Female Reproductive Physiology

PITUITARY GLAND

The pituitary gland lies below the hypothalamus at the base of the brain within a bony cavity (sella turcica) and is separated from the cranial cavity by a condensation of dura mater overlying the sella turcica (diaphragma sellae). The pituitary gland is divided into two major portions (Figure 4-1). The neurohypophysis, which consists of the posterior lobe (pars nervosa), the neural stalk (infundibulum), and the median eminence, is derived from neural tissue and is in direct continuity with the hypothalamus and central nervous system. The adenohypophysis, which consists of the pars distalis (anterior lobe), pars intermedia (intermediate lobe), and pars tuberalis—which surrounds the neural stalk—is derived from ectoderm.

FIGURE 4-1 Hypophyseal-pituitary portal circulatory system.

The arterial blood supply to the median eminence and the neural stalk (pituitary portal system) represents a major avenue of transport for hypothalamic secretions to the anterior pituitary.

The neurohypophysis serves primarily to transport oxytocin and vasopressin (antidiuretic hormone) along neuronal projections from the supraoptic and paraventricular nuclei of the hypothalamus to their release into the circulation.

The anterior pituitary contains different cell types that produce six protein hormones: follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyroid-stimulating hormone (TSH), prolactin, growth hormone (GH), and adrenocorticotropic hormone (ACTH).

The gonadotropins, FSH and LH, are synthesized and stored in cells called gonadotrophs, whereas TSH is produced by thyrotrophs. FSH, LH, and TSH are glycoproteins, consisting of α and β subunits. The α subunits of FSH, LH, and TSH are identical. The same α subunit is also present in human chorionic gonadotropin (hCG). The β subunits are individual for each hormone. The half-life for circulating LH is about 30 minutes, whereas that of FSH is several hours. The difference in half-lives may account, at least in part, for the differential secretion patterns of these two gonadotropins.

Prolactin is secreted by lactotrophs. Unlike the case with other peptide hormones produced by the adenohypophysis, pituitary release of prolactin is under tonic inhibition by the hypothalamus. The half-life for circulating prolactin is about 20 to 30 minutes. In addition to its lactogenic effect, prolactin may directly or indirectly influence hypothalamic, pituitary, and ovarian functions in relation to the ovulatory cycle, particularly in the pathologic state of chronic hyperprolactinemia (see Chapter 32).

GONADOTROPIN SECRETORY PATTERNS

A normal ovulatory cycle can be divided into a follicular and a luteal phase (Figure 4-2). The follicular phase begins with the onset of menses and culminates in the preovulatory surge of LH. The luteal phase begins with the onset of the preovulatory LH surge and ends with the first day of menses.

FIGURE 4-2 Hormone levels during a normal menstrual cycle.

Decreasing levels of estradiol and progesterone from the regressing corpus luteum of the preceding cycle initiate an increase in FSH by a negative feedback mechanism, which stimulates follicular growth and estradiol secretion. A major characteristic of follicular growth and estradiol secretion is explained by the two-gonadotropin (LH and FSH), two-cell (theca cell and granulosa cell) theory of ovarian follicular development. According to this theory, there are separate cellular functions in the ovarian follicle wherein LH stimulates the theca cells to produce androgens (androstenedione and testosterone) and FSH then stimulates the granulosa cells to convert these androgens into estrogens (androstenedione to estrone and testosterone to estradiol), as depicted in Figure 4-3. Initially, at lower levels of estradiol, there is a negative feedback effect on the ready-release form of LH from the pool of gonadotropins in the pituitary gonadotrophs. As estradiol levels rise later in the follicular phase, there is a positive feedback on the release of storage gonadotropins, resulting in the LH surge and ovulation. The latter occurs 36 to 44 hours after the onset of this midcycle LH surge. With pharmacologic doses of progestins contained in contraceptive pills, there is a profound negative feedback effect on gonadotropin-releasing hormone (GnRH) so that none of the gonadotropin pool (ready-release or storage) is released. Hence, ovulation is (generally) blocked (see Chapter 26).

FIGURE 4-3 The two-gonadotropin (LH and FSH), two-cell (theca cell on top and granulosa cell below) theory of follicular development. Each cell is theorized to perform separate functions; LH stimulates the production of androgens (androstenedione and testosterone) in the theca cell, and FSH stimulates the aromatization of these androgens to estrogens, estrone, and estradiol in the granulosa cell.

