Developmental Genetics of the Mammalian Ovary

哺乳动物雌性生殖细胞必须成功地完成发育程序，以初始卵泡发生，导致能够受精的成熟配子和遗传物质转移到下一代。出生时，卵巢含有完整的生殖细胞，每个生殖细胞被单层颗粒细胞包围，颗粒细胞共同形成原始卵泡。我们已经鉴定了一种新的卵母细胞特异性碱性螺旋-环-螺旋转录因子FIG-alpha（Factor In the Germline，alpha），并且已经建立了单拷贝FIG-alpha基因被破坏的小鼠品系。缺乏FIG-alpha的雌性小鼠是不育的，因为生殖细胞耗竭继发于不能形成原始卵泡。FIG-alpha的下游靶点的鉴定应该为卵泡形成的分子基础提供额外的见解。在卵泡发生开始后，FIG-alpha还调节编码ZP 1、ZP 2和ZP 3的单拷贝基因的表达。正常情况下，这三种卵母细胞糖蛋白在卵泡发生期间分泌以形成透明卵母细胞，透明卵母细胞是一种介导订单特异性精子与卵子结合的细胞外基质（例如，人类精子不会与小鼠透明质酸结合）。使用转基因，我们已经确定，小鼠ZP 1是不需要的透明卵母细胞，订单特定的精子结合或生育能力的形成。相比之下，ZP 2和ZP 3都是稳定基质形成所需的，没有ZP 2和ZP 3小鼠是不育的。尽管用它们的人类同源物替换小鼠ZP 2、小鼠ZP 3或两者都能恢复透明卵母细胞基质，但它不影响精子结合的特异性。目前正在进行建立表达所有三种人β-淀粉样蛋白或小鼠β-淀粉样蛋白突变形式的小鼠品系的育种研究，以进一步研究精卵相互作用的分子基础。在卵子发生的后期，卵母细胞变得转录不活跃，并且在减数分裂成熟和排卵期间，许多母体RNA被降解。在受精时，两个配子都是转录惰性的，胚胎基因组的主要激活发生在两细胞阶段。精子除了其基因组外，几乎没有给受精带来什么，早期发育程序的激活必须取决于母体因素。MATER（胚胎需要的母体抗原）是一种细胞质蛋白，存在于生长的卵母细胞中，并持续存在于早期胚胎中。缺乏MATER的小鼠具有正常的卵泡发生和排卵卵。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代表对哺乳动物发育至关重要的母性效应基因产物。研究正在进行中，以确定其在终末分化的生殖细胞向早期胚胎全能干细胞转变中的作用。

Mammalian female germ cells must successfully complete developmental programs to initial folliculogenesis that lead to mature gametes capable of fertilization and the transfer of genetic material to the next generation. At birth the ovary contains its full complement of germ cells, each surrounded by a single layer of granulosa cells which together form the primordial follicles. We have identified a novel, oocyte-specific, basic helix-loop-helix transcription factor, FIG-alpha (Factor In the Germline, alpha) and mouse lines have been established in which the single-copy Fig-alpha gene has been disrupted. Female mice lacking FIG-alpha 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. After the onset of folliculogenesis, FIG-alpha also modulates the expression of the single-copy genes that encode ZP1, ZP2 and ZP3. Normally, the three zona glycoproteins are secreted during folliculogenesis to form the zona pellucida, an extracellular matrix that mediates order-specific sperm binding to the egg (e.g., human sperm will not bind to the mouse zona pellucida). Using transgenesis, we have determined that mouse ZP1 is not required for formation of the zona pellucida, order-specific sperm binding or fertility. In contrast, both ZP2 and ZP3 are required for stable matrix formation without which mice are infertile. Although the replacement of either mouse ZP2, mouse ZP3 or both with their human homologue restores the zona pellucida matrix, it does not affect the specificity of sperm binding. Breeding studies to establish mouse lines expressing all three human zona proteins or mutant forms of the mouse zona proteins are being pursued to further investigate the molecular basis of sperm-egg interactions. 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.