Abstract
Egg overactivation occurs with a low frequency in the populations of naturally ovulated frog eggs. At present, its natural inducers, molecular mechanisms, and intracellular events remain unknown. Using microscopic and biochemical analyses, we demonstrate here that high levels of hydrogen peroxide-induced oxidative stress can cause time- and dose-dependent overactivation of Xenopus eggs. Lipofuscin accumulation, decrease of soluble cytoplasmic protein content, and depletion of intracellular ATP were found to take place in the overactivated eggs. Progressive development of these processes suggests that egg overactivation unfolds in a sequential and ordered fashion.
Highlights
Oocytes and eggs of the African clawed frog Xenopus laevis provide the most common model for studying oogenesis, fertilization, meiotic and mitotic cell cycle progression, and apoptosis because of their large size and high biochemical tractability
The term “eggs” is generally used in the frog model for mature ovulated oocytes arrested in metaphase of the second meiotic division by high activity of the key meiotic regulators, such as the maturation promoting factor (MPF) and the cytostatic factor (CSF) [1]
They include the eggs arrested in the second meiotic metaphase, as it can be judged by the presence of a white spot on the dark animal hemisphere (Figure 1(a)); apoptotic eggs that lost the white spot after activation and experience progressive decoloring of the pigment layer (Figure 1(b)); overactivated eggs that lost their pigmentation and became near completely white (Figure 1(c)); and the eggs with the contracted pigment layer of the animal hemisphere (Figure 1(d))
Summary
Oocytes and eggs of the African clawed frog Xenopus laevis provide the most common model for studying oogenesis, fertilization, meiotic and mitotic cell cycle progression, and apoptosis because of their large size and high biochemical tractability. The meiotic metaphase arrest prevents cell cycle progression and parthenogenesis prior to fertilization. Arrested eggs awaiting fertilization can experience various injuries leading to the loss of their quality. The stress- and age-triggered damage leads to decreased rates of fertilization, polyspermy, parthenogenesis, and abnormal development of embryos. Spontaneous egg activation and exit from the meiotic metaphase arrest make successful fertilization impossible [4, 5]. It was reported that unfertilized Xenopus eggs spontaneously activate, exit the meiotic arrest, and degrade by a robust apoptotic process within 48 hours after ovulation [6, 7]
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