Background

The reproductive health of an animal is an important parameter of overall health and physiological function. In female laboratory rodents, assessment of estrous cycle stage over several consecutive days allows for short- and long-term tracking of each animal’s cyclicity, and thus reproductive health. In mice and rats, there are typically four stages that comprise the estrous cycle (in order): proestrus; estrus; metestrus; and diestrus, with diestrus often spanning two days (diestrus I and diestrus II). A healthy animal will move through the stages successively across 4-5 days on average (Byers et al., 2012; Cora et al., 2015).

Proper assessment of estrous cycle stage is important in producing timed pregnancies for developmental studies, as female mice are most receptive to mating when introduced to a male on proestrus or estrus (Mader et al., 2009; Smarr et al., 2016). Furthermore, certain animal behavior, physiology, and pharmacology parameters can change with estrous cycle stage in rodents (Maguire et al., 2005; Cordeau et al., 2008; Milad et al., 2009; Lebron-Milad et al., 2013; Cushman et al., 2014; McHenry et al., 2017; Yagi et al., 2017; Broestl et al., 2018; Cordeira et al., 2018; Hirsch et al., 2018; Kaur et al., 2018; Santos et al., 2018; Johnson et al., 2019). Impaired mouse estrous cyclicity can provide a readout of compromised reproductive function, for example in toxicological studies (Hannon et al., 2014), models of endocrine disorders (Sullivan and Moenter, 2004; Novaira et al., 2014; Babwah et al., 2015), and in models of neurological disorders (Fawley et al., 2012; Jaini et al., 2015; Li et al., 2017; Li et al., 2018). Changes in vaginal wall electrical impedance in rats and mice can be measured with specialized monitors, but these changes may lack accuracy and may only be detected on certain cycle stages (Singletary et al., 2005). Similarly, changes in the vaginal opening can be determined by visual observation, but again only on certain stages, such as proestrus and estrus (Byers et al., 2012). By contrast, estrous cycle stage can be readily assessed in a minimally invasive manner through vaginal cytology, as has been documented for a century (Allen, 1922). Although these samples may be dried and stained for preservation and examination, such as with Giemsa or Toluidine blue stains (Cora et al., 2015), this process is time-consuming and may not be amenable to continual estrous cycle monitoring in large numbers of animals. With proper training and practice, however, it is possible to determine cycle stage based on direct examination of cell morphology and number without additional staining or cell preservation procedures. Here, we provide a protocol for this estrous cycling process for mice.

Funding agencies, including the U.S. National Institutes of Health and the Canadian Institutes of Health Research, have recently implemented policies requiring researchers to address sex as a biological variable, necessitating the increased inclusion of female animals in biomedical research (Clayton and Collins, 2014; Clayton, 2018; Canadian Institutes of Health Research, 2019). With greater use of female animals in rodent studies, and larger numbers of animals that may require estrous cycle monitoring, there is an added need for tools that expedite the processes of estrous cycle data analysis. To this end, we also present a novel software program that facilitates a greater throughput of mouse estrous cycle data acquisition, analysis, and visualization, which can also be used for other common laboratory rodents, such as rats. This tool allows for improved understanding of patterns or changes that are occurring in each animal across large cohorts.