The number of sperm that reaches the oocytes in mammalian species is limited. In mice, 8–10 oocytes are ovulated, a similar number of sperm reaches the oocytes, and nearly all oocytes are fertilized via natural mating. Meanwhile, our improved superovulation technique (ultrasuperovulation: administration of inhibin antiserum and equine chorionic gonadotropin [IASe]) produced 100 oocytes from a single female C57BL/6 mouse but resulted in only approximately 20 fertilized oocytes via mating. We hypothesized that sperm shortage in the ampulla might cause this low fertilization rate. Mice were mated in the proestrus stage or after hormone injection, but ovulation timing was not considered. In clinical application, the rhythm method supports fertilization by testing the ovulation period and synchronizing the ovulation and copulation timings. Therefore, this study examined the effects of ovulation and copulation timings on in vivo fertilization in female mice with IASe. Synchronization of the ovulation and copulation timings increased fertilization efficiency in female mice with ultrasuperovulation. The number of embryos obtained post ovulation was three times higher than that obtained pre ovulation. This study suggests that synchronized ovulation and copulation timings improve the efficiency of in vivo fertilization in IASe-treated female mice. This technique can be used to produce genetically modified mice and develop technologies for infertility treatment.

Capacitation is an important event in the completion of fertilization by mammalian sperm. Cholesterol efflux is a trigger of capacitation. In general, cholesterol acceptors of albumin and β-cyclodextrins are used to induce capacitation during in vitro fertilization. Previously, we reported that methyl-β-cyclodextrin (MBCD), which is composed of seven glucoses, had a higher ability to induce capacitation than bovine serum albumin (BSA) in frozen–thawed mouse sperm. Comparison of albumin and cyclodextrins is helpful for understanding the mechanism of capacitation. In this study, we examined the effects of albumin, MBCD, and a different type of cyclodextrin, dimethyl-α-cyclodextrin (DMACD), which is composed of six glucoses, on several events of sperm capacitation. We showed that DMACD induced sperm capacitation and promoted fertilization ability. The time required to increase the fertilization rate differed among BSA, MBCD, and DMACD. BSA and MBCD enhanced cholesterol and phospholipid efflux, whereas DMACD enhanced only phospholipid efflux. BSA, MBCD, and DMACD increased sperm membrane fluidity, rearrangement of the lipid raft, and the acrosome reaction. These findings suggest that phospholipid efflux is a novel trigger of capacitation. Increasing the choice of sperm capacitation inducers may be useful for improving in vitro fertilization (IVF) techniques not only in mice, but also in various species in which it has been difficult to produce embryos by IVF.