Chapter 9. Female Reproductive System

• Describe oogenesis, its relationship to follicular maturation, and the roles of pituitary and ovarian factors in their regulation.
• Describe gonadotropin control of ovarian function.
• List the target organs and principal physiologic actions of estrogen and progesterone and how they interact with each other.
• Describe the cellular mechanisms of action for estrogen and progesterone.
• Describe the endometrial (proliferative and secretory phases) and ovarian events that occur throughout the menstrual cycle and correlate them with the changes in blood levels of pituitary and ovarian hormones.
• Identify the pathways of sperm and egg transport required for fertilization and for movement of the embryo to the uterus.
• Describe the principal endocrine functions of the placenta, particularly in rescue of the corpus luteum and maintenance of pregnancy, and the fetal adrenal-placental interactions involved in estrogen production.
• Understand the roles of oxytocin, relaxin, and prostaglandins in the initiation and maintenance of parturition.
• Explain the hormonal regulation of mammary gland development during puberty, pregnancy, and lactation, and explain the mechanisms that control milk production and secretion.
• Explain the physiologic basis for the effects of steroid hormone contraceptive methods.
• Describe the age-related changes in the female reproductive system, including the mechanisms responsible for these changes, throughout life from fetal development to senescence.

The principal functions of the female reproductive system are to produce the ova for sperm fertilization and to provide the appropriate conditions for embryo implantation, fetal growth and development, and birth. Endocrine regulation of the reproductive system is directed by the hypothalamic-pituitary-ovarian axis. Ovarian-derived hormones regulate the hypothalamic-pituitary-gonadal axis in a classical negative feedback pattern. Throughout the ovarian cycle, a selected follicle is stimulated to undergo growth and development, culminating in ovulation. The remnants of the follicle undergo reorganization into the corpus luteum, a temporary endocrine organ that plays a central role in preparation and maintenance of the initial stages of pregnancy. Parallel changes occur in endometrial morphology and function throughout the ovarian cycle in preparation for implantation of a fertilized ovum. Ovarian and placental hormones maintain pregnancy and prepare the breast for lactation. This chapter describes the basic principles of the neuroendocrine regulation of this hypothalamic-pituitary-ovarian axis.

The female reproductive organs include the ovaries, the uterus and fallopian tubes, and the breasts or mammary glands (Figure 9–1). Their growth, development, and function are under hormonal regulation. The ovaries store and release the ovum and produce the 2 principal female sex hormones, estrogen and progesterone. Functionally, the ovaries consist of an outer cortex layer containing different-size follicles and an inner medulla consisting of vascular connective tissue and hilar cells. The primordial follicle contains a primary oocyte surrounded by epithelial (pregranulosa) cells separated from the ovarian stroma by a basement membrane. During follicular development, the epithelial cells differentiate into granulosa cells, and a layer of cells from the ovarian stroma is transformed into theca cells. The larger, more mature follicles are filled with a transparent albuminous fluid and consist of an external fibrovascular coat, connected with the surrounding stroma of the ovary by a network of blood vessels; and an internal coat, consisting of several layers of nucleated cells (granulosa cells) anchored in the zona pellucida, a glycoprotein-rich eosinophilic material surrounding the oocyte. The zona pellucida forms the corona radiata, which close to the time of ovulation is separated from the granulosa cells and expelled with the oocyte. Formation of the follicles begins before birth, and their development and maturation continue uninterrupted from puberty until the end of a woman’s reproductive life, as discussed later.

The female genital tract is derived from the müllerian ducts (see Chapter 8; Figure 8–6) and consists of the uterus, fallopian tubes, and vagina (see Figure 9–1). The fallopian tubes extend from each of the superior angles of the uterus and consist of the isthmus, the ampulla, and the infundibulum, which opens into the abdominal cavity and is surrounded by ovarian fimbriae and attached to the ovary (see Figure 9–1). The epithelial lining of the fallopian tubes has secretory and ciliated cells that contribute to sperm movement, aiding in fertilization and facilitating the movement of the zygote (fertilized ovum) to the uterus for implantation and fetal development. These functions are also aided by the rhythmic contraction of the smooth muscle walls.