During the luteal phase, both LH and FSH are significantly suppressed through the negative feedback effect of elevated circulating estradiol and progesterone. This inhibition persists until progesterone and estradiol levels decline near the end of the luteal phase as a result of corpus luteal regression, should pregnancy fail to occur. The net effect is a slight rise in serum FSH, which initiates new follicular growth for the next cycle. The duration of the corpus luteum’s functional regression is such that menstruation generally occurs 14 days after the LH surge in the absence of pregnancy.

HYPOTHALAMUS

Five different small peptides or biogenic amines that affect the reproductive cycle have been isolated from the hypothalamus. All exert specific effects on the hormonal secretion of the anterior pituitary gland. They are GnRH, thyrotropin-releasing hormone (TRH), somatotropin release-inhibiting factor (SRIF) or somatostatin, corticotropin-releasing factor (CRF), and prolactin release-inhibiting factor (PIF). Only GnRH and PIF are discussed in this chapter.

GnRH is a decapeptide that is synthesized primarily in the arcuate nucleus. It is responsible for the synthesis and release of both LH and FSH. Because it usually causes the release of more LH than FSH, it is less commonly called LH-releasing hormone (LH-RH) or LH-releasing factor (LRF). Both FSH and LH appear to be present in two different forms within the pituitary gonadotrophs. One is a releasable form and the other a storage form. GnRH reaches the anterior pituitary through the hypophyseal portal vessels and stimulates the synthesis of both FSH and LH, which are stored within gonadotrophs. Subsequently, GnRH activates and transforms these molecules into releasable forms. GnRH can also induce immediate release of both LH and FSH into the circulation. Some recent research that found receptors for GnRH in other tissues including the ovary suggests that GnRH may have a direct effect on ovarian function as well.

GnRH is secreted in a pulsatile fashion throughout the menstrual cycle as depicted in Figure 4-4. The frequency of GnRH release, as assessed indirectly by measurement of LH pulses, varies from about every 90 minutes in the early follicular phase to every 60 to 70 minutes in the immediate preovulatory period. During the luteal phase, pulse frequency decreases while pulse amplitude increases. A considerable variation among individuals has been identified.

FIGURE 4-4 The pulsatile release of GnRH during the normal menstrual cycle.

Intravenous and subcutaneous administration of exogenous pulsatile GnRH has been used to induce ovulation in selected women who are not ovulating as a result of hypothalamic dysfunction. A continuous (nonpulsatile) infusion of GnRH results in a reversible inhibition of gonadotropin secretion through a process of “downregulation” or desensitization of pituitary gonadotrophs. This represents the basic mechanism of action for the GnRH agonists (nonapeptides, containing only nine amino acids) that have been successfully used in the therapy of such ovarian hormone–dependent disorders as endometriosis, leiomyomas, hirsutism, and precocious puberty.

Several mechanisms control the secretion of GnRH. Estradiol appears to enhance hypothalamic release of GnRH and may help induce the midcycle LH surge by increasing GnRH release or by enhancing pituitary responsiveness to the decapeptide. Gonadotropins have an inhibitory effect on GnRH release. Catecholamines may play a major regulatory role as well. Dopamine is synthesized in the arcuate and periventricular nuclei and may have a direct inhibitory effect on GnRH secretion through the tuberoinfundibular tract that projects onto the median eminence. Serotonin also appears to inhibit GnRH pulsatile release, whereas norepinephrine stimulates it. Endogenous opioids suppress release of GnRH from the hypothalamus in a manner that may be partially regulated by ovarian steroids.

The hypothalamus produces PIF, which exerts chronic inhibition of prolactin release from the lactotrophs. A number of pharmacologic agents (e.g., chlorpromazine) that affect dopaminergic mechanisms influence prolactin release. Dopamine itself is secreted by hypothalamic neurons into the hypophyseal portal vessels and inhibits prolactin release directly within the adenohypophysis. Based on these observations, it has been proposed that hypothalamic dopamine may be the major PIF. In addition to the regulation of prolactin release by PIF, the hypothalamus may also produce prolactin-releasing factors (PRFs) that can elicit large and rapid increases in prolactin release under certain conditions, such as breast stimulation during nursing. All PIFs and PRFs have not been clearly characterized biochemically as of 2008. TRH serves to stimulate prolactin release as well. This phenomenon may explain the association between primary hypothyroidism (with secondary TRH elevation) and hyperprolactinemia. The precursor protein for GnRH, called GnRH-associated peptide (GAP), has been identified to be both a potent inhibitor of prolactin secretion and an enhancer of gonadotropin release. These findings suggest that this GnRH-associated peptide may also be a physiologic PIF and could explain the inverse relationship between gonadotropin and prolactin secretions seen in many reproductive states.

