The technique of ova transplantation was used to study the causes of the decline in litter size in ageing mice. Only 14% of morulae and blastocysts obtained from the uteri of young donors survived to term when transplanted into the uteri of old hosts (13 to 24 months of age), whereas 48% survived in young hosts (2 to 7 months of age). A slightly higher percentage of grossly abnormal ova was recovered on the 4th day of pregnancy from the uteri of old mice than from young mice (13% compared with 5%) However, normal-appearing morulae and blastulae from old mice survived as well as those from young mice when transferred into uteri of young hosts. It was concluded that the initial decline in litter size in aged mice is probably due to an unfavourable uterine environment.

No abstract

Summary. C57B1 female mice aged 4 to 7 months (controls) and 8 to 9, 10 to 11, 12 to 13, and 14 to 15 months were mated and killed at 12 hr, 60 hr or 7\ m=1/ 2\ days post coitum (p.c.). Corpora lutea were counted and evaluated in serially sectioned ovaries. Implantation sites were counted in the animals killed 7\ m=1/ 2\ days p.c. The mean number of implantations in 10-to 11-month-old mice was less than in the control animals. This decrease was closely associated with morphological degeneration of corpora lutea of pregnancy. By 12 to 13 months of age most animals had neither implantation sites nor recognizable corpora lutea of pregnancy 7\ m=1/ 2\ days after mating. These two changes were highly correlated and were not due to a decrease in the number of ovulations as determined by examination of the ovaries of animals killed 12 or 60 hr p.c.It was concluded that the initial decline in fertility with increased maternal age in this strain of mouse may be due to failure of luteal support of the uterus.

Summary. Postimplantation reproductive failure was studied in young adult, 9-, 11-, 13-and 15-month-old C57BL/6J mice. Loss of all implanted embryos between the 8th and 18th days of pregnancy increased with advancing maternal age. The percentage of individual postimplantation deaths increased from 9% in young adults to 23% in 9-month-old mice and further increased to 45% in 11-month-old animals. Evidence is presented that deaths most commonly occurred in the early postimplantation period. Harman & Talbert (1970) showed that there was a marked decline in the number of implantation sites in the uterus of C57BL/6J mice associated with increasing maternal age. This decline began when the mice were 8 to 9 months old and continued until about 15 months of age, when implantation sites were found in only a small percentage of mice which were known to have mated.The present investigation is an extension of this work and was designed to study the influence of maternal age on the survival of embryos and foetuses of this strain of mice between the 8th and 18th days of pregnancy.Female mice of the C57BL/6J strain were obtained from the Jackson Memor¬ ial Laboratory, Bar Harbor, Maine. Ageing animals were retired breeders, each of which had delivered a minimum of three litters before they were received in this laboratory. Young adult control mice were obtained from the same source and delivered one litter before inclusion in the study.Ageing female mice and young adult controls were placed with young adult AJ-strain males late in the afternoon and were examined for the presence of a vaginal plug before 10.00 hours the following morning. Females that mated were isolated and laparotomy under Avertin anaesthesia was performed 7 days later. The number and location of implantation sites in each uterine horn were observed through a mid-ventral incision. Care was taken not to handle the implantation swellings. The incision was closed in the pregnant mice and they were killed on the 18th day of gestation. At this time, the number of foetuses and résorption sites in each horn were noted and each foetus was examined with a dissecting microscope for evidence of externally visible anomalies. The ovaries were prepared for histological study.Animals in which no implantation sites were found at laparotomy on the 8th 449