Canis familiaris, domestic dog facts at GeoChemBio: reproductive cyle (estrus cycle)

Canis lupus familiaris
dog reproductive cycle

Introduction

The dog is a monoestral polyovulatory non-seasonal species. As a result, bitches spontaneously ovulate only once or twice per year and ovulation can occur at any time of the year. However, there are few exceptions, such as the Tibetan Mastiff and the Basenji that have single annual estrus cycle, which may be influenced by photoperiod.

Peculiarities of dog reproductive cycle

Unusual endocrinology: blood concentration of progesterone starts to increase a few days before ovulation resulting in quite high levels when ovulation occurs.
Oocytes are ovulated at an immature diploid stage (germinal vesicle) and they complete their meiotic maturation in the uterine tube where they can survive for several days.
The canine oocyte/embryo spends a long time (8-10 days) in the uterine tube, as compared with many other mammals (for example, 2 days in pigs and 3-4 days in cattle and mice).
Embryonic development is slow, and the total pre-implantation period is particularly long (18-20 days).

Estrous cycle The period of cyclic physiological and behavior changes in non-primate female mammals that exhibit estrus. The estrus cycle of the bitch is considerably longer than in most other domestic animals.
- Proestrus Precedes estrus. Lasts for 5-20 days (averages 9 days). During proestrus, the Graafian follicles undergo maturation. Proestrus is diagnosed clinically by the vaginal swelling and vaginal discharge that contains mucus and blood. There is a progressive decline in aggressive response to interested males. Refusals of mounting attempts progressively change from aggressive to ambivalent to playful to passively lying down. Proestrus ends with the onset of receptive behavior typically occurring 0.5–3 days after the peak in estradiol and within a day of the preovulatory LH surge. Endocrinologically, physiological proestrus ends with the preovulatory LH surge.
- Estrus The period (commonly called "heat") in the reproductive cycle associated with maximum sexual receptivity and fertility in non-primate female mammals. In dogs, estrus occurs once or twice a year and can persist for 5-15 days. This period is defined as the onset of either behavioral signs or vaginal epithelial exfoliative cytology (>90% cornification). Estrus behavior is characterized by proactive receptivity to mounting by males and increased male-seeking behavior. Sometimes bitches exhibit minimal outward signs of estrus.
  During estrus, follicle diameter increases, reaching pre-ovulatory stage (0.5-0.7 mm or 0.8-1.0 mm, according to different sources).
  Ovulation in dogs is not induced by mating as in cats and can occur before (up to 3 days) or up to 5 days after mating (unlike in cats who are induced ovulators). Time of ovulation can be more precisely determined by profiling of luteinizing hormone (LH, also known as lutropin) and progesterone in blood. Ovulation occurs in 2-days after the preovulatory LH surge with little variation, however, the oocytes are still at the immature germinal vesicle stage (GV, prophase I) at that time. Determination of LH peak is usually limited to research protocols. Blood progesterone concentration of 5-7 ng/ml (or > 2 ng/ml) is relatively reliable parameter to predict ovulation and can be used routinely. Oocytes in uterine tube can remain fertilizable until up to 4-5 days after ovulation. Viable spermatozoa can be found up to 11 days after mating.
  Ovulation rate as well as litter size are variable, depending on the breed. Mean total number in small-size breeds (less than 10 kg) is 4-7 whereas in medium (10-20 kg) and large breeds (>20 kg) is about 7-9 and 9-12 corpora lutea, correspondingly. Ovulation lasts for 24-36 hours.
- Metestrus The period following estrus defined as post-estrus portion of luteal phase. Metestrus is considered to last until evidence of the ongoing luteal phase becomes minimal. The end of metestrus, and anestrus onset, are variably defined as when uterine endometrium has undergone histological "repair", when mammary enlargement in response to luteal phase progesterone recedes, and when serum progesterone declines to levels below 1 or 2 ng/ml (∼3–6 nmol/l).
- Interestrus The period between estrus without signs of sexual behavior when bitch is non-ovulatory. The interestrus interval averages 31 weeks (16-56 weeks). The variation is due differences in variable duration of anestrus.
  - Diestrus The term "diestrus" is used in some veterinary texts as a substitute for and synonymous with metestrus in bitches, thereby applying "diestrus" across species to generally refer to the period of luteal function and avoiding possible misconceptions that "metestrus" might refer to only a short period after estrus as in descriptions of artiodactyl and rodent cycles.
  - Anestrus An obligate state of sexual inactivity in female dogs. In dogs aseasonal anestrus varies in duration from 2 to 10 months depending on breed and on individual after progesterone declines below 1–2 ng/ml.
    During anestrus, follicular growth occurs, but terminal follicular differentiation is absent and maximum follicle diameter is only 0.6-1 mm.