Physiology of menopause

The postmenopausal ovary is small and essentially devoid of follicles. The appearance of the postmenopausal ovary, coupled with the observation that oophorectomy is associated with menopausal symptoms, led to the original theory that follicular depletion was responsible for menopause. More recent evidence suggests that menopause has origins in both the central nervous system and the ovary. In addition, men appear to experience a similar, albeit later and more subtle change, called andropause. Both changes can be referred to as “gonadopause” and associated mechanisms in the central nervous system and gonads seem to be quite extensive and to reflect the general aging process.

Fertility decreases dramatically in women beginning at about age 35 but accelerating after the age of 40. The accelerated fall after 40 may be the first sign of impending ovarian failure. Although ovarian follicles remain visible on ultrasound, attempts at artificial induction of ovulation with injected gonadotropins are largely unsuccessful after about age 45 years. This suggests that a physiologic defect develops within the oocytes or follicles prior to their depletion. About 3–4 years before menopause is apparent, serum follicle-stimulating hormone (FSH) levels begin to rise subtly and ovarian estrogen, anti-Müllerian hormone, inhibin and progesterone production falls. Menstrual cycle length tends to decrease as the follicular phase progressively shortens. Ultimately, ovulation and menstruation cease entirely. The age of onset of menopause has changed very little over time – even the Ancient Greeks mention the age of 50 as typical. Age of menopause is affected by multiple factors. Maternal menopausal age is predictive of a daughter’s menopausal age. Age of menarche does not affect age of menopause. Most agree that race and parity have no effect. Smokers enter menopause at an earlier age than nonsmokers.

Although ovarian failure is a major component of menopause, functional alterations also occur at the level of the pituitary. Changes arise in the intrinsic rhythms that control sleep and the neuroendocrine axes. Such changes in the circadian oscillator lead to diminished nocturnal melatonin secretion and altered sleep, decreased responsiveness of the gonadotropin axis to steroid feedback and decreased adrenal steroid production. Aging is also associated with a more general decline in central dopaminergic and noradrenergic neuronal function. Estrogen deficiency further exacerbates the dopamine deficiency by increasing the ratio of norepinephrine to dopamine.

During menopause, the decrease in ovarian estrogen and inhibin production reduces negative feedback signals to the pituitary and hypothalamus and results in a progressive rise in gonadotropin levels. Because inhibin acts exclusively to regulate FSH (Chapter 1), FSH levels rise disproportionately to luteinizing hormone (LH) levels. When in doubt, persistent elevation of serum FSH levels confirms the diagnosis of menopause. Although ovarian estrogen production essentially ceases, the ovary continues to make the androgens testosterone and androstenedione. Most of this steroid biosynthesis occurs in the hilar cells of the medulla of the gland and very little occurs in the stroma. Hilar cells share a common embryologic origin with testicular Leydig cells, the main androgen-secreting cells in the male (Chapter 5).

Although ovarian estrogen production ceases at menopause, postmenopausal women are not completely estrogen deficient. Peripheral tissues such as fat, liver and kidney express the enzyme aromatase and can convert circulating androgens to estrogens. The major difference between direct ovarian estrogen secretion and peripheral conversion is that most of the estrogen produced by the latter process is estrone. Estrone is the estrogen produced from aromatization of androstenedione, the major androgen secreted by the postmenopausal ovary and adrenal gland (Chapter 2). Estrone is a very weak estrogen compared with estradiol. In the typical concentrations found in postmenopausal women, estrone does not provide protection against the long-term consequences of estrogen deficiency. Obese postmenopausal women are somewhat protected from this. Fat is a particularly rich source of aromatase activity and obese postmenopausal women can produce substantial amounts of estrone. These high quantities of endogenous estrone provide some protection against the risk of menopausal vasomotor symptoms and osteoporosis but at a cost. Prolonged exposure of the endometrium to estrogen stimulation that is unopposed by postovulatory progesterone will increase the risk for the development of endometrial hyperplasia and carcinoma (Chapter 42). The endometrium is never converted from proliferative physiology to secretory morphology and this unregulated growth favors neoplastic change. A similar risk of endometrial stimulation is present in women receiving estrogen alone for postmenopausal hormone replacement. For this reason, women who still have their uterus but require or choose postmenopausal estrogen replacement should also be given progesterone in a continuous or cyclic fashion.