The estrous cycle consists of the reproductive cycle of females. It lasts four to five days
and is divided into four phases: proestrus, estrous, metaestrus, and diestrus [103]. It can be
identified by cytological analysis according to the proportion and cell types in the
vaginal secretion [104].
Each phase of the cycle lasts around 24 hours and is mainly linked to fluctuations in
estradiol levels, which begins at puberty and ends in senescence (about 12 months in
female rats). The metaestrus and diestrus phases have low level concentrations of serum
estradiol. The proestrus is characterized by the highest level of estradiol followed by the
estrous phase, where levels of the hormone start to decrease, coinciding with ovulation
and corpus luteum formation [105].
Box 1: Estrous cycle.