Estrous Cycle

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Mouse ovarian follicle

The estrous cycle (British spelling, oestrous) is the main reproductive cycle of other species females of non-primate vertebrates, for example rats, mice, horses, pig have this form of reproductive cycle. Also do not confuse with "estrus", which is a phase of the cycle.

There are also a variety of different forms:

  • Polyestrous Animals - Estrous cycles throughout the year (cattle, pigs, mice, rats).
  • Seasonally Polyestrous Animals - Animals that have multiple estrous cycles only during certain periods of the year (horses, sheep, goats, deer, cats).
  • Monestrous Animals - Animals that have one estrous cycle per year (dogs, wolves, foxes, and bear)

Links: estrous cycle | mouse estrous cycle | ovary | oocyte | uterus | menstrual cycle

Some Recent Findings

  • Bradykinin during estrous cycle of mouse[1] "Thus, this study indicates that the levels of bradykinin and bradykinin B(2)-receptor both simultaneously regulate estrous cycle and are important components for the reproductive process."
More recent papers  
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Estrous Cycle Stages

The descriptions below refer to the "typical" mammalian cycle.

proestrus - estrus - metestrus - diestrus


The first stage in the estrous cycle immediately before estrus characterized by development of both the endometrium and ovarian follicles.


The second stage in the estrous cycle immediately before metestrus characterized by a receptivity to a male and to mating, often referred to as "heat" or "in heat". Pheromones may also be secreted only at this stage of her cycle.


The third stage in the estrous cycle immediately before diestrus characterized by sexual inactivity and the formation of the corpus luteum.


The last stage in the estrous cycle immediately before the next cycle proestrus characterized by a functional corpus luteum and an increase in the blood concentration of progesterone.


Not a stage in the estrous cycle, but a prolonged period of sexual rest where the reproductive system is quiescent.

Vaginal Smear Comparison

Vaginal Smear Comparison Table
Guinea pig Rat (Long and Evans)
I. Superficial squamous cells with pyknotic nuclei; progressive leucopenia. 1. Small round nucleated cells; disappearance of leucocytes.
Intermediate (cornification) period
a. Many cornified cells, usually mixed with type I cells;
b. Prevalence of cornified cells with cheesy masses; leucopenia
Cornification period
2. Early cornified cell stage; leucocytes scarce.
3. Late cornified cell stage with large cheesy masses; leucopenia
II, III, IV. Appearance of deep layer cells; reappearance and great exodus of leucocytes; gradual disappearance of cornifiecl cells; sometimes erythrocytes present. 4. Leucocytic-cornified cell stage; reappearance of leucocytes; gradual disappearance of the cornified cells.
V. Dioestrus: Leucocytes and atypical atypical vaginal cells. 5. Dioestrus: Leucocytes and vaginal cclls.
Reference: Papanicolaou GN. The Sexual Cycle in the Human Female as revealed by Vaginal Smears. Am J Anat. 1933;52: 519–637.
Links: Estrous Cycle | Guinea Pig Development | Rat Development

Rat Estrous Cycle


One of the best characterised polyestrous reproductive cycles, though different species of rats may differ in reproduction. In general, puberty occurs at 6-8 weeks when the estrous cycle commences each cycle is 4-5 days. The estrous cycle is polyestrous, more than one estrous cycle during a specific yearly time, with an estrous period of approximately 12 hours.

See the review of the rat estrous cycle.[2]

Links: rat | PubMed - rat estrous cycle

Mouse Estrous Cycle


The mouse oestrus cycle is 4-6 days, with oestrus lasting less than 1 day. The estrous cycle stops during lactation except for one oestrus 12-20 hours postpartum. The information below refers to determining the stage of the estrous cycle in the mouse by the appearance of the vagina.[3]

  • Estrous Diestrus - Vagina has a small opening and the tissues are bluish-purple in color and very moist.
  • Proestrus - Vagina is gaping and the tissues are reddish-pink and moist. Numerous longitudinal folds or striations are visible on both the dorsal and ventral lips.
  • Estrus - Vaginal signs are similar to proestrus, but the tissues are lighter pink and less moist, and the striations are more pronounced.
  • Metestrus-1 - Vaginal tissues are pale and dry. Dorsal lip is not as edematous as in estrus.
  • Metestrus-2 - Vaginal signs are similar to metestrus-1, but the lip is less edematous and has receded. Whitish cellular debris may line the inner walls or partially fill the Vagina.

