Biochemical studies of mammalian oogenesis: Protein synthesis during oocyte growth and meiotic maturation in the mouse

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Abstract
Using oocytes isolated from juvenile and adult mice, we have examined the qualitative patterns of protein synthesis during growth and during meiotic maturation of these oocytes. Oocytes were cultured in a defined medium in the presence of [35S]methionine and radioactively labelled proteins were separated by SDS-polyacrylamide gel electrophoresis and detected by fluorography. The results of these studies demonstrate that: (i) the patterns of protein synthesis are very similar in individual oocytes which are at the same stage of growth or of meiotic maturation, indicating a high degree of biochemical homogeneity in a given population of isolated mouse oocytes, (ii) the linear increase in protein content of growing mouse oocytes (with respect to oocyte volume) is accompanied by significant qualitative changes in the size classes of proteins synthesized, and (iii) meiotic maturation (germinal vesicle dissolution and nuclear progression to the second metaphase) is characterized by several discrete qualitative changes in the pattern of protein synthesis in the oocyte, especially during the period following germinal vesicle breakdown. Experiments carried out with oocytes cultured in the presence of drugs which have been shown to inhibit meiotic maturation at specific stages of nuclear progression suggest that: (i) protein synthesis is not required for germinal vesicle breakdown to take place; (ii) mixing of the oocyte’s nucleoplasm and cytoplasm must occur in order for those changes in the pattern of protein synthesis which characterize meiotic maturation to take place; and (iii) failure of nuclear progression to proceed beyond the circular bivalent stage does not prevent those changes in the pattern of protein synthesis which characterize meiotic maturation from taking place. The latter observations suggest that there are basic differences in the control of meiotic maturation in oocytes isolated from mammalian, as compared to non-mammalian, animal species.