The uterus is composed of 3 layers: serous, muscular, and mucous. The muscular coat accounts for the bulk of the uterus and consists of bundles of smooth muscle fibers, organized in layers and intermixed with loose connective tissue, blood vessels, lymphatic vessels, and nerves. The mucous membrane, or endometrium, is lined by columnar ciliated secretory epithelium that undergoes cycles of proliferation, differentiation, and breakdown every 28 days in preparation for embryo implantation. The arterial blood supply to the female reproductive tract is provided by branches of the hypogastric and ovarian arteries from the abdominal aorta. The veins correspond with the arteries and end in the uterine plexuses. The nerves are derived from the hypogastric and ovarian plexuses and from the third and fourth sacral nerves.

The breast consists of glandular tissue organized in lobes connected by fibrous tissue, with fat deposits interspersed between the lobes (see Figure 9–1). The mammary lobes themselves are made of lobules connected by connective tissue, blood vessels, and ducts. The lobules consist of a cluster of rounded alveoli, which open into excretory lactiferous ducts and unite to form larger ducts made of longitudinal and transverse elastic fibers. These ducts converge toward the areola, beneath which they form ampullar dilatations, which serve as reservoirs for the milk. The arterial blood supply to the breast is derived from the thoracic branches of the axillary, the intercostal, and the internal mammary arteries. The veins drain into the axillary and internal mammary veins.

Follicular Phase

During the follicular phase, FSH stimulates follicular recruitment and growth and estrogen synthesis. Before the selection of the follicle for ovulation, granulosa cells are responsive only to FSH. As follicular maturation progresses, the coupling between FSH receptor stimulation and the activation of adenylate cyclase becomes more and more efficient, leading to a steady increase in cAMP production. The accumulation of FSH-induced cAMP results in upregulation of LH receptors, allowing LH to act as a surrogate for FSH in granulosa cells. Low gonadotropin levels (especially FSH) lead to granulosa cell death and follicular atresia.

Luteal Phase

LH is responsible for ovulation and corpus luteum formation and for progesterone and estrogen production by the corpus luteum. Activation of the LH receptors in theca cells stimulates androstenedione production, providing the substrate for the enzymatic conversion to 17β-estradiol that is mediated by the enzyme aromatase in granulosa cells.

Ovarian production of steroid hormones (progesterone, estrogen, and testosterone) and peptide hormones (inhibins) varies throughout the ovarian cycle. The production and secretion rates of the principal ovarian steroid hormones are summarized in Table 9–1.

Estrogen

Androgens

Female androgens are derived from the adrenal glands (dehydroepiandrosterone and androstenedione), from the ovaries (androstenedione and testosterone), and from peripheral conversion of androstenedione and dehydroepiandrosterone to testosterone. Ovarian androgen secretion parallels that of estrogen throughout the menstrual cycle, whereas adrenal androgen production does not fluctuate throughout the menstrual cycle. Most of the circulating testosterone in females is derived from the peripheral conversion of androstenedione by 17β-hydroxysteroid dehydrogenase. The conversion of testosterone to dihydrotestosterone in peripheral tissues is limited in females because of higher levels of sex hormone-binding globulin in females than in males, as well as by the peripheral conversion to estrogen by aromatase, protecting females from virilization by dihydrotestosterone.

Progesterone

The preovulatory LH surge results in luteinization of granulosa and theca cells, altering the steroidogenic pathway so that progesterone becomes the primary steroid hormone produced by each of these cell types after luteinization. The changes leading to the ability to produce progesterone include increased expression of the enzymes involved in the conversion of cholesterol to progesterone (cholesterol side-chain cleavage cytochrome P450 complex and 3β-hydroxysteroid dehydrogenase) and decreased expression of the enzymes that convert progesterone to estrogens (17α-hydroxylase cytochrome P450 and aromatase cytochrome P450).

Inhibins, Activins, and Follistatin

Activin production by the granulosa cells changes during folliculogenesis and its effects are probably limited to paracrine action on granulosa cells. Activin promotes granulosa cell proliferation, upregulates FSH receptor expression on granulosa cells, and modulates steroidogenesis in both granulosa and theca cells. In the pituitary-ovarian axis, activin is a physiologic antagonist to inhibin and specifically stimulates pituitary FSH synthesis and secretion.

Activin-binding protein, follistatin, is a product of granulosa cells. Its basal expression increases with differentiation of granulosa cells. Its function is to neutralize the effect of activin on steroid production. The physiologic endocrine role of follistatin is not completely understood, but its effects are most likely autocrine or paracrine on ovarian steroidogenesis.

The ovarian cycle is divided into a follicular and a luteal phase (Figure 9–4).

