Staging of the estrous cycle and induction of estrus in experimental rodents: an update

Abstract

Background: Determination of the phases of the estrous cycle and induction of estrus (heat) in experimental animals remains useful, especially in reproductive function research.

Main body of the abstract: This review provides a detailed description and discusses extensively the variations observed in different phases of the estrous cycle in laboratory animals using rats and mice as examples. It also illustrates how these phases can be determined and how to induce estrus 'heat' when required. The phases of the estrous cycle can be determined using various methods such as visual assessment, vaginal smear/cytology, histology of female reproductive organs (vagina, uterus and ovaries), vaginal wall impedance assessment and determination of urine biochemical parameters. Female animals can be artificially brought to estrus phase 'heat' to make them receptive to male counterparts.

Conclusion: Determination of the length and phases of the estrous cycle and induction of estrus are useful in teaching and research and evaluating the effects of drugs/chemicals on the reproductive functions.

Keywords: Estradiol; Estrous cycle; Menstrual cycle; Progesterone; Reproductive cycle.

Fig. 1

Appearance of the vagina in different phases of estrous cycle of a Swiss albino strain mouse. a-Proestrus, b-Estrus, c- Metestrus, d- Diestrus (Ekambaram et al., [9])

Fig. 2

Appearance of the vagina in different phases of estrous cycle of an agouti strain mouse. a-Proestrus, b-Estrus, c- Metestrus, d- Diestrus (Byers et al., [6])

Fig. 3

Appearance of the vagina in different phases of estrous cycle of a non-agouti strain mouse. a-Proestrus, b-Estrus, c- Metestrus, d- Diestrus (Byers et al., [6])

Fig. 4

Oestrous cycle wheel

Fig. 5

Vaginal smears at proestrus in Sprague-Dawley rats

Fig. 6

Vaginal smears at estrus in mice

Fig. 7

Vaginal smears at estrus in Sprague Dawley rats

Fig. 8

Vaginal smears at metestrus in mice

Fig. 9

Vaginal smears at metestrus in Sprague Dawley rats

Fig. 10

Vaginal smears at diestrus in mice (a-e) and Sprague Dawley rats (f)

Fig. 11

Histology of the rat reproductive organs at proestrus (West wood, 2008)

Fig. 12

Histology of rat reproductive organs at estrus (West wood, 2008)

Fig. 13

Histology of the rat reproductive organs at Metestrus (Westwood, 2008)

Fig. 14

Histology of the rat reproductive organs at diestrus (Westwood, 2008)

Fig. 15

A standard impedance monitor

Fig. 16

Male terminal of a standard stereo audiocable used as a resistance-measuring probe (Ramos et al., [20])