The oestrous cycle is the reproductive cycle found in most mammalian placental females whereby there are recurring periods when the female is fertile and sexually receptive (oestrus) interrupted by periods in which the female is not fertile and sexually receptive (anoestrus). Animals that have oestrous cycles reabsorb the endometrium (inner membrane of the mammalian uterus) if conception does not occur during that cycle. The oestrous cycle is contrasted with the menstrual cycle in which the endometrium is shed through menstruation when pregnancy does not occur and in which the female may be sexually receptive at any time during the cycle.
There are many variations among animals in terms of their oestrous cycles. Some may undergo oestrous only one time a year during a particular season (white-tailed deer, foxes), while others may undergo a succession of cycles during a certain time of the year if they do not become pregnant (horses, sheep), and others may undergo cycles throughout the whole year (mice, cows, pigs).
Below is an overview of the hormonal changes during the oestrous cycle:
Phases of the Oestrous Cycle
The oestrous cycle can be split into different stages:
This is the stage before oestrus; there is increased follicular growth and regression of the corpus luteum. This means that progesterone levels are decreasing (due to corpus luteum regressing) so that the hypothalamus is no longer inhibited and oestrogen levels are rising (due to the formation of follicles), oestrogen promotes GnRH secretion. At this point of the oestrus cycle the female is not sexually receptive.
This is the period when the female is sexually receptive. Ovarian follicles are maturing causing oestrogen levels to be at their highest, which stimulates pulse secretions of FSH/LH to reach peak levels. Oestrogen levels continue to rise until ovulation – when the mature follicle ruptures on the side of the ovary, releasing the ovum (egg) for fertilisation. The female may show signs of ‘heat’ during oestrus.
After ovulation the corpus luteum begins to develop from the ruptured follicle, which causes progesterone levels to rise, inhibiting the cycle. The uterus lining may also secrete progesterone.
The corpus luteum becomes a dominant structure, meaning progesterone levels will be at their highest. The progesterone inhibits the hypothalamus, preventing it from secreting GnRH.
This is the stage of rest and ‘reset’ for the reproductive system; any follicular development at this time will be minimal. The process of ‘resetting’ begins with the uterus secreting prostaglandin F2a, (stimulated by the high levels of progesterone due to the development of the corpus luteum) this causes the degeneration of the corpus luteum when levels of prostaglandin are high. Degeneration of the corpus luteum results in a rapid decline of progesterone; hypothalamic secretions of GnRH are no longer inhibited. New follicles develop resulting in a rise of oestrogen and the cycle returns to pro-oestrus.
In seasonal breeders, the length of anoestrus is controlled by exposure to light by the pineal gland. Longer days mean more light exposure and low levels of melatonin. Melatonin secretion is controlled by the pineal gland, low light levels mean more melatonin secretion and vice versa in long day breeders. When melatonin levels are low enough (during longer days) pro-oestrus sets in.
Selection of a Dominant Follicle for Ovulation
A follicular wave is a group of immature follicles that begin the maturation process. Several of these waves occur during one oestrus cycle yet only one mature follicle arises for ovulation (in normal circumstances). The reason only one mature follicle arises is due to the inhibiting effects of progesterone (due to the presence of the corpus luteum from the previous ovulation). When progesterone levels are high, a follicle cannot complete the maturation process. Therefore only after luteolysis can a follicle fully mature.
- During the first few days of the oestrous cycle, a group of follicles are recruited from the follicular pool (all the primordial follicles which are yet to mature). This selected group is the start of a follicular wave.
- From the initial group of recruited follicles only a number of them develop into selected follicles. (There are moderate levels of FSH/LH at this stage)
- From the selected follicles only 1 will become a dominant follicle, however due to the high levels of progesterone this dominant follicle becomes atretic and degenerates
- Another follicular wave will begin as before, but will end the same due to still high levels of progesterone. However around the time that the second wave dominant follicle degenerates luteolysis occurs. The degeneration of the corpus luteum causes progesterone levels to fall.
- Another follicular wave begins just as before, yet due to the low levels of progesterone the resulting dominant follicle is able to develop into the preovulatory follicle and go on to be ovulated.
- The stage at which the preovulatory follicle is ovulated is oestrus.
- Seasonally mono-oestrous – Undergo oestrus one time a year at a predictable time
- Dioestrous – Undergo oestrus twice per year, such as most dogs.
- Polyoestrous – Polyoestrous species are those can go through a succession of oestrous cycles during the year, for example, cats, cows, and pigs.
- Seasonally Polyoestrous – Undergo oestrous cycles during a particular time of the year. For example, horses, sheep and goats go into oestrus more than one time during a particular season if not mated. These can be divided into short-day and long-day breeders:
- Short-day breeders, such as sheep are sexually active in fall or winter.
- Long-day breeders, such as horses are sexually active in spring and summer.
Frequency of Cycles
A female dog is dioestrous and goes into heat typically twice every year. The pro-oestrus is relatively long at 5–7 days, while the oestrus may last 4–13 days. With a dioestrous of 7–10 days, a typical cycle lasts about 3 weeks followed by about 150 days of anoestrus. They bleed during this time, which will usually last from 7–13 days, depending on the size and maturity of the dog.
A mare may be 4 to 10 days in heat and about 14 days in dioestrous. Thus a cycle may be short, i.e. 3 weeks. Horses mate in spring and summer, autumn is a transition time, and anoestrus rules the winter.
Horses are seasonal long day breeders, meaning they require long days (long periods of light) for their oestrous cycle to begin. This is because light inhibits the production of melatonin by the pineal gland. Melatonin at high levels (during the shorts days due to lack of light) inhibits the oestrous cycle by preventing the secretion of GnRH, so when melatonin levels are low, GnRH secretion is no longer inhibited and the oestrous cycle can begin cycling. The reason for their seasonality is due to their gestation period, given a gestation period of about eleven months, it prevents them from having young when the cold of winter would make their survival risky. This is why animals can reproduce during only certain times of the year.
The oestrous cycle of the cow is generally about 21 days long, but it can range from 17 to 24 days in duration. Each cycle consists of a long luteal phase (days 1-17) where the cycle is under the influence of progesterone and a shorter follicular phase (days 18-21) where the cycle is under the influence of oestrogen. The cycle begins with standing heat, or oestrus. This time of peak oestrogen secretion can last from 6 to 24 hours, with ovulation occurring 24 to 32 hours after the beginning of oestrus.
The female cat in heat has an oestrus of 14 to 21 days and is an induced ovulator. Without copulation, she may enter interoestrus before re-entering oestrus. With copulation and in the absence of pregnancy, cycles occur about every three weeks. Cats are polyoestrous but experience a seasonal anoestrus in autumn and late winter.
- Ewe—17 days
- Goat—21 days
- Sow—21 days
- Elephant—16 weeks