics of the Mammalian Ovary

哺乳动物雌性生殖细胞必须成功完成发育程序，启动卵泡发生，从而产生能够受精的成熟配子，并将遗传物质传递给下一代。出生时，卵巢含有完整的生殖细胞，每个生殖细胞都被一层颗粒细胞包围，这些颗粒细胞共同形成原始卵泡。我们已经确定了一种基本的螺旋-环-螺旋转录因子fig - α（种系因子，α），缺乏这种调节因子的雌性小鼠由于生殖细胞耗竭而无法形成原始卵泡而不育。对fig - α下游靶点的鉴定，将有助于进一步了解卵泡形成的分子基础，以及其他对成功发生卵子至关重要的发育过程。在卵泡发生开始后，FIG-alpha还调节编码ZP1、ZP2和ZP3的单拷贝基因的表达。它们一起形成了小鼠卵细胞周围的透明带，其中两个（ZP2和ZP3）被报道为“精子受体”。受精后，透明带被ZP2的分裂修饰，精子不再结合。在哺乳动物中精子的跨分类结合是有限的，人类精子不与小鼠卵子结合。利用转基因技术将小鼠的ZP2和/或ZP3替换为人类的同源物，建立了具有人-小鼠嵌合透明带的小鼠系。出乎意料的是，人类精子不结合huZP2和huZP2/huZP3拯救卵子，但小鼠精子结合，体外受精卵进展为双细胞胚胎，拯救小鼠是可育的。同样出乎意料的是，人类ZP2在受精后仍未分裂，小鼠精子继续结合早期抢救胚胎。这些观察结果与精子结合所需的透明带的超分子结构由ZP2的分裂状态调节的模型一致。在卵子发生后期，卵母细胞的转录变得不活跃，在减数分裂成熟和排卵过程中，母体RNA的大部分被降解。受精时，两个配子都处于转录惰性，胚胎基因组的主要激活发生在双细胞阶段。精子给受精带来的只有基因组，而早期发育程序的激活必须依赖于母体因素。MATER（胚胎所需的母体抗原）是一种细胞质蛋白，存在于生长中的卵母细胞中，并持续存在于早期胚胎中。缺乏MATER的小鼠卵泡发育正常，排卵正常。虽然受精发生，但胚胎不会超过早期卵裂阶段。雄性不受影响，MATER代表了对哺乳动物发育至关重要的母体效应基因产物。研究正在进行中，以确定其在晚期分化生殖细胞向早期胚胎的全能干细胞转变中的作用。

Mammalian female germ cells must successfully complete developmental programs to initiate folliculogenesis that lead to mature gametes capable of fertilization and the transfer of genetic material to the next generation. At birth the ovary contains a full complement of germ cells, each surrounded by a single layer of granulosa cells which together form the primordial follicles. We have identified a basic helix-loop-helix transcription factor, FIG-alpha (Factor In the Germline, alpha) and female mice lacking this regulatory factor are sterile because of germ cell depletion secondary to an inability to form primordial follicles. Identification of downstream targets of FIG-alpha should provide additional insights into the molecular basis of follicle formation and other developmental programs important for successful oogenesis. After the onset of folliculogenesis, FIG-alpha also modulates the expression of the single-copy genes that encode ZP1, ZP2 and ZP3. Together they form the zona pellucida surrounding ovulated mouse eggs and two (ZP2 and ZP3), are reported 'sperm receptors'. After fertilization, the zona pellucida is modified by cleavage of ZP2 and sperm no longer bind. Cross-taxa sperm binding is limited among mammals and human sperm do not bind to mouse eggs. Using transgenesis to replace mouse ZP2 and/or ZP3 with human homologues, mouse lines with human-mouse chimeric zonae pellucidae have been established. Unexpectedly, human sperm do not bind to huZP2 and huZP2/huZP3 rescue eggs, but mouse sperm bind, in vitro fertilized eggs progress to two-cell embryos, and rescue mice are fertile. Also unanticipated, human ZP2 remains uncleaved after fertilization and mouse sperm continue to bind early rescue embryos. These observations are consistent with a model in which the supramolecular structure of the zona pellucida necessary for sperm binding is modulated by the cleavage status of ZP2. Late in oogenesis, the oocyte becomes transcriptionally inactive and much of the maternal RNA is degraded during meiotic maturation and ovulation. At fertilization, both gametes are transcriptionally inert and the major activation of the embryonic genome occurs at the two-cell stage. The sperm brings little but its genome to fertilization and the activation of early development programs must depend on maternal factors. MATER (Maternal Antigen That Embryos Require) is a cytoplasmic protein that is present in growing oocytes and persists in early embryos. Mice lacking MATER have normal folliculogenesis and ovulate eggs. Although fertilization takes place, embryos do not progress beyond the early cleavage stage. Males are unaffected and MATER represents a maternal effect gene product critical for mammalian development. Investigations are underway to determine its role in transition of the terminally differentiated germ cell into the totipotent stem cells of the early embryo.