Animal Reproduction (AR)
Animal Reproduction (AR)
Original Article

The rat estrous cycle revisited: a quantitative and qualitative analysis

C.C. Paccola, C.G. Resende, T. Stumpp, S.M. Miraglia, I. Cipriano

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The rat has been elected as the main animal model in several studies involving reproduction. However, there are scarce and conflicting data related to its estrous cycle. It comprises phases characterized by different cell types in vaginal smears (proestrus, estrus, metestrus and diestrus). Nevertheless, this classification and the time span of each phase are controversial and may induce misleading interpretations. In addition, there are no reports regarding the quantification of all cell types in each phase, including pre-acidophilic cells and leukocytes. The goal of this study was to revisit the literature about the rat estrous cycle and to perform a detailed quantitative and qualitative description of its phases and the transitional periods among them. Vaginal smears were obtained twice daily for 20 days from Wistar rats and stained using the Shorr method. Cells were classified as small (SBC) or large (LBC) basophilic cells, nucleated (NAC) or enucleated (EAC) acidophilic cells, pre-acidophilic cells (PAC) or leukocytes. Ten fields per smear were analyzed and cellular frequencies were determined to distinguish the phases. Enucleated acidophilic cells were observed in all phases. The number of PAC was high during proestrus, but none were found during estrus. Frequency of NAC was higher during the transitional period between metestrus II and diestrus than during other phases. Leukocytes were first observed during metestrus and showed very high frequency during diestrus. This study demonstrated that the quantitative analysis of cell populations in vaginal smears improves the identification of the estrous cycle phases and may contribute to a more precise detection of cyclical alterations.


estrous cycle, morphometric analysis, rat, Shorr method, vaginal smear
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