Embryo development, hormonal requirements and maternal responses during canine pregnancy.
The events of canine gestation appear to occur consistently among bitches relative to the time of the preovulatory LH surge. The interval from fertilization to the eight-cell stage was 5 days after insemination before oocyte maturation and only 3 days following insemination after oocyte maturation. Sixteen-cell embryos were observed at day 11 (day 0 = day of the LH surge) after either early or late insemination. Apparently, embryonic cleavage between the two-cell and 16-cell stages occurs more rapidly after fertilization of more mature oocytes. This finding, together with the narrow window for fertilization, may explain why the duration of gestation is similar whether mating occurs before or a few days after oocyte maturation. Observations also indicate that cessation of migration and final situating of embryos occurs between day 16 and day 20 and that uterine lumen vesicles are > 1 mm in diameter at days 17-19; vesicles are > 2 mm in diameter and elongated to 3-6 mm by days 20-22. Some blastocyst enlargement occurs between day 14 and day 20, and expansion inside lemon-shaped uterine vesicles prevents flushing of intact embryos from the uterus after day 20 or 21. Blastocysts can be enclosed in the zona pellucida as late as day 19 and loss of zona pellucida with further expansion occurs on days 19-20. Uterine swellings can be observed in vivo, albeit inconsistently, at days 21-22 at the time of embryo attachment, and even before invasion of the embryo into the endometrium. The uterine responses to embryo localization may be detected via uterine transillumination by day 21, even in the absence of gross swelling. Blastocysts remain unattached as late as days 21-22; invasion of placental trophectoderm occurs as early as day 22 and as late as day 23, and only 1-2 days before heartbeats are detected by sonography. Assay of canine relaxin by canine relaxin-specific radioimmunoassay detected increases in serum relaxin concentrations as early as days 26-30 and no earlier than the concurrent increase in serum prolactin concentrations at days 26-30; the increase in serum relaxin concentrations was also no earlier than increases in the concentrations of serum acute phase proteins, including fibrinogen. It is not known whether relaxin can stimulate prolactin secretion in dogs. When natural progesterone alone was provided by injection and subcutaneous implants before and after ovariectomy performed before implantation, implantation occurred normally, and pregnancy was maintained to term. The increase in prolactin was not different from that of control pregnancy, despite the absence of effective systemic concentrations of oestrogen, as observed by a typical castration response in LH and FSH. Lack of oestrogen may have compromised mammary development and lactation. Therefore, the pregnancy-associated increase in prolactin concentrations does not require an increase in or the presence of maternal oestrogen. These observations extend our knowledge of canine pregnancy and indicate several areas worthy of further investigation.