Allen C. Schroeder

The developmental capacity of mouse oocytes that matured spontaneously in vitro is normal

The aim of this project was to compare the developmental capacities of mouse oocytes matured in vivo and in vitro. The frequencies of fertilization, preimplantation development, and birth of live offspring after transfer of morulae to uteri of pseudopregnant foster mothers were compared after germinal vesicle stage oocytes underwent spontaneous maturation in vitro, and after gonadotropin-induced maturation in vivo and ovulation. Both groups of matured ova were fertilized in vitro, and preimplantation development was carried out in vitro. Equivalent developmental capacities were observed for all comparisons between the two groups of oocytes. The acquisition of normal developmental capacity depended on the presence of serum in the oocyte maturation medium. The expansion (mucification) of the cumulus oophorus was not required for fertilization or normal development. The frequency of fertilization was lower in oocytes that matured while denuded of cumulus cells. However, when fertilization did occur in these oocytes, a normal percentage developed to live offspring. It is concluded that a normal developmental program occurs during spontaneous maturation of mouse oocytes, and that the presence of cumulus cells during spontaneous maturation may affect the oocytes fertilizability rather than its subsequent developmental capacity.

Culture systems for mammalian oocyte development: progress and prospects.

Progress in the development of systems for murine oocyte growth and maturation invitro is described. Fully grown germinal vesicle stage oocytes undergo spontaneous meiotic maturation when liberated from Graafian follicles and cultured in appropriate medium. When compared to ova induced to mature invivo by the administration of exogeneous gonadotropins, equivalent percentages of spontaneously matured ova were fertilized and developed to live offspring. The presence of a high molecular weight component of serum was found to be essential during oocyte maturation in culture in order for the ova to become fertilizable. All of the sera tested contained similar activity except for human cord serum. None of the hormones or growth factors that were tested could substitute for serum. A higher percentage of ova became fertilizable when the cumulus cells were present during oocyte maturation.

Factors affecting the developmental capacity of mouse oocytes undergoing maturation in vitro.

One of the major problems encountered by in vitro fertilization ( I V F ) clinics is to recognize the maturational state of the oocytes obtained from stimulated follicles, and to treat them appropriately if they are judged to be too immature for fertilization. For example, at the Norfolk clinic, more than half of the recovered oocytes were judged to be immature, and about 73% of these were still in the germinal vesicle stage.’ Immature ova are usually cultured for some period of time with the hope that the maturational program will be completed normally so that fertilization can be attempted. This protocol is followed even though various investigators using several species have had difficulty in achieving fertilization and development of oocytes matured in vitro. 2-s While the nuclear events of maturation, namely germinal vesicle breakdown (GVBD) and polar body formation, progressed normally, the cytoplasm of the oocytes usually failed to promote male pronuclear formation after sperm penetration. Accordingly, it was concluded that full “cytoplasmic maturation ” did not occur during spontaneous maturation in vitro. 2.5 It was suggested that either the environment of the intact follicle was needed to actively promote cytoplasmic maturation, or that culture systems were inadequate or toxic, or both. Successful fertilization and development of oocytes matured in vitro were achieved in some and these studies suggested that general culture deficiencies were the source of previous difficulties rather than the lack of a special follicular factor( s ) needed to support normal cytoplasmic maturation. Our approach therefore