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

  • Abundances and localizations of Claudin-1 and Claudin-5 in the domestic cat (Felis catus) ovary during the estrous cycle[1] "Claudins (CLDNs) are major Ca2+-independent cell adhesion molecules functioning at tight junctions (TJ). The presence and localization of cell adhesion molecules are important for understanding the mechanisms associated with follicular and luteal development in the ovary. In this study, there was an examination of whether CLDN-1 and CLDN-5 are present in a cell- and stage-specific manner during follicular and luteal development in the domestic cat ovary using immunohistochemistry and Western blot analysis. While results from immunoblot analyses revealed there were relatively similar abundances of CLDN-5 protein in three phases of the ovarian cycle, the abundance of CLDN-1 in the luteal phase was greater than those measured in the follicular and anestrous phases (P < 0.01). Results with immunohistochemistry indicate CLDN-1 and -5 are mainly localized in the cell nuclei and cytoplasm of all tissues of the cat ovary. In follicles, throughout the development from primordial to large antral follicles, CLDN-1 and -5 were present in oocytes, and the granulosa and theca cell layers. In follicles at all stages of atresia, there were cell-type and stage-specific protein distributions with immunostaining present in granulosa, thecal interstitial, and fibroblast-like cells. In corpora lutea, both small and large luteal cells stained positively for both claudins. In conclusion, the specific presence and localization patterns of CLDN-1 and -5 in the cat ovary is suggestive that these TJ proteins could have local functions in the regulation of most ovarian functions such as follicle development and atresia, ovulation, and corpus luteum formation and regression."
  • Estrous Cycle Manipulation in Dogs[2] "Since 1939, scientists have studied estrous cycle manipulation in dogs resulting in more articles published in this field that any other area of canine reproduction. Estrous cycle manipulation in dogs must be safe and reliable. Dopamine agonists, gonadotropin-releasing hormone agonists, and gonadotropins are hormones that have been used for estrus induction in bitches but each treatment has advantages and disadvantages. Despite widespread availability of these medications throughout the rest of the world, there are no drugs currently labeled for canine estrus induction in the United States."
  • Vaginal Cytology of the Laboratory rat and mouse: Review and Criteria for the Staging of the Estrous Cycle Using Stained Vaginal Smears[3] "Microscopic evaluation of the types of cells present in vaginal smears has long been used to document the stages of the estrous cycle in laboratory rats and mice and as an index of the functional status of the hypothalamic-pituitary-ovarian axis. The estrous cycle is generally divided into the four stages of proestrus, estrus, metestrus, and diestrus. On cytological evaluation, these stages are defined by the absence, presence, or proportion of 4 basic cell types as well as by the cell density and arrangement of the cells on the slide. Multiple references regarding the cytology of the rat and mouse estrous cycle are available. Many contemporary references and studies, however, have relatively abbreviated definitions of the stages, are in reference to direct wet mount preparations, or lack comprehensive illustrations. This has led to ambiguity and, in some cases, a loss of appreciation for the encountered nuances of dividing a steadily moving cycle into 4 stages. The aim of this review is to provide a detailed description, discussion, and illustration of vaginal cytology of the rat and mouse estrous cycle as it appears on smears stained with metachromatic stains."
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Search term: Estrous Cycle

Older papers  
These papers originally appeared in the Some Recent Findings table, but as that list grew in length have now been shuffled down to this collapsible table.

See also the Discussion Page for other references listed by year and References on this current page.

  • Bradykinin during estrous cycle of mouse[4] "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."

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.[5]

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.[6]

  • 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


1937 - Etymology and pronunciation of the word "oestrus" and its derivatives[7]

This word seems to offer more difficulties as to pronunciation and spelling than any other technical word in biology. Derived originally from the Greek oloxpos, signifying the gadfly, and taken over into Latin as oestrus, the word came secondarily to mean frenzy or strong desire. The Latin derivative is properly of masculine gender, following the Greek, but we are told by Tyson[8] that some grammarians gave it the neuter form oestrum as early as 400 a.v. In its more general senses the word became naturalized in English with the spelling oestrum and has been so used in prose and poetic literature by many writers (see Tyson’s article and the Oxford English Dictionary).

In the original Greek and Latin the meaning of the word already included, among other forms of excitement, the recurrent sexual impulse of animals. We owe its present definite technical use, however, to the late Walter Heape,[9] whose analysis and terminology of the phenomena of the reproductive cycle form the basis of research on that subject in the present century. As pointed out by Asdell,[10] Heape was not using the well-naturalized English word oestrum, which in English signifies any form of recurrent excitement (e.g., the poetic frenzy), but was deliberately adopting the Latin word oestrus for use as a specific technical term meaning in English “periodic sexual excitement of the female.” Writers having the latter significance in mind should, for the sake of precision, respect the difference and use the word oestrus.

It is scarcely necessary to point out that the nominative form is oestrus, and the adjectival form oestrous (cf. fungus, fungous; mucus, mucous).

As to pronunciation, the Greek and Latin diphthong of the first syllable has become in English merely a digraph, and in England is pronounced like long e, as in thief. Wyld’s Dictionary of “Received Standard English” gives this pronunciation only, The Oxford English Dictionary gives also the short e, as in yet, as an alternative pronunciation, but by the time the Shorter Oxford Dictionary reached the letter O, the compilers had discovered that the short e is an American usage. The word oestrum seems to have first appeared in the American dictionaries in the 1860 edition of Worcester and the 1864 Webster. In both eases the short pronunciation of e was alone given. Webster continued to give preference to this pronunciation, but since the 1909 revision cites also the long e as a non-preferred pronunciation. The Century Dictionary of 1911 gives the long e only, but on the other hand the 1913 Funk and Wagnalls gives the short e only.

It is evident, therefore, that the pronunciation of the non-technical word oestrum, and consequently of the technical term oestrus, oestrin, oestrogenic, ete., is ... (truncated)


  1. Liman N & Ateş N. (2020). Abundances and localizations of Claudin-1 and Claudin-5 in the domestic cat (Felis catus) ovary during the estrous cycle. Anim. Reprod. Sci. , 212, 106247. PMID: 31864490 DOI.
  2. Kutzler MA. (2018). Estrous Cycle Manipulation in Dogs. Vet. Clin. North Am. Small Anim. Pract. , 48, 581-594. PMID: 29709316 DOI.
  3. Cora MC, Kooistra L & Travlos G. (2015). Vaginal Cytology of the Laboratory Rat and Mouse: Review and Criteria for the Staging of the Estrous Cycle Using Stained Vaginal Smears. Toxicol Pathol , 43, 776-93. PMID: 25739587 DOI.
  4. 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.
  5. 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.
  6. 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
  7. Corner GW. Etymology and pronunciation of the word "oestrus" and its derivatives. (1937) Science. 85(2199):197-198. PMID 17844622
  8. Stuart L. Tyson, Sctexce, 612: 74, 1931.
  9. Walter Heape, Quart. Jour, Micros, Soi,, ns, 44: 1, 1901,
  10. Sidney A, Asdell, Screncr, 75: 131, 1932.


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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