Asymmetrical Distribution of Implantation Sites in the Rat Uterus

Implantation sites in 175 primigravid CFE rats showed a significant (p < 0.001) dextral bias, the proportion in the right horn being about 55% for essentially all litter sizes. The bias was not absolute but occurred twice as often as a sinistral bias, and larger between-sides differences were found in rats with a dextral bias. The bias was lost in subsequent pregnancies, but there was no tendency toward alternation between sides. Distribution of conceptuses was negatively correlated, except that rats with six or fewer conceptuses in the right horn (the majority of which had below average total numbers of conceptuses) showed a positive correlation. Statistically the negative correlation was due to the low variance, reflecting the uniformity between rats with respect to litter size. At the same time, distribution overall was more equal than binomial, significantly so (p < 0.001) for the 156 rats with a total of 12 or more conceptuses. In 38 of 45 rats, the ovary with more corpora lutea was ipsilateral to the horn with the larger number of conceptuses, even though about one-third of the rats had excessive numbers of corpora lutea. Moreover, the left-right differences between both corpora lutea and conceptuses showed a very close correspondence for all rats, including the “spontaneous superovulators.” The results suggest that one ovary, usually the right, has a larger population of follicles capable of maturing and ovulating, and that ovulation, albeit biased, conforms to binomial expectations. It is further suggested that implantation is more equal than binomial due to factor(s) operating on the whole uterus to limit litter size, but the side with the larger number of corpora lutea remains physiologically superior, possibly due to a local effect of the ovaries on the uterine horns.