Physiology of the Menstrual Cycle
In prepubertal females, the ovaries are small and composed of stroma and primordial germ cells; the normal female ovary at birth has the full complement of germ cells required for the entire reproductive life of the individual. The uterus too is small before puberty and is lined by inactive thin endometrium.
The onset of menstruation (menarche) signifies onset of puberty in the female. The age at which menarche occurs usually varies between 10 and 14 years of age and is dictated by pituitary secretion of follicle-stimulating hormone (FSH), which in turn is controlled by the hypothalamus via gonadotropin-releasing hormones (Figure 52-1). Hypothalamic initiation of secretion of releasing hormones is influenced by a variety of higher psychosocial stimuli. After its initial stimulation by the hypothalamus, FSH secretion is controlled primarily by feedback from ovarian hormones, and cyclic menstruation begins (Figure 52-1).
Changes in the ovary, endometrium, and blood hormone levels during the menstrual cycle.
Repeated cycles of menstruation, sometimes interrupted by pregnancy, continue to the menopause (cessation of menstruation), which marks the end of reproductive life in the female. Menopause appears to be caused by primary failure of the ovary, which fails to respond to FSH stimulation. It is characterized by (1) failure of ovulation, (2) marked decrease in estrogen and absence of progesterone secretion by the ovary, (3) atrophy of the endometrium, and (4) marked increase of pituitary FSH secretion (due to decreased negative feedback by estrogen). Elevated FSH is responsible for many of the symptoms of menopause such as hot flushes.
The ovaries are paired ovoid structures weighing 5–8 g each situated in the retrouterine space in relation to the lateral part of the uterine tube on each side. Each ovary is covered by germinal epithelium except where it is attached to the broad ligament of the uterus, at which point the germinal epithelium covering the ovary is continuous with the peritoneum (both are derived from the embryonic coelom).
The bulk of the ovary consists of dense mesenchymal ovarian stromal cells plus germinal follicles and, after puberty, corpora lutea at various stages of maturation (Figure 52-2). The exact appearance of the ovary depends on the age of the patient and the phase of the menstrual cycle. If the patient is closer to menarche, there are more primordial follicles; if she is closer to menopause, there are more regressed, hyalinized corpora lutea (corpora albicantia). Scattered embryonic epithelial remnants and hilar cells are often present; hilar cells possess abundant lipid-filled cytoplasm and are believed to be analogous to the testicular interstitial cells of Leydig.