The follicular phase begins on day 1 of the cycle, the first day of menses, and corresponds to the growth and development of a dominant follicle. Throughout the reproductive life span of the woman (from puberty to menopause), a single mature oocyte is produced each month. Most of the human oocytes (germ cells) present during uterine development are destined to undergo apoptosis, or programmed cell death. Only follicles that are responsive to FSH stimulation (approximately 350) will enter the final stage of development and progress to ovulation. At mid cycle (day 14), the rising levels of estrogen stimulate a surge in LH release, which stimulates ovulation 24–36 hours later.

The luteal phase begins after ovulation, with the reorganization of the remnants of the ovulatory follicle and the formation of the corpus luteum (see Figure 9–4). The corpus luteum is a transient endocrine organ that produces progesterone, and to a lesser extent estradiol and inhibin A. The corpus luteum is under LH regulation. The luteal phase ends when there is no fertilization and placental-mediated survival of the corpus luteum discussed below.

Ovarian Regulation of Gonadotropin Release

Gonadotropin release is under negative and positive feedback regulation by estradiol, progesterone, and inhibins A and B. Progesterone and 17β-estradiol act both in the hypothalamus and the pituitary, and inhibins act at the level of the pituitary (see Figure 9–2). The contributions of these ovarian hormones vary according to the stage of the ovarian cycle.

Follicular Phase

During this phase, the dominant follicle produces high concentrations of 17β-estradiol and inhibin B. Although initially estradiol exerts negative feedback on FSH and LH release, as concentrations of estradiol increase, toward the end of the follicular phase, a switch from negative to positive feedback occurs. High estradiol levels in the hypothalamus and pituitary lead to low-amplitude, high-frequency pulses (every 90 minutes) of LH, resulting in a midcycle LH surge. The estradiol-mediated stimulation of the LH surge results from an increased responsiveness of gonadotropic cells to GnRH (following exposure to increasing estradiol levels), an increase in GnRH receptor number, and a GnRH surge, triggered by the effect on the hypothalamus of increasing estradiol concentrations (see Figures 9–2 and 9–4). Inhibin B levels rise during the follicular phase and decrease immediately before the LH peak, with a brief surge occurring 2 days after ovulation. Inhibin A levels increase in the late follicular phase to reach a peak concentration on the day of the LH and FSH surge. The concentration then falls briefly before rising to reach a maximum concentration during the midluteal phase.

Luteal Phase

The midcycle surge in LH levels induces ovulation, resumption of meiosis, and promotes the formation and survival of the corpus luteum during the luteal phase. During the luteal phase, high circulating concentrations of progesterone (produced by the corpus luteum) suppress the frequency and the amplitude of LH release, resulting in an overall decrease in LH by blocking the surges of GnRH, downregulating pituitary GnRH receptor expression, and decreasing gene expression of the α- and β-subunits of both LH and FSH. Thus, negative feedback regulation by progesterone during the luteal phase prevents a second LH surge. The marked suppression of GnRH and LH pulse frequency achieved by high progesterone levels during the luteal phase allows enrichment of gonadotroph FSH levels. Inhibin B levels remain low during the luteal phase. Inhibin A is secreted by the granulosa cells during the luteal phase, and its concentration falls during luteal regression synchronously with estradiol and progesterone, remaining low during the early follicular phase.

Unless the corpus luteum is stimulated by human chorionic gonadotropin (hCG), a placental hormone (described later), it regresses. Regression or lysis of the corpus luteum and the associated decrease in progesterone levels leads to an increase in FSH release toward the beginning of the next ovarian cycle. During the luteal-follicular transition, following the midcycle rise in FSH secretion, the inhibin B concentration rises and peaks 4 days after the peak FSH concentration is reached.

