The estrous cycle is the female reproductive cycle that different species of mammals have, only in primates is it known as the menstrual cycle.
The difference in name lies in the differences that exist in the phase of the cycle that is related to ovulation, from where a new cycle will begin.
Stages of the estrous cycle of the cow
It is divided into 4 phases that are characterized by very specific hormonal conditions, which we will study next.
STROGENIC, FOLICULAR OR PROLIFERATIVE PHASE: Includes Proestro and Estro.
It is the stage of the estrous cycle where the follicles (oocytes) of the ovary begin to grow by stimulation of FSH. These follicles begin to produce Estradiol (E2).
It includes days 17, 20 and 21 of the cycle.
Predominant hormones: FSH
Hormones that are low: Progesterone (P4) begins to decrease.
It is the phase of the cycle that is characterized by the highest levels of estradiol (which have been increasing since the Proestro), and which are now secreted in greater quantity by the Follicle that has become Dominant. This hormone is responsible for the appearance of the signs of heat or heat: such as mucus, sexual receptivity of the male, restlessness, hyperemic vulva, rides and is allowed to ride, among others.
Estradiol also stimulates the release of LH; and the increasing increase of this hormone until reaching a peak that triggers ovulation (of the dominant follicle), is favored by the low concentrations of Progesterone (P4) in this phase of the cycle, because of a corpus luteum that has been destroyed (luteolysis) in the previous phase (Right) with what stopped producing it. When Progesterone (P4) is decreased, it does not exert an inhibition on GnRh (which is what it would normally do) and thus Estradiol (E2) can freely stimulate its production, but especially LH, since FSH begins to be inhibited by Inhibin (so that no more follicles grow).
It lasts from 12 to 24 hours.
Predominant hormones: Estradiol and LH.
Hormones that are low: Progesterone (P4).
PROGESTATIONAL, LUT OR SECRETOR PHASE: Includes the Metaestro and the Right-handed.
Day 0 of the cycle marks the beginning of the metaestro. It is characterized because all hormonal circumstances of the previous phase (Estro) begin to change. Therefore, Progesterone (P4) begins to increase and gonadotropins decrease. This happens because the LH Peak has just caused ovulation, so that the follicle will not produce more Estrogen (which does not stimulate LH anymore). Similarly, when ovulation occurs, there is a hemorrhagic body that will become a corpus luteum and begin with the increased production of Progesterone (P4), until it is maximum at day 7.
It includes days 2 and 3.
Predominant hormones: Progesterone (P4) begins to increase.
Hormones that are low: Gonadotropin LH begins to decrease.
It is the phase of the cycle where the corpus luteum has already developed to its fullest since ovulation occurred, therefore Progesterone levels are the highest. The increase in this hormone causes a decrease in gonadotropins FSH and LH, by inhibiting GnRh in the hypothalamus.
It goes from day 4 to day 16 of the cycle.
Predominant hormones: Progesterone.
Hormones that are low: FSH, LH and Estradiol.
After this phase, the cycle can be followed two paths:
Continue with the Proestro to repeat the cycle.
It is a period of ovarian inactivity, in which there is no manifestation of zeal. It may be due to physiological conditions (such as pre-pubertal, pregnancy and lactation) or external factors (environmental, nutritional and management factors or specific pathologies) that affect cyclicity.
In this article you can see in detail the causes of anestrous.
In the following video you will see the hormonal interaction of each phase of the estrous cycle: