Abstract
Although oocytes from prepubertal animals are found less competent than oocytes from adults, the underlying mechanisms are poorly understood. Using the mouse oocyte model, this paper has tested the hypothesis that the developmental potential of prepubertal oocytes is compromised due mainly to their impaired potential for glutathione synthesis. Oocytes from prepubertal and adult mice, primed with or without eCG, were matured in vitro and assessed for glutathione synthesis potential, oxidative stress, Ca2+ reserves, fertilization and in vitro development potential. In unprimed mice, abilities for glutathione synthesis, activation, male pronuclear formation, blastocyst formation, cortical granule migration and polyspermic block were all compromised significantly in prepubertal compared to adult oocytes. Cysteamine and cystine supplementation to maturation medium significantly promoted oocyte glutathione synthesis and blastocyst development but difference due to maternal age remained. Whereas reactive oxygen species (ROS) levels increased, Ca2+ storage decreased significantly in prepubertal oocytes. Levels of both catalytic and modifier subunits of the γ-glutamylcysteine ligase were significantly lower in prepubertal than in adult oocytes. Maternal eCG priming improved all the parameters and eliminated the age difference. Together, the results have confirmed our hypothesis by showing that prepubertal oocytes have a decreased ability to synthesize glutathione leading to an impaired potential to reduce ROS and to form male pronuclei and blastocysts. The resulting oxidative stress decreases the intracellular Ca2+ store resulting in impaired activation at fertilization, and damages the microfilament network, which affects cortical granule redistribution leading to polyspermy.
Highlights
The production of viable embryos from oocytes obtained from prepubertal animals would decrease the generation interval and lead to a significant increase in the annual genetic gain over conventional progeny testing programs [1]
Oocytes obtained from prepubertal animals have produced viable offspring after in vitro maturation and fertilization [2,3,4,5], the developmental competence of these embryos was lower than that of embryos produced from adult animals [2,3,6,7,8,9,10,11,12,13]
The present study showed that in unstimulated mice, blastocyst rates of adult oocytes were significantly higher than that of prepubertal oocytes, and maternal priming with eCG increased blastocyst rates of both prepubertal and adult oocytes to the same level (Fig. 4)
Summary
The production of viable embryos from oocytes obtained from prepubertal animals would decrease the generation interval and lead to a significant increase in the annual genetic gain over conventional progeny testing programs [1]. Oocytes obtained from prepubertal animals have produced viable offspring after in vitro maturation and fertilization [2,3,4,5], the developmental competence of these embryos was lower than that of embryos produced from adult animals [2,3,6,7,8,9,10,11,12,13]. In vitro-matured oocytes of prepubertal bovine showed abnormal chromatin microtubule configurations and protein synthesis [9,14,15]. The number of oocytes that failed to show Ca2+ oscillations after fertilization was significantly higher in calf than in cow [9]. While calf oocytes showed abnormal CGs distribution [9), CGs distribution in oocytes from prepubertal goat and sheep was normal following in vitro maturation [11,17]
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