Oogenesis and Formation of the Dominant Follicle

Folliculogenesis, or formation of the dominant follicle, consists of 2 stages: the gonadotropin-independent (preantral) period and the gonadotropin-dependent (antral or graafian) period. Primordial follicular growth up to the antral stage (up to 0.2 mm) occurs during fetal life and infancy and is gonadotropin independent (see Figure 9–5). Primary follicles are formed when the flattened epithelial cells become cuboidal and undergo mitosis. The antral follicle growth phase is characterized by granulosa cell proliferation, expression of FSH and steroid hormone receptors, and association of the theca cells with the growing follicle and granulosa cells, leading to formation of the secondary follicles. Tertiary follicles are formed following further theca cell hypertrophy and development. Their antrum is filled with estrogen-rich fluid, and the theca interstitial cells start to express FSH and LH receptors. The mechanisms that trigger initiation of follicular growth are still not completely understood, but are thought to involve bidirectional communication between germ and somatic cells through gap junctions and paracrine factors, including cytokines and growth factors (insulin-like growth factor 1 [IGF-1], epidermal and fibroblast growth factors, and interleukin 1β). When follicles reach a size of 2–5 mm, approximately 50% enter the selection growth phase and are rescued from apoptosis. This final developmental phase of follicular growth begins approximately 85 days before ovulation in the luteal phase of the cycle preceding ovulation (see Figure 9–5).

AMH or müllerian-inhibiting substance discussed in Chapter 8 as it pertains to sex differentiation in the male embryo, is a peptide growth factor and member of the large transforming growth factor beta family of growth and differentiation factors. AMH is expressed in the granulosa cells of the recruited primordial follicles, and continues to be expressed in the growing follicles that have undergone recruitment from the primordial follicle pool but that have not been selected for dominance. This pattern of expression suggests an important role in the regulation of both the number of growing follicles and their selection for ovulation. Because the number of growing follicles is correlated to the size of the primordial follicle pool size, a marker such as AMH that reflects all follicles that have made the transition from the primordial follicle pool to the growing pool has been proposed to be a good indirect marker of ovarian reserve. FSH and inhibin B levels have also been proposed to serve as predictors of the ovarian reserve.

The average time between primary follicle development and ovulation is 10–14 days (see Figure 9–5).

During follicular recruitment, the oocyte enters a growth phase that leads to the completion of the first meiotic division. The resumption of meiosis is mediated by the midcycle surge in LH. LH acts on mature follicles to terminate the program of gene expression associated with folliculogenesis. The transcription of genes that control granulosa cell proliferation (ie, IGF-1, FSH and estrogen receptors, cyclin D2) and those that encode steroidogenic enzymes is rapidly turned off by LH-mediated increases in intracellular cAMP. In addition, LH induces genes involved in ovulation (ie, progesterone receptor, cyclooxygenase-2) and luteinization (ie, cell cycle inhibitors, steroidogenic enzymes, transcription factors, and protein kinases). At this stage, mRNA synthesis virtually ceases and does not resume until 1–3 days after the egg has been fertilized, when the final phases of meiosis are completed. One preovulatory follicle is selected every cycle, approximately 350 times during the female reproductive life span.

Ovulation

Formation of the Corpus Luteum

If fertilization occurs, the corpus luteum continues to grow and function for the first 2–3 months of pregnancy. Regression of the corpus luteum is prevented by placental production of hCG during the initial gestational period. hCG stimulates the granulosa-lutein cells to produce progesterone, 17-hydroxyprogesterone, estrogen, inhibin A, and relaxin, a polypeptide hormone from the insulin/IGF hormone family. Relaxin regulates the synthesis and release of metalloproteinases, mediators of tissue (uterus, mammary gland, fetal membranes, birth canal) growth and remodeling, in preparation for birth and lactation. After the first trimester of pregnancy, the corpus luteum slowly regresses as the placenta assumes the role of hormone biosynthesis for the maintenance of pregnancy.

Luteolysis

Luteolysis is the process of lysis or regression of the corpus luteum if fertilization does not occur within 1–2 days of ovulation. Luteolysis marks the end of the female reproductive cycle and involves an initial decline in progesterone secretion (functional luteolysis), followed by changes in the cellular structure leading to gradual corpus luteum involution (structural or morphologic luteolysis) to form a small scar of connective tissue known as the corpus albicans.

Proliferative Phase

The proliferative phase corresponds to the follicular phase of the ovary (see Figure 9–4). It is characterized by estrogen-induced endometrial epithelial cell proliferation and upregulation of estradiol and progesterone receptor expression. The preovulatory endometrial proliferation leads to relative hypertrophy of the uterine mucosa. This is the initial phase of endometrial maturation in preparation for implantation of the embryo.

Secretory Phase

This phase corresponds to the ovarian luteal phase and is characterized by progesterone-induced differentiation of the endometrial epithelial cells into secretory cells. During the secretory phase, there is a short, well-defined period of uterine receptivity for embryo implantation (referred to as the “implantation window”).

