Protocol for Providing Additional Pseudo-Pregnant Recipient Mice for Embryo Transfer and Intra-Uterine Insemination by Plugging in the Middle of the Day

The fact that 10% of female mice enter oestrus and allow mating in the middle of the day is an old observation that has been more or less forgotten. We here show that this old knowledge can be used to improve the efficacy of both embryo transfer and insemination protocols. The present technical paper shows that rapid re-arrangements of mating cages, to achieve pseudo-pregnant recipients in the middle of the day, can be of great advantage in emergency situations. Such emergency situations occur repeatedly, i.e. when a scientist has forgotten to re-arrange her/his mating cages, and the last important male suddenly has become ill and may die within a few hours. A rapid technique for uterine artificial insemination in mice in such situations is extremely valuable. An artificial intra-uterine insemination requires only a minimum of planning, a minimum of instrumentation and a minimum of surgical training. The artificial insemination must be performed shortly after mating due to rapid constriction of the utero-tubual junction (UTJ). This means that the timing of the insemination is very important. We here show that the success rate for embryo transfers, when using recipients plugged in the middle of the day, was the same as for ordinary overnight mating protocols. In addition, it: should be noted that the success rate (frequency of pregnancies) for uterine inseminations was 55% if using F1 recipients of C57BL/6J (considerable lower if using recipients of inbred C57BL/6J), which is amazingly high, since inseminations in mice is known to be tricky to perform in a reproducible manner.