[Vaginal smear cycle in 4-day cyclic Chinese hamsters, Cricetulus griseus]

The contents of vaginal smear of 4-day cyclic Chinese hamster (Cricetulus griseus) was investigated every 3 hours for 5 days. A light-dark cycle of 14--10 hr was used with the lights turned on at 6 : 00 a.m. Estrous cycle of the Chinese hamster determined by vaginal smears can be divided into 6 periods. The proestrous phase started at about 0 : 00 of day 1, the day of the proestrous phase was designated as day 1 of the estrous cycle. In the afternoon of the same day 1, nucleated epithelial cells gradually increased in number (proestrus : I), and the vaginal contents became to consist solely of nucleated epithelial cells at about 18 : 00 to 21 : 00 (estrus : II). At about 0 : 00 of day 2, however, nucleated epithelial cells were superseded suddenly by cornified epithelial cells, and this phase lasted for 9 to 12 hr (metestrus I : III). Towards the end of the cornified stage, nucleated cells appeared in short duration (metestrus II : IV). And then, in the evening of day 2, leucocytes gradually increased in number with degeneration of nucleated cells (diestrus I : V-1). On day 3, vaginal smear contained a large amount of mucus as well as degenerated nucleated cells and leucocytes (diestrus II : V-2). At about 21 : 000 of day 4, some cornified epithelial cells were seen and then proestrous stage was returned. The females were mated with 3 to 5 males in the evening of day 1, copulation was confirmed in 83.7% females in the next morning,thus the copulation in the Chinese hamster may be thought to occur during the vaginal smear stage of nucleated epithelial cells (estrous phase), i.e. about 18 : 00 to 24 : 00 of day 1.