Menstrual Phase

The menstrual period is characterized by shedding of the endometrium, resulting from proteolysis and ischemia in its superficial layer. Proteolytic enzymes accumulate in membrane-bound lysosomes during the first half of the postovulatory period. The integrity of the lysosomal membrane is lost with the decline in estrogen and progesterone on day 25, resulting in lysis of the glandular and stromal cells and the vascular endothelium. Ischemia caused by vasoconstriction of endometrial vessels during the early part of the menstrual period results in rupture of the capillaries, leading to bleeding. In addition, a significant increase in prostaglandin F2α in the late secretory endometrium stimulates release of acid hydrolases from lysosomes and enhances myometrial contractions, aiding in the expulsion of degenerated endometrium.

Fertilization

Fertilization is the union of the 2 germ cells, the ovum and the sperm, restoring chromosome number and initiating the development of a new individual. The final steps of mammalian oogenesis (and of spermatogenesis) prepare eggs (and sperm) for fertilization. In preparation for ovulation, fully grown oocytes undergo “meiotic maturation,” preparing them to interact with sperm. A very low proportion (approximately 0.002%) of the sperm deposited into the vagina migrates up the female reproductive tract to the ampullary-isthmic junction of the fallopian tubes (see Figure 9–6), the site of fertilization. During this trajectory, sperm undergo activation or “capacitation,” a series of changes in the sperm plasma membrane that increase its affinity for the zona pellucida, enabling the sperm to bind to the ovum and to undergo the acrosome reaction. In the fallopian tubes, sperm bind to the zona pellucida, leading to fusion of the ovum and sperm plasma membranes to form a single “activated” cell, the zygote (see Figure 9–6). This simple process requires several events.

Sperm Acrosome Reaction and Penetration of the Ovum’s Zona Pellucida

Sperm binding to the zona pellucida primes the sperm cell to initiate the acrosome reaction. The acrosome reaction involves fusion of the acrosome with the sperm plasma membrane and exocytosis of its enzymatic contents (proteases and glycosidases) required for sperm penetration. During or after the acrosome reaction, the fertilizing sperm penetrates through the zona pellucida and fuses with the ovum plasma membrane. In this process, the sperm initially binds with the tip of its head to the ovum, which is followed by release in binding of the tip of the sperm head and an equatorial (parallel) binding, allowing incorporation of the acrosomal membrane into the ovum cytoplasm and fusion of the 2 cells.

Cortical and Zona Reaction

Fusion of the sperm and the ovum triggers the second meiotic division of the ovum, leading to the formation of the mature oocyte and the second polar body. In addition, this fusion triggers mechanisms that prevent fertilization of the ovum by multiple sperm, such as exocytosis of cortical granules (cortical reaction) from the oocyte, resulting in proteolysis of zona pellucida glycoproteins, as well as cross-linking of proteins forming the perivitelline barrier. The fusion of the sperm and ovum reconstitutes a diploid cell, called the zygote.

During migration of the zygote through the fallopian tubes toward the site of implantation in the uterine cavity, mitosis yields a morula and then a blastocyst. The outer cells of the blastocyst; the trophoblast cells, participate in the implantation process and form the fetal components of the placenta.

Implantation

The human embryo (blastocyst) enters the uterus 3 days before implantation. As mentioned earlier, the “window of implantation” corresponds to the short period of endometrial receptivity for the embryo, between days 20 and 24 of the menstrual cycle. Morphologically, this optimal period for implantation is characterized by the presence of columnar epithelium with microvilli and an increase in stromal cell proliferation. Implantation involves cytokine induction of adhesion molecule expression in endometrial cells, followed by invasion requiring enzymatic digestion of the extracellular matrix with simultaneous control of hemostasis and angiogenesis within decidual tissues. These enzymatic processes are mediated by proteinases, metalloproteinases, collagenases, gelatinases, stromelysins, metallolastases, and urokinase. Both the growth of the trophoblast and invasion are required for successful embryo implantation and placental development.

Estrogen

Estrogen Synthesis, Transport, and Metabolism

Most of the estradiol released into the blood circulates bound to sex hormone-binding globulin and to albumin, with only 2%–3% circulating in the free form. Estradiol (as well as androstenedione) is converted to estrone (a weak estrogen) in peripheral tissues (Figure 9–7). Estrone is converted to estriol, primarily in the liver. Estrogens are metabolized by sulfation or glucuronidation, and the conjugates are excreted into the urine. Estrogen can also be metabolized through hydroxylation and subsequent methylation to form catechol- and methoxy-estrogens.

