UTERINE MASS AND UTERINE BLOOD-VOLUME IN MICE SELECTED 21 GENERATIONSFOR ALTERNATIVE CRITERIA TO INCREASE LITTER SIZE

Lines of mice, selected for 21 generations using alternative criteria to increase litter size, were evaluated for uterine mass and uterine b lood volume to help explain differences in uterine capacity. For this study, mice were sampled from Generation 27, the sixth generation afte r relaxation of selection. Mice came from all four criteria of selecti on (LS = selection on number born to unaltered females; IX = selection on an index of ovulation rate and ova success; UT = selection on uter ine capacity; and LC = unselected control) in each of three replicates (a total of 12 lines). Measurement was at one of two stages, either 3 d or 6 d of gestation. Matings were at 10 wk of age, and a total of 5 08 mice (17 to 26 per line-day of pregnancy subclass) were measured. T he mean of the three selected groups exceeded the control in uterine m ass (P < .001), uterine blood volume (P < .002), uterine mass/body mas s (P < .03), and uterine blood volume/body mass (P < .04) but not in u terine blood volume/uterine mass. Greater uterine mass and concomitant ly greater uterine blood volume may have been partly responsible for g reater uterine capacity resulting from LS, IX, and UT selections.