Interlitter variability and developmental stage of day 11 rat embryos produced by overnight and morning short‐period breeding regimens

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Abstract
Day 11 rat embryos produced by overnight and morning short‐period breeding regimens were compared for differences in interlitter variability and stage of development. Female Sprague‐Dawley rats were mated either overnight (5 PM‐9 AM, n = 22) or for 2 hr in the morning (8–10 AM, n = 20), and the presence of a vaginal plug was determined (day 0). At 11 AM on day 11 of gestation, rats were killed and embryos were examined for crown‐rump length, somite number, and general morphology; protein content was determined later. There were no differences in mean number of implantations, live embryos, and abnormalities between the two regimens. There were significant differences in mean crown‐rump length, somite number, and protein content but not in the magnitude of their variances. The difference in mean somite number was 1.2 (23.7 vs. 24.9), which corresponds to a time difference in development of approximately 2 hr. The range of mean somite number in morning short‐period breeding group was 3.2 (22.2–25.4), corresponding to approximately 5.3 hr. These results are inconsistent with the popularly held hypothesis that the timing of fertilization is the major cause of interlitter variability.