Comparison of day 21 vaginal exfoliative cytology lavage and vaginal histology in the evaluation of oestrous cycle stages in group-housed CD-1 mice.

Vaginal lavage cytology is commonly used in rodents to determine oestrous cyclicity and is a requirement for regulatory reproductive toxicity studies. While rats are the preferred species for these studies, where scientifically justified, the mouse is an alternative species. The current study sought to evaluate the utility of day 21 vaginal lavage cytology in the CD-1 mouse for assessment of changes in the murine oestrous cycle. It compared the vaginal lavage technique, using either fresh or fixed and stained cytology smears, with the histological evaluation of the vagina on day 21 of a murine study and examined the results in the context of interpreting data from reproductive toxicity studies conducted in mice. On day 21, concordance between methanol-fixed, Wright-Giemsa-stained vaginal lavage cytology slides and histological examination of the vagina was recorded in 17/24 animals (70.83%). The results also show that vaginal lavage cytology in mice yields variable data with the stages of the oestrous cycle being less clearly discernible. Compared with vaginal lavage cytology, terminal histological examination of the vagina was found to be the most accurate method for determining stage of oestrous. Routine assessment of oestrous cyclicity in mice by vaginal lavage cytology is of questionable value and has animal welfare implications. [ABSTRACT FROM AUTHOR]