Estrous and menstrual cycles

Reproduction in female mammals is cyclic in nature occurring either as estrous cycles or menstrual cycles. These cycles depend on a tightly regulated interplay between the hypothalamus-pituitary-ovarian axis and involve both negative and positive feedback regulation of the GnRH pulse generator by the ovarian steroid hormones LH and FSH. Females differ from males in that estradiol can have both negative and positive feedback effects on the hypothalamus and pituitary. The positive feedback of estradiol is manifested in the hypothalamus at the kisspeptin neurons, which respond to rapidly rising estradiol levels by stimulating increased pulsatility of GnRH. This increased GnRH pulse pattern leads to stimulation of LH production by pituitary gonadotropes and results in the ovulatory surge. The corpus luteum formed from the ovulated follicle secretes progesterone which exerts negative feedback on the GnRH pulse generator. The menstrual cycle differs from the estrous cycle in that the follicular phase is significantly longer and there is shedding of the uterine endometrium in response to declining progesterone levels resulting in menses (menstruation). The endometrium undergoes characteristic changes in morphology during the menstrual cycle which allows accurate dating of the endometrium. Mammalian species may be polyestrous, or seasonally polyestrous or monoestrous.