Cycle:
- It is exhibited by all eutherian mammalian females except the order -primate.
- The mammalian females which exhibit one estrus cycle in one breeding season are called as monoestrous.
Example: Dogs, Fox, Bats, Deer etc.
- The mammalian females which exhibit two or more oestrous cycles in one breeding season are called polyestrous.
Example: Rat, squirrel, cow, pig, horse, cat, sheep rabbit etc.
- The oestrous cycle is studied with the help of vaginal smears.
- The duration of various phases of the oestrous cycle varies from species to species. In the rat, it is from 4 to 5 days and in the rabbit is 16 to 17 days.
The oestrous cycle has 4 phases –
1. Diestrus phase:
- It is also called as anestrus phase.
- It is the longest phase & its duration is of 60 to 70 hours.
- The ovary is without Graffian follicles.
- The uterine gland is thin, small & inactive.
- The vaginal smear shows leucocytes.
- During the non-breeding season, the female remains in the diestrus phase.
2. Proestrus phase:
- It is the smallest phase. Its duration is 1/2 to 5 hours.
- There is no growth of ovarian follicles.
- Vaginal smear exhibits epithelial cells.
- The uterine glands begin to increase in number & size.
- The thickness & contractability of the uterine wall begins to increase.
3. Oestrous phase:
- Its duration in the rat is 9 to 15 hours.
- It is also called a heat period.
- The basal temperature of the body is increased.
- Formation of Graffian follicles in the ovary.
- The vaginal smear shows the presence of cornified cells.
- The maximum level of estrogen in the blood.
- Ovulation takes place.
4. Post-Estrus or metestrus phase:
- Its duration is of 10 to 14 hours.
- The vaginal smear shows cornified cells & leucocytes.
- The maximum level of progesterone in the blood.
- Formation of Corpus luteum in the ovaries.
- If there is fertilization, the female remains in metestrus till the parturition.
- The oestrous cycle repeats if there is no fertilization and after parturition.