ESTROGENS

During early follicular development, circulating estradiol levels are relatively low. About 1 week before ovulation, levels begin to increase, at first slowly, then rapidly. The conversion of testosterone to estradiol in the granulosa cell of the follicle occurs through an enzymatic process called aromatization and is depicted in Figure 4-3. The levels generally reach a maximum 1 day before the midcycle LH peak. After this peak and before ovulation, there is a marked and precipitous fall. During the luteal phase, estradiol rises to a maximum 5 to 7 days after ovulation and returns to baseline shortly before menstruation. Estrone secretion by the ovary is considerably less than secretion of estradiol but follows a similar pattern. Estrone is largely derived from the conversion of androstenedione through the action of the enzyme aromatase (Figure 4-5).

FIGURE 4-5 Steroidogenic pathways showing aromatization in red. Cmpd B, corticosterone; cmpd S, II-deoxycortisol; DOC, desoxycorticosterone; OH, hydroxylase.

PROGESTINS

During follicular development, the ovary secretes only very small amounts of progesterone and 17α-hydroxyprogesterone. The bulk of the progesterone comes from the peripheral conversion of adrenal pregnenolone and pregnenolone sulfate. Just before ovulation, the unruptured but luteinizing graafian follicle begins to produce increasing amounts of progesterone. At about this time, a marked increase also occurs in serum 17α-hydroxyprogesterone. The elevation of basal body temperature is temporally related to the central effect of progesterone. As with estradiol, secretion of progestins by the corpus luteum reaches a maximum 5 to 7 days after ovulation and returns to baseline shortly before menstruation. Should pregnancy occur, progesterone levels and therefore basal body temperature remain elevated.

ANDROGENS

Both the ovary and the adrenal glands secrete small amounts of testosterone, but most of the testosterone is derived from the metabolism of androstenedione, which is also secreted by both the ovary and the adrenal gland. Near midcycle, an increase occurs in plasma androstenedione, which reflects enhanced secretion from the follicle. During the luteal phase, a second rise occurs in androstenedione, which reflects enhanced secretion by the corpus luteum. The adrenal gland also secretes androstenedione in a diurnal pattern similar to that of cortisol. The ovary secretes small amounts of the very potent dihydrotestosterone (DHT), but the bulk of DHT is derived from the conversion of androstenedione and testosterone. The majority of dehydroepiandrosterone (DHEA) and virtually all DHEA sulfate (DHEA-S), which are weak androgens, are secreted by the adrenal glands, although small amounts of DHEA are secreted by the ovary.

SERUM-BINDING PROTEINS

Circulating estrogens and androgens are mostly bound to specific sex hormone–binding globulins (SHBG) or to serum albumin. The remaining fraction of sex hormones is unbound (free), and this is the biologically active fraction. It is unclear whether steroids bound to serum proteins (e.g., albumin) are accessible for tissue uptake and utilization. The synthesis of SHBG in the liver is increased by estrogens and thyroid hormones but decreased by testosterone.

PROLACTIN

Serum prolactin levels do not change strikingly during the normal menstrual cycle. Both the serum level of prolactin and prolactin release in response to TRH are somewhat more elevated during the luteal phase than during the mid-follicular phase of the cycle. This suggests that high amounts of circulating estradiol and progesterone may enhance prolactin release. Prolactin release varies throughout the day, with the highest levels occurring during sleep.

Prolactin may participate in the control of ovarian steroidogenesis. Prolactin concentrations in follicular fluid change markedly during follicular growth. The highest prolactin concentrations are seen in small follicles during the early follicular phase. Prolactin concentrations in the follicular fluid may be inversely related to the production of progesterone. In addition, hyperprolactinemia may alter gonadotropin secretion. Despite these observations, the physiologic role of prolactin during the normal menstrual cycle has not been clearly established.

FOLLICULAR DEVELOPMENT

Primordial follicles undergo sequential development, differentiation, and maturation until a mature graafian follicle is produced. The follicle then ruptures, releasing the ovum. Subsequent luteinization of the ruptured follicle produces the corpus luteum.

