Determining the Stage of the Estrous Cycle in the Mouse by the Appearance of the Vagina

Abstract

A simple method is described by which the various stages of the estrous cycle of the mouse can be easily determined by the appearance of the vagina. Data are presented which show that this method of determining estrous state is at least as accurate and reliable as the customary method of vaginal smearing. The method is faster, and less stressful to the animals than the vaginal smear procedure. Since the initial study of the estrous cycle in the guinea pig by Stockard and Papanicolaou in 1917, the vaginal smear technique has been the accepted procedure for determining the estrous state in various mammalian species. The technique involves an identification of the cell types and their relative quantities present in preparations obtained from the vagina by either scraping or swabbing the vaginal walls, or by vaginal washings. The technique is not without its disadvantages. Emery and Schwabe (1936) have reported that frequent vaginal smearing of both castrate and intact rats using cotton swabs resulted in an abnormally high incidence of cornified or estrus-like smears. Frequent smearing has also been found to result in cornification of the vaginal epithelium in mice (Wade and Doisy, 1935). Vaginal smearing, particularly during estrus, could induce pseudopregnancy and result in abnormally long cycles. Gross changes in the appearance of the