Estrogen Receptor-Mediated (Genomic) Effects

Estrogen receptors are mostly nuclear, but are also found in the cytoplasm. The process of hormone-receptor binding and translocating to the nucleus is similar to that used by adrenal steroid hormones testosterone.

Physiologic Actions of Estrogen at Target Organs

Reproductive System

Estrogen exerts multiple effects in reproductive organs (Figure 9–8).

Uterus

Estrogen promotes endometrial proliferation, sensitizes uterine smooth muscle to the effects of oxytocin by increasing the expression of oxytocin receptors and contractile proteins, and increases watery cervical mucus production.

Ovary

Estrogen exerts potent mitotic effects on granulosa cells.

Breast

Estrogen stimulates growth and differentiation of the ductal epithelium, induces mitotic activity of ductal cylindric cells, and stimulates the growth of connective tissue. The density of estrogen receptors in breast tissue is highest in the follicular phase of the menstrual cycle and falls after ovulation. Estrogen can also indirectly affect mammary gland development by elevating prolactin and progesterone levels and inducing progesterone receptors in mammary epithelium. The growth-promoting effects of estrogen have been implicated in breast and endometrial cancer.

Other Body Systems

Liver—Estrogen affects the expression of apoprotein genes and increases lipoprotein receptor expression, resulting in a decrease in serum concentrations of total cholesterol and low-density lipoprotein (LDL) cholesterol, increases in serum high-density lipoprotein (HDL) cholesterol and triglyceride concentrations, and decreases in serum lipoprotein A concentrations. Estrogen regulates the hepatic expression of genes involved in coagulation and fibrinolysis. Estrogen decreases plasma concentrations of fibrinogen, antithrombin III and protein S, and plasminogen activator inhibitor type 1. Elevated plasma estrogen levels are associated with an overall increase in the potential for fibrinolysis. Estrogen stimulates the synthesis of hormone-binding proteins (thyroxine- and cortisol-binding globulins).

Central Nervous System

Estrogen has neuroprotective actions, and its age-associated decline is associated with a decline in cognitive function (Figure 9–9).

Bone

Overall, the effects of estrogen are antiresorptive. Estrogen promotes bone maturation and closure of epiphysial plates in long bones. It conserves bone mass by suppressing bone turnover and maintaining balanced rates of bone formation and bone resorption. Estrogen affects the generation, life span, and functional activity of both osteoclasts and osteoblasts. It promotes synthesis of osteoprotegerin, decreases osteoclast formation and activity, and increases osteoclast apoptosis.

Nongenomic Effects of Estrogen

Some rapid effects of estrogen cannot be explained by a transcriptional mechanism (nongenomic) and are the result of direct estrogenic action on cell membranes mediated by cell membrane forms of estrogen receptor (see Figure 9–7). Although these receptors remain largely uncharacterized, they are thought to resemble their intracellular counterparts. Examples of some of the rapid nongenomic effects of estrogen include direct effects on the vasculature and activation of growth factor–related signaling pathways. Estrogen can cause short-term vasodilatation by both endothelium-dependent and endothelium-independent pathways. Estrogen produces short-term vasodilatation and reduces vascular smooth muscle tone through the increased formation and release of endothelial nitric oxide and prostacyclin and the opening of calcium channels mediated by cyclic guanosine monophosphate. Estrogen inhibits apoptosis of endothelial cells and promotes their angiogenic activity in vitro and may protect against atherosclerosis. At pharmacologic concentrations, estrogen inhibits the influx of extracellular calcium into vascular smooth muscle cells by an effect on L-type calcium channels.

Progesterone

Progesterone is the predominant ovarian hormone produced in the luteal phase. It is produced by both theca and granulosa cells of the corpus luteum in response to LH stimulation. The majority of progesterone secreted circulates bound to albumin. The principal targets of progesterone are the reproductive tract and the hypothalamic-pituitary axis. The degradation of progesterone is similar to that of androgens and estrogens and occurs primarily in the liver.

