The aging brain in elderly

In Heape’s (1900) publication, the term estrus was first used. Estrus defined as the special period of sexual desire in the female. A Greek word oistros which means madness or frenzy has been adapted into Estrus which refers to the period of heat or sexual receptivity (Goldman et al., 2007). By making the Estrus as a base word, Heape (1900) defined the rest of the stages as proestrus, metestrus and diestrus.

In Goldman et al. (2007) and Westwood (2008), the authors documented that morphological changes in ovaries, uterus and vagina, characterized the stages of estrous cycle. In proestrus stage, an animal is coming into heat while in metestrus is a short period where conception is absent and diestrus is a period of short rest where reproductive tracts prepare for receipt of the ovum (Heape, 1900). The ovulation occurs from the start of proestrus stage to the end of estrus stage (Young et al., 1941; Schwarts. 1964). The mean cycle length in the female rat is 4 days, predicted from the onset of sexual maturity up to the age of 12 months (Long & Evans, 1922; Freeman, 1988).

The proestrus and estrus phases last for 12 hours respectively while metestrus lasts for 21 hours and diestrus lasts for 57 hours (Paccola et al., 2013). However, Long and Evans (1922) classified the estrous cycle in five phases. The phases are termed as proestrus, estrus, metestrus I, metestrus II and diestrus (Gronroos & Kaupilla, 1959). Lasted about 15- 18 hours, the stage of metestrus I was also known as early metestrus while metestrus II or known as late metestrus lasts for 6 hours (Hebel & Stromberg, 1986). Maeda et al. (2000) and Westwood (2008) recorded the cycle has also been divided into 5 phases as proestrus, estrus, diestrus I or metestrus, and diestrus II. In a 4- day cycle and 5- day cycle, diestrus lasts for 2 days (dies