Table 1.

Stages of ovarian aging in a female rat as described by Lu et al. (1979) [24].

Age (months)	Stages of ovarian aging in female rats
5 months	Reproductive maturity
	(i) Estrous cycle (4-5 days)
	(ii) Elevated and constant levels of estradiol
	(iii) Low levels of progesterone
	(iv) Lack of hormone surges and ovulation
6–8 months	Ovarian decline
	(i) Decreased levels of estradiol and progesterone
	(ii) Little or no follicles
10–12 months	Irregularity of estrous cycles
19 months	Constant estrous cycle
	(i) Low to medium levels of serum estradiol, estrone, testosterone, androstenedione, and progesterone
	(ii) Very low levels of 20α-hydroxyprogesterone
	(iii) No preovulatory release of gonadotropin and prolactin
	(iv) Increased levels of FSH
24 months	Prolactin
	(i) Increased levels of prolactin (abolished by ovariectomy)
	(ii) Retained ability to develop follicles and corpora
	(iii) Retained ability to secrete steroid hormones