Comparing the estrus cycle with the menstrual cycle
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DVM MS PhD
Meigs Distinguished Teaching Associate Professor, Department of Physiology and Pharmacology, College of Veterinary Medicine, University of Georgia, Athens, GA
Cyclic changes of reproductive activities, including the physiological functions and behavior, are observed in the female animals. This is because that the female animals have to accommodate two seemingly contradictory roles in order to achieve the success of reproducing their offspring. At one period of time the female gametes grow and trigger the excitatory urge for mating. On the other hand, after the mating process the female animals need to provide a quiescent, calm uterine environment suitable for nurturing the growth and development of embryos/fetuses. The cyclic changes in most mammalian females is referred as estrous cycle whereas the term menstrual cycle is used for primates including humans.
1. Why the different expressions are used?
Animals with estrous cycle generally do not menstruate, the endometrium is resorbed if fertilization does not occur. In a specific time of the cycle, those animals are in heat or estrus. During this brief window of time female animas become sexually receptive to male animal’s mating bid. The day of beginning of such sexual receptivity is thus designated as Day 0 of the estrous cycle.
In primates the thickened endometrium is sloughed off through menstruation. There is no apparent variation in estrus or sexual receptivity in these animals. As such, the beginning of menstruation is designated as Day 1 of the menstrual cycle.
2. What are the main differences between animals with estrous cycle and menstrual cycle?
The time of ovulation in animals with estrous cycle occurs early in the cycle, as the high estrogen levels stimulate sexual behavior as well as exerting a positive feedback to induce ovulation through the hypothalamic-pituitary-ovarian axis. In primates ovulation occurs in mid-menstrual cycle to allow primary follicles to reach ovulatory stage.
The durations of estrous cycle vs. menstrual cycle are also different. This is mainly because the hormonal secretion from primate’s corpus luteum is different from non-primate animals.
There is also difference in reproductive senescence. In women and some primates, there is menopause. In other animals, Reproductive senescence has not been recognized.
The main differences between animals in estrous vs. menstrual cycles and their consequence will be discussed in presentation.