Abstract
A decline in fertility rate is observed in aged mice of an inbred IVCS strain. This study was conducted if the rate of in vitro fertilization is also decreased with age. If male and female mice of the same age were used for natural matings, the highest (7.0) reproduction efficiency (no. of neonates / no. of mated mice) was obtained at 4 months of age. No live neonates were born from dams over 9 months of age. The frequency of post-implantation embryonic death was as high as 43-63% at 6-8 months of age.
From females at 9 or 10 months of age, 6.3 or 2.0 fertilized eggs were recovered after natural matings. Of these eggs, 92-100% developed into blastocysts in culture. Superovulation was induced in females at age of 2-11 months, and 51-95% of the recovered oocytes were fertilized in vitro. The majority of these embryos passed through the 2-cell stage within 24 h and 70-99% of them developed to the blastocyst stage 96 h after insemination. Of these embryos obtained from 5, 10 or 11 months old mice, 52, 25 or 21% of the transferred embryos were developed to neonates. These results indicate that most of eggs are still viable, even if they are derived from "infertile" aged mice. Thus, infertility occurring in aged mice of this strain over 9-months of age is primarily due to the dysfunction of the reproductive tract rather than the low quality of eggs. In addition, neonates can be obtained from aged mice by transferring in vitro fertilized embryos to younger recipients.