At about 8 to 10 weeks of fetal development, oocytes become progressively surrounded by precursor granulosa cells, which then separate themselves from the underlying stroma by a basal lamina. This oocyte–granulosa cell complex is called a primordial follicle. In response to gonadotropin and ovarian steroids, the follicular cells become cuboidal, and the stromal cells around the follicle become prominent. This process, which takes place in utero (i.e., in the fetal ovary) at between 20 and 24 weeks’ gestation, results in a primary follicle. As granulosa cells proliferate, a clear gelatinous material surrounds the ovum, forming the zona pellucida. This larger unit is called a secondary follicle.

In the adult ovary, a graafian follicle forms as the innermost three or four layers of rapidly multiplying granulosa cells become cuboidal and adherent to the ovum (cumulus oophorus). In addition, a fluid-filled antrum forms among the granulosa cells. As the liquor continues to accumulate, the antrum enlarges, and the centrally located primary oocyte migrates eccentrically to the wall of the follicle. The innermost layer of granulosa cells of the cumulus, which are in close contact with the zona pellucida, become elongated and form the corona radiata. The corona radiata is released with the oocyte at ovulation. Covering the granulosa cells is a thin basement membrane, outside of which connective tissue cells organize themselves into two coats: the theca interna and theca externa.

During each cycle, a cohort of follicles is recruited for development. Among the many developing follicles, only one usually continues differentiation and maturation into a follicle that ovulates. The remaining follicles undergo atresia. On the basis of in vitro measurement of local steroid levels, growing follicles can be classified as either estrogen predominant or androgen predominant. Follicles greater than 10 mm in diameter are usually estrogen predominant, whereas smaller follicles are usually androgen predominant. Mature preovulatory follicles reach mean diameters of about 18 to 25 mm. Furthermore, in estrogen-predominant follicles, antral FSH concentrations continue to rise while serum FSH levels decline beginning at the mid-follicular phase. In smaller, androgen-predominant follicles, antral fluid FSH values decrease while serum FSH levels decline; thus, the intrafollicular steroid milieu appears to play an important role in determining whether a follicle undergoes maturation or atresia. Additional follicles may be “rescued” from atresia by administration of exogenous gonadotropins.

Follicular maturation is dependent on the local development of receptors for FSH and LH. FSH receptors are present on granulosa cells. Under FSH stimulation, granulosa cells proliferate, and the number of FSH receptors per follicle increases proportionately. Thus, the growing primary follicle is increasingly more sensitive to stimulation by FSH; as a result, estradiol levels increase. Estrogens, particularly estradiol, enhance the induction of FSH receptors and act synergistically with FSH to increase LH receptors.

During early stages of folliculogenesis, LH receptors are present only on the theca interna layer. LH stimulation induces steroidogenesis and increases the synthesis of androgens by thecal cells. In nondominant follicles, high local androgen levels may enhance follicular atresia. However, in the follicle destined to reach ovulation, FSH induces aromatase enzyme and its receptor formation within the granulosa cells. As a result, androgens produced in the theca interna of the dominant follicle diffuse into the granulosa cells and are aromatized into estrogens. FSH also enhances the induction of LH receptors on the granulosa cells of the follicle that is destined to ovulate. These are essential for the appropriate response to the LH surge, leading to the final stages of maturation, ovulation, and the luteal phase production of progesterone. Thus, the presence of greater numbers of FSH receptors and granulosa cells and increased induction of aromatase enzyme and its receptors may differentiate between the follicle of the initial cohort that will develop normally and those that will undergo atresia.

Growth factors such as insulin, insulin-like growth factor (IGF), fibroblast growth factor (FGF), and epidermal growth factor (EGF) may also play significant mitogenic roles in folliculogenesis, including enhanced responsiveness to FSH.

OVULATION

The preovulatory LH surge initiates a sequence of structural and biochemical changes that culminate in ovulation. Before ovulation, a general dissolution of the entire follicular wall occurs, particularly the portion that is on the surface of the ovary. Presumably this occurs as a result of the action of proteolytic enzymes. With degeneration of the cells on the surface, a stigma forms, and the follicular basement membrane finally bulges through the stigma. When this ruptures, the oocyte, together with the corona radiata and some cumulus oophora cells, is expelled into the peritoneal cavity, and ovulation takes place.

Ovulation is now known from ultrasonic studies to be a gradual phenomenon, with the collapse of the follicle taking from several minutes to as long as an hour or more. The oocyte adheres to the surface of the ovary, allowing an extended period during which the muscular contractions of the fallopian tube may bring it in contact with the tubal epithelium. Probably both muscular contractions and tubal ciliary movement contribute to the entry of the oocyte into, and the transportation along, the fallopian tube. Ciliary activity may not be essential because some women with immotile cilia also become pregnant.

