Developmental capacity of mouse oocytes matured in vitro: Effects of gonadotrophic stimulation, follicular origin and oocyte size

Abstract

Development of mammalian oocytes is usually correlated with ovarian follicular development. This correlation was tested by determining whether gonadotrophic stimulation of follicular development in immature mice resulted in a coordinated increase in the embryonic developmental capacity of the oocytes. Oocyte cumulus cell complexes were isolated at the germinal vesicle stage from small, medium and large antral follicles of 26-day-old mice and matured and fertilized in vitro. The frequency with which embryos from oocytes from small follicles completed the two-cell to blastocyst transition was lower than for embryos from oocytes from large follicles (33% and 79%, respectively). Germinal-vesicle stage oocyte-cumulus cell complexes were isolated from 22-26-day-old mice that were unprimed or primed by injection of equine chorionic gonadotrophin 48 h before isolation. Oocytes were matured in control medium, or in medium containing 1 μg follicle-stimulating hormone (FSH) ml-1, and then fertilized in vitro. Priming did not increase the number of embryos completing the two-cell stage to blastocyst transition in the 22-day-old group nor did FSH treatment of maturing oocytes when the oocytes were isolated from unprimed 22-day-old mice. In contrast, priming increased the percentage of embryos completing the two-cell stage to blastocyst transition in the 26-day-old group by 20%. FSH treatment of maturing oocytes from the unprimed, 26-day-old group increased the number of embryos completing the transition to the same level as those in the primed 26-day-old group, but FSH did not increase the frequency of transition in the primed 26-day-old group. The developmental capacity of eggs superovulated by 18- and 22-day-old mice was 34% and 10% less, respectively, than in the 26-day-old group. Germinal vesicle stage oocytes of the same size were grouped after isolation from primed 20- and 26-day-old mice and were matured and fertilized in vitro. Two-cell stage embryos derived from oocytes isolated from 26-day-old mice developed to the blastocyst stage more frequently than embryos derived from 20-day-old mice, despite the fact that the oocytes were the same size. It is concluded that oocytes of the same size can be qualitatively different in a way that is manifested in their capacity to complete the two-cell stage to blastocyst transition. Furthermore, the preimplantation developmental capacity of mouse oocytes is affected by (i) the size of the follicle and (ii) stimulation of follicular development in vivo by exogenous gonadotrophins. The ability of FSH to increase the developmental capacity of oocytes maturing in vitro varied depending on the age of the mice and prior gonadotrophin priming in vivo. The processes of gonadotrophin-stimulated follicular and oocyte development are, therefore, separable.