Cui Wei
Department of Veterinary and Animal Sciences, Animal Models Core Facility, Institute for Applied Life Sciences (IALS), University of Massachusetts Amherst, Amherst, MA, United States.
Front Cell Dev Biol. 2021 Mar 8;9:648057. doi: 10.3389/fcell.2021.648057. eCollection 2021.
In mammals, including humans, mature oocytes are ovulated into the oviduct for fertilization. Normally, these oocytes are arrested at metaphase of the second meiosis (MII), and this arrest can be maintained for a certain period, which is essential for fertilization and oocyte manipulations , such as assisted reproduction in clinics and nuclear/spindle transfer in laboratories. However, in some species and under certain circumstances, exit from MII occurs spontaneously without any obvious stimulation or morphological signs, which is so-called oocyte spontaneous activation (OSA). This mini-review summarizes two types of OSA. In the first type (e.g., most rat strains), oocytes can maintain MII arrest , but once removed out, oocytes undergo OSA with sister chromatids separated and eventually scattered in the cytoplasm. Because the stimulation is minimal (oocyte collection itself), this OSA is incomplete and cannot force oocytes into interphase. Notably, once re-activated by sperm or chemicals, those scattered chromatids will form multiple pronuclei (MPN), which may recapitulate certain MPN and aneuploidy cases observed in fertility clinics. The second type of OSA occurs in ovarian oocytes (e.g., certain mouse strains and dromedary camel). Without ovulation or fertilization, these OSA-oocytes can initiate intrafollicular development, but these parthenotes cannot develop to term due to aberrant genomic imprinting. Instead, they either degrade or give rise to ovarian teratomas, which have also been reported in female patients. Last but not the least, genetic models displaying OSA phenotypes and the lessons we can learn from animal OSA for human reproduction are also discussed.
在包括人类在内的哺乳动物中，成熟的卵母细胞会排卵至输卵管进行受精。通常情况下，这些卵母细胞停滞于第二次减数分裂中期（MII），并且这种停滞状态能够维持一段时间，这对于受精以及卵母细胞操作（如临床辅助生殖和实验室中的核/纺锤体转移）至关重要。然而，在某些物种以及特定情况下，卵母细胞会在没有任何明显刺激或形态学迹象的情况下自发地从MII期退出，这就是所谓的卵母细胞自发激活（OSA）。本综述总结了两种类型的OSA。在第一种类型中（例如，大多数大鼠品系），卵母细胞能够维持MII期停滞状态，但一旦取出，卵母细胞就会发生OSA，姐妹染色单体分离，最终分散在细胞质中。由于刺激极小（卵母细胞采集本身），这种OSA是不完全的，无法迫使卵母细胞进入间期。值得注意的是，一旦被精子或化学物质重新激活，那些分散的染色单体将形成多个原核（MPN），这可能重现了生育诊所中观察到的某些MPN和非整倍体病例。第二种类型的OSA发生在卵巢卵母细胞中（例如，某些小鼠品系和单峰骆驼）。在没有排卵或受精的情况下，这些发生OSA的卵母细胞能够启动卵泡内发育，但由于异常的基因组印记，这些孤雌胚不能发育至足月。相反，它们要么退化，要么引发卵巢畸胎瘤，这在女性患者中也有报道。最后但同样重要的是，还讨论了显示OSA表型的遗传模型以及我们可以从动物OSA中吸取的关于人类生殖的经验教训。