Progesterone Receptor-Mediated Effects

Progesterone exerts most of its effects by directly regulating gene transcription through 2 specific progesterone receptor proteins, termed A and B, located in target tissues such as the breast, uterus, brain, central nervous, and cardiovascular systems. These progesterone receptor proteins arise from a single gene and act as ligand-inducible transcription factors, regulating the expression of genes by binding specific progesterone responsive elements on the DNA (as described for estrogen and adrenal steroid hormones). The expression of progesterone receptors is upregulated by estrogen and downregulated by progesterone in most target tissues. Thus, progesterone receptor expression in the uterus is increased during the first half of the menstrual cycle and decreased during the luteal phase.

In general, progesterone acts on the reproductive tract to prepare it for initiation and maintenance of pregnancy. The major physiologic roles of progesterone are mediated in the uterus and ovary, where it stimulates the release of mature oocytes, facilitates implantation, and maintains pregnancy through stimulation of uterine growth and differentiation and suppression of myometrial contractility. In the brain, progesterone modulates sexual behavior and regulates body temperature. Increased progesterone levels during the luteal phase increase both core and skin temperatures. This results in a biphasic pattern of body core temperature throughout the menstrual cycle, with a higher temperature in the luteal phase of the cycle.

Physiologic Actions of Progesterone at Target Organs

Effects on the Uterus during Early Pregnancy

Progesterone induces stromal differentiation; stimulates glandular secretions, and modulates cyclic proliferation during the menstrual cycle.

Progesterone induces uterine cell proliferation and differentiation in early pregnancy producing an environment that supports early embryonic development.

Promotion and Maintenance of Implantation

Progesterone plays a major role in preparing the endometrium for implantation of a fertilized ovum, stimulates the synthesis of enzymes responsible for lysis of the zona pellucida.

Effects on Uterine Contractility

Progesterone induces quiescence of the myometrium (Figure 9–10). Several mechanisms are involved including: increasing resting membrane potential and preventing electrical coupling between myometrial cells; decreasing extracellular calcium influx required for contraction by downregulating gene expression of subunits of voltage-dependent calcium channels; blocking the ability of estradiol to induce membrane expression of α-adrenergic receptors (α-adrenergic activation causes contractions); decreasing prostaglandin synthesis; increasing prostaglandin inactivation; and opposing the stimulatory effects of estrogen on endometrial prostaglandin F2α expression. Progesterone maintains the levels of relaxin, inhibiting spontaneous or prostaglandin-induced myometrial contraction, and contributes to the maintenance of implantation and early pregnancy by increasing the collagen framework and distensibility of the uterus.

At the end of pregnancy, the decrease in progesterone levels is associated with increased prostaglandin synthase activity and prostaglandin F2α production, enhancing uterine contractility. The antiprogestin mifepristone antagonizes all of the actions of progesterone on prostaglandin synthesis and catabolism and stimulates prostaglandin production, thereby producing its abortive effects.

Effects on Lactation

In the mammary gland, progesterone stimulates lobular-alveolar development in preparation for milk secretion. Progesterone antagonizes prolactin’s effects in mid- to late pregnancy, preventing milk protein synthesis before parturition. The sudden fall in circulating progesterone that occurs with parturition is associated with a concurrent increase in prolactin secretion and the onset of lactation.

Antiestrogen Actions

Progesterone antagonizes estrogen induction of many of the known hormone-responsive genes. This effect is mediated by downregulation of cytoplasmic and nuclear estrogen receptor protein concentrations, decreasing the active estrogen concentration (and antagonizing the action of estrogen receptor at the molecular level), particularly in the uterus.

The Placenta

Structure and Physiologic Function

The placenta is derived from 2 major cell types, which are the source of the principal placental hormones. The outer cell mass of the blastocyst, the precursor to the trophoblast, is in contact with the endometrium and undergoes proliferation and tissue penetration during implantation. The trophoblast has 2 cell populations: an inner cytotrophoblast and an outer invasive syncytiotrophoblast. The maternal side of the placenta contains fetal chorionic villi that provide an extensive surface area for nutrient and gas exchange between the fetal and maternal circulation. The villi are covered with multinucleated syncytiotrophoblast and trophoblast stem cells, stromal cells, and blood vessels. The villous cytotrophoblast cells are entirely secluded from maternal elements, with the exception of any molecules that might be transported across the placenta by the syncytiotrophoblast. By contrast, the extravillous trophoblast cells are continuously exposed to maternal tissues. The middle layer of the placenta consists of densely packed cytotrophoblast cell columns and serves as structural support for the underlying villi.

