Strain-specific spontaneous activation during mouse oocyte maturation

Objective: To examine whether spontaneous oocyte activation is determined by genetic differences and interacts with culture environment.

Design: Experimental study.

Setting: Temple University School of Medicine.

Animal(s): C57BL/6, DBA/2, C3H/HeJ, and A/J strains, along with reciprocal F1 hybrid female mice (5-6 weeks).

Intervention(s): Immature oocytes from different mouse strains collected and cultured in different maturation conditions, including different serum, serum replacement, bovine serum albumin (BSA), and follicle-stimulation hormone (FSH).

Main outcome measure(s): The emission of first polar body, pronucleus formation, meiotic arrest, spontaneous activation, and expression of maturation regulators.

Result(s): Oocytes from C57BL/6 mice display a high rate of delayed first meiotic division and spontaneous activation after the first meiotic division with in vitro maturation (IVM), and the second meiotic division with in vivo maturation (VVM) after superovulation. Spontaneous activation with IVM is sensitive to culture environment. Oocytes that spontaneously activated during the first meiotic division with IVM have unusual replicated sister chromatid pairs with slight connections at centromeres at first mitosis, whereas oocytes that activated in vivo display haploidization from the second meiotic division. Spontaneous activation is also seen in F1 hybrid oocytes, indicating a dominant trait from C57BL/6. Delayed meiosis was associated with reduced cyclin B and securin expression.

Conclusion(s): Both mouse strain and culture environment have a statistically significant effect on the incidence of meiotic defects and spontaneous activation. Reduced expression of meiotic regulators may underlie this effect.