Embryonic developmental stages in relation to the LH peak in dogs

Blood samples were collected from nine Beagle bitches every 6 h during pro-oestrus and oestrus to measure plasma concentrations of progesterone, oestradiol and LH. The number of ovarian follicles was estimated once a day using transcutaneous ultrasonography. According to the concentrations of plasma progesterone, the bitches were mated once and subsequently ovariohysterectomized 3-7 days after mating. The number of corpora lutea was counted, and oocytes and embryos were collected by flushing of the oviducts. In ovaries that had more than three follicles, the number of follicles observed using ultrasonography was underestimated, whereas the recovery rate (number of oocytes and embryos flushed/number of corpora lutea counted) was 99%. Embryos were processed for sectioning and the developmental stages were determined by counting the number of blastomeres under bright field microscopy. Potentially fertilized oocytes and a zygote were observed on day 7 after the LH peak and the developmental rate was about one cell cycle in 24 h until day 12 after the LH peak. The timing of embryonic development was significantly correlated with the time of the LH peak.