Reproductive performance of young female mice following vaginal opening.

Abstract

Mirskaia & Crew (1930) have reported that the first vaginal oestrus in young female mice was frequently not associated with mating. In addition, pregnancy followed first mating in only 24% of cases whereas in the same mice at 3 to 6 months of age, 80 to 90 % of females became pregnant following mating. Since it seemed not unlikely that genetic and/or environmental factors may have contributed to this poor reproductive performance, it was decided to reexamine the fertility of young mice. Mice of the Quackenbush (QS) strain were used in this study. They were housed under light- and temperature-controlled conditions and given free access to a commercial pelleted mouse food and tap water. The 'young' female mice were 21 days old at the start of the experiment while the 'mature' females were virgin mice approximately 10 weeks old. To determine their reproductive performance, two female mice, one young and one mature, were placed in each of fifty cages containing a mature male QS mouse of unknown fertility. The young mice were examined each morning for vaginal opening; when it was first noted, the age and body weight of the mouse were recorded. A vaginal smear was taken each morning from all unmated female mice, commencing for young mice on the day of vaginal opening. Day 1 of pregnancy was indicated by the presence of a copulation plug or spermatozoa in the vaginal smear. Body weight on Day 1 of pregnancy was recorded. The mice were checked in this fashion for mating for 28 days. During this period, the males were reassigned at random to cages on every 7th day. Mice which mated were removed from the mating cages before reallocation of the males in order to avoid the possibility of the 'Bruce-effect' operating (Bruce, 1960), and killed on Day 18 of pregnancy. The number of CL of pregnancy was used as an estimate of ovulation rate while an estimate of litter size was obtained from the counts of the number of normally developing fetuses (referred to as viable fetuses). During the experiment, 6/50 young and 1/50 mature mice died from unknown causes; age had no significant effect on these proportions ( \\] = 2-46, P>0-1). Mean ( + S.E.) age and body weight at vaginal opening for young mice which survived were 23-5 + 0-2 days and 12-4 + 0-2 g, respectively. For mice which survived, the proportions of mice mating, of mice pregnant at Day 18, of mated mice pregnant at Day 18 and of mice mating at first vaginal oestrus for both age groups are presented in Table 1. Age had a significant effect (P<0-05)