At birth, primary oocytes are in the prophase of the first meiotic division. They continue in this phase until the next maturation division occurs in conjunction with the midcycle LH surge. A few hours preceding ovulation, the chromatin is resolved into distinct chromosomes, and meiotic division takes place with unequal distribution of the cytoplasm to form a secondary oocyte and the first polar body. Each element contains 23 chromosomes, each in the form of two monads. The second maturation spindle forms immediately, and the oocyte remains at the surface of the ovary. No further development takes place until after ovulation and fertilization have occurred. At that time, and before the union of the male and female pronuclei, another division occurs to reduce the chromosomal component of the egg pronucleus to 23 single chromosomes (22 plus X or Y), each composed of the one monad. The ovum and a second polar body are thus formed. The first polar body may also divide.

LUTEINIZATION AND CORPUS LUTEUM FUNCTION

After ovulation and under the influence of LH, the granulosa cells of the ruptured follicle undergo luteinization. These luteinized granulosa cells, plus the surrounding theca cells, capillaries, and connective tissue, form the corpus luteum, which produces copious amounts of progesterone and some estradiol. The normal functional life span of the corpus luteum is about 9 to 10 days. After this time it regresses, and unless pregnancy occurs, menstruation ensues, and the corpus luteum is gradually replaced by an avascular scar called a corpus albicans. The events occurring in the ovary during a complete cycle are shown in Figure 4-6.

FIGURE 4-6 Schematic representation of the sequence of events occurring in the ovary during a complete follicular cycle.

(Adapted from Yen SC, Jaffe R [eds]: Reproductive Endocrinology. Philadelphia, WB Saunders, 1978.)

MENSTRUAL PHASE

Because it is the only portion of the cycle that is visible externally, the first day of menstruation is taken as day 1 of the menstrual cycle. The first 4 to 5 days of the cycle are defined as the menstrual phase. During this phase, there is disruption and disintegration of the endometrial glands and stroma, leukocyte infiltration, and red blood cell extravasation. In addition to this sloughing of the functionalis, there is a compression of the basalis due to the loss of ground substances. Despite these degenerative changes, early evidence of renewed tissue growth is usually present at this time within the basalis of the endometrium.

PROLIFERATIVE PHASE

The proliferative phase is characterized by endometrial proliferation or growth secondary to estrogenic stimulation. Because the bases of the endometrial glands lie deep within the basalis, these epithelial cells are not destroyed during menstruation.

During this phase of the cycle, the large increase in estrogen secretion causes marked cellular proliferation of the epithelial lining, the endometrial glands, and the connective tissue of the stroma (Figure 4-7). Numerous mitoses are present in these tissues, and there is an increase in the length of the spiral arteries, which traverse almost the entire thickness of the endometrium. By the end of the proliferative phase, cellular proliferation and endometrial growth have reached a maximum, the spiral arteries are elongated and convoluted, and the endometrial glands are straight, with narrow lumens containing some glycogen.

FIGURE 4-7 Early proliferative phase endometrium. Note the regular, tubular glands lined by pseudostratified columnar cells.

SECRETORY PHASE

Following ovulation, progesterone secretion by the corpus luteum stimulates the glandular cells to secrete glycogen, mucus, and other substances. The glands become tortuous and the lumens are dilated and filled with these substances. The stroma becomes edematous. Mitoses are rare. The spiral arteries continue to extend into the superficial layer of the endometrium and become convoluted (Figure 4-8).

FIGURE 4-8 Late secretory phase endometrium. Note the tortuous, saw-toothed appearance of the endometrial glands with secretions in the lumens. The stroma is edematous and necrotic during this stage, leading to sloughing of the endometrium at the time of menstruation.

The marked changes that occur in endometrial histology during the secretory phase permit relatively precise timing (dating) of secretory endometrium.

If pregnancy does not occur by day 23, the corpusluteum begins to regress, secretion of progesterone and estradiol declines, and the endometrium undergoes involution. About 1 day before the onset of menstruation, marked constriction of the spiral arterioles takes place, causing ischemia of the endometrium followed by leukocyte infiltration and red blood cell extravasation. It is thought that these events occur secondary to prostaglandin production by the endometrium. The resulting necrosis causes menstruation or sloughing of the endometrium. Thus, menstruation, which clinically marks the beginning of the menstrual cycle, is actually the terminal event of a physiologic process that enables the uterus to be prepared to receive another conceptus.