VARIATION IN THE NUMBER OF CORPORA LUTEA AND IN THE NUMBER AND SIZE OF 6-DAY BLASTOCYSTS IN RABBITS SUBJECTED TO SUPEROVULATION TREATMENT

Abstract

SUMMARY The numbers of corpora lutea and the numbers and sizes of 6-day blastocysts have been studied in rabbits, widely varying in ages and weights, that had been subjected to a superovulation treatment. For all does, the average value of each characteristic on the left sides of the animals was similar to that on the right, the average left-to-right difference being not significant. An average of thirty-one corpora lutea and thirteen blastocysts per doe was obtained. Maximum numbers per doe were one hundred and fifty-three corpora lutea and seventy-three blastocysts. Average blastocyst diameter was 2·203 mm, the average volume being 6·780 mm3. There was a fairly large variation from one doe to another in the relative numbers of corpora lutea (or in the relative numbers of blastocysts) on the two sides of the animal. The number of blastocysts per doe was positively related to the number of corpora lutea. The number of corpora lutea per doe was inversely related to the age of the doe. An inverse relation between the number of blastocysts and the age of the doe was presumably indirect, being mediated by the number of corpora lutea. All these relationships were highly significant. There was no demonstrable relationship between the number of corpora lutea and the weight of the doe. No firm evidence was found for a direct relationship (independent of weight and number of corpora lutea) between the number of blastocysts and age, or for any relationship with weight. Diameter of blastocysts seemed more appropriate than volume as a measure of size suitable for analysis. Transformations tending to give linear relationships between variables have been studied. Size differences between blastocysts within the same uterine horn formed a large source of variation, probably not attributable to the superovulation procedure. Differences in blastocyst size between uterine horns within does were significant but introduced little extra variation. Mean blastocyst size varied greatly from one doe to another, but none of this variation was related significantly to known factors such as the weight of the dam or sire, or the age of the dam. The experiment appeared to be fairly sensitive in detecting these relationships. The relationships found between the number of corpora lutea and the age or weight of the doe can be reconciled with the previous literature. The lack of demonstrable connexion between blastocyst size and the weight of dam or sire is in conflict with the results of other authors, and no specific reconciliation can be made. It is suggested that uniformity in age is more important than uniformity in weight when test animals are selected for assay of follicle stimulating hormone under conditions comparable with those of the present work.