Links: mouse estrous cycle

Pig Estrous Cycle


The feedback systems between the ovaries, uterus, hypothalamus and pituitary gland govern a cycle of events that takes 18-21 days. If conception occurs this cyclic pattern is interrupted and pregnancy is maintained for approximately 114 days. Removal of the sucking stimulus at weaning triggers a new sequence of events.

Links: pig | PubMed- pig estrous cycle

Dog Estrous Cycle

  • Proestrus (9 days) - Precedes estrus, estradiol concentration increases as ovarian follicules mature and the uterus enlarges. The vaginal epithelium proliferates accompanied by diapedesis of erythrocytes (most cells in vaginal smear) from uterine capillaries.
  • Estrus (9 days) - Accompanied by female mating behaviour, glandular secretions increase, the vaginal epithelium becomes hyperemic, and ovulation occurs. Cycle is influenced mainly by estrogens and the interval between successive estrus cycles is about 7 months.
  • Diestrus (70-80 days) - Accompanied by female non-mating behaviour, corpus lutea present and secretes progesterone. Uterine glands undergo hypertrophy and hyperplasia, vaginal secretions and the cervix constricts.
  • Anestrus - Anestrus is a prolonged period of sexual rest where the reproductive system is quiescent.

Links: dog

Bovine Estrous Cycle

Cow and calf

Bovine estrous cycle hormone graph.jpg

Specific hormone concentrations are not shown in the above graph, only the relative hormone levels at different times during the cycle.

Links: cow


  1. Singh P, Krishna A & Sridaran R. (2011). Changes in bradykinin and bradykinin B(2)-receptor during estrous cycle of mouse. Acta Histochem. , 113, 436-41. PMID: 20546864 DOI.
  2. Hubscher CH, Brooks DL & Johnson JR. (2005). A quantitative method for assessing stages of the rat estrous cycle. Biotech Histochem , 80, 79-87. PMID: 16195173 DOI.
  3. Champlin AK, Dorr DL & Gates AH. (1973). Determining the stage of the estrous cycle in the mouse by the appearance of the vagina. Biol. Reprod. , 8, 491-4. PMID: 4736343


Westwood FR. (2008). The female rat reproductive cycle: a practical histological guide to staging. Toxicol Pathol , 36, 375-84. PMID: 18441260 DOI.

Goldman JM, Murr AS & Cooper RL. (2007). The rodent estrous cycle: characterization of vaginal cytology and its utility in toxicological studies. Birth Defects Res. B Dev. Reprod. Toxicol. , 80, 84-97. PMID: 17342777 DOI.


Caligioni CS. (2009). Assessing reproductive status/stages in mice. Curr Protoc Neurosci , Appendix 4, Appendix 4I. PMID: 19575469 DOI.

Hubscher CH, Brooks DL & Johnson JR. (2005). A quantitative method for assessing stages of the rat estrous cycle. Biotech Histochem , 80, 79-87. PMID: 16195173 DOI.

Su P, Wu JC, Sommer JR, Gore AJ, Petters RM & Miller WL. (2005). Conditional induction of ovulation in mice. Biol. Reprod. , 73, 681-7. PMID: 15917351 DOI.

Marcondes FK, Bianchi FJ & Tanno AP. (2002). Determination of the estrous cycle phases of rats: some helpful considerations. Braz J Biol , 62, 609-14. PMID: 12659010

Champlin AK, Dorr DL & Gates AH. (1973). Determining the stage of the estrous cycle in the mouse by the appearance of the vagina. Biol. Reprod. , 8, 491-4. PMID: 4736343

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Cite this page: Hill, M.A. (2019, September 23) Embryology Estrous Cycle. Retrieved from

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