The physiologic functions of the placenta can be classified as follows:

• Supportive, allowing embryo implantation into the uterus and transporting nutrients and oxygen necessary for fetal growth
• Immune, suppressing the local immune system to prevent immunologic rejection of the fetus by the mother
• Endocrine, including hormone synthesis, transport, and metabolism to promote fetal growth and survival

Inability of the placental unit to perform these functions leads to multiple complications of human pregnancy, including abortion, miscarriage, impaired fetal growth, and preeclampsia.

Endocrine Function of the Placenta

The placenta produces cytokines, hormones, and growth factors that are essential for regulation of the feto-maternal unit. In addition, the placenta expresses enzymes involved in hormone metabolism, playing an important role in protection of the fetus from maternal adrenal-derived androgens through aromatase activity and from glucocorticoids through the activity of 11β-hydroxysteroid dehydrogenase type II (see Chapter 6). The principal placental hormones are as follows.

Human Chorionic Gonadotropin (HCG)

hCG is a heterodimeric glycoprotein from the same hormone family as LH, FSH, and TSH. It is produced by the syncytiotrophoblast and released into the fetal and maternal circulation. It is known as the hormone of pregnancy and is the basis for the pregnancy test. hCG is detected in serum at day 6–8 after implantation, and its levels peak at 60–90 days of gestation, declining thereafter. hCG has structural and functional similarity to LH, and exerts its physiologic effects through binding to the LH receptors. The main function of hCG is to maintain the corpus luteum to ensure the production of progesterone until placental production takes over. In addition, hCG plays an important role in fetal development through the regulation of testosterone production by the fetal Leydig cells (see Chapter 8). Regulation of hCG release from the placenta is not completely understood, but evidence indicates that its paracrine regulation involves placenta-derived GnRH, activin, and inhibin. Maternal hCG levels are a useful index of functional status of the trophoblast (placental health).

Human Placental Lactogen and Growth Hormone

The human growth hormone (hGH) and human placental lactogen (hPL) gene family is important in the regulation of maternal and fetal metabolism and the growth and development of the fetus. hPL is produced by the syncytiotrophoblast and is secreted into both the maternal and fetal circulations after the sixth week of pregnancy. In the fetus, hPL modulates embryonic development; regulates intermediary metabolism; and stimulates the production of IGFs, insulin, adrenocortical hormones, and pulmonary surfactant. During pregnancy, hGH-V, a growth hormone (GH) variant expressed by the placenta, becomes the predominant GH in the mother. This hormone has structural and functional similarity to pituitary GH (it differs by 13 amino acids) and is not released into the fetus. Starting from the 15th to the 20th week of gestation up to term of pregnancy, placental GH gradually replaces maternal pituitary GH, which becomes undetectable. hGH-V stimulates IGF-1 production and modulates maternal intermediary metabolism, increasing the availability of glucose and amino acids to the fetus. Placental GH is not detectable in the fetal circulation, and thus it does not appear to have a direct effect on fetal growth. However, its physiologic role is thought to involve modulation of placental development through an autocrine or paracrine mechanism because of GH receptor expression by the placenta.

Progesterone

The major source of progesterone during the initial phase of pregnancy is the corpus luteum, which is under hCG regulation. Starting from about week 8 of gestation, the placenta (syncytiotrophoblast) becomes the principal source of progesterone, leading to increasing levels of maternal progesterone from 25 ng/mL during the luteal phase to 150 ng/mL during the last trimester of pregnancy. Because the placenta is unable to produce cholesterol from acetate, cholesterol for placental progesterone synthesis is derived from circulating LDL. LDL binds to the LDL receptor in trophoblast cells and undergoes endocytosis; cholesterol is released and processed through the steroidogenic hormone pathway (see Chapter 6). As discussed earlier, progesterone plays an important role in maintaining uterine quiescence during pregnancy, inhibiting prostaglandin synthesis and modulating the immune response to preserve pregnancy.

Estrogen

The main source of estrogen during the initial phase of pregnancy is the corpus luteum, being replaced later by placental production. The principal estrogen produced by the syncytiotrophoblast cells of the placenta is estriol. The production of estrogen by the placenta requires coordinated interaction between fetal and maternal adrenal gland steroid hormone production (feto-placental unit of steroid biosynthesis). The placenta lacks 17α-hydroxylase and 17, 20-desmolase and is thus unable to convert progesterone to estrogen or to produce androgens. This lack of placental androgen production protects the female fetus from masculinization; protection is also aided by the strong aromatase activity that inactivates maternal and fetal adrