The Other 51%: An Updated Characterization of the Female Rat’s Estrous Cycle

Abstract

The estrous cycle in female rats is analogous to the menstrual cycle in humans, lasting 4–5 days on average, and is composed of 4 main stages (proestrus, estrus, metestrus and diestrus) and 4 transitional stages. Each stage differs in length and is characterized by distinctive cell types with unique morphologies, which helps in the histological identification of each specific stage. Generally due to added expense and/or increased variables, animal research has historically been conducted on male animal models, and those findings are applied across the whole population.The most notable limitation in the utilization of the female animal model is the added unknown variability caused by hormonal fluctuation on the experiment. The main aim of this study was to create estrous synchronization of each experimental cohort, limiting the introduction of these variables. Although group housing has been shown to induce spontaneous synchronization (in mice), the inability to select for a specific stage, as well as limited success in rats, required an alternative method. We used luteinizing hormone‐releasing hormone analog (LHRHa) one week prior to exposure to induce estrous synchronization. Estrous stage was assessed via vaginal lavage and crystal violet staining. Slide comparisons of lavage cells collected prior to and after chemical manipulation showed differences in quantity and/or morphology of cells in each stage. In our study, we targeted a successful synchronization to the diestrus stage at 168 hours post‐injection. In our assessment of over 140 animals, we found that synchronization was successful 49.3% of the time. Overall, the use of LHRHa to induce estrous synchronization maximized experimental time management and limited estrous‐cycle‐dependent physiological and metabolic changes, enabling biochemical data to be collected and interpreted with minimal extraneous variables. These data are extremely beneficial to the drug discovery process, as researchers now look for more gender‐balanced models to assess new therapeutics.DISCLAIMERSThe views expressed in this poster are those of the author(s) and do not reflect the official policy of the Department of Army, Department of Defense, or the U.S. Government. The experimental protocol was approved by the Animal Care and Use Committee at the United States Army Medical Research Institute of Chemical Defense, and all procedures were conducted in accordance with the principles stated in the Guide for the Care and Use of Laboratory Animals (National Research Council, 2011), and the Animal Welfare Act of 1966 (P.L. 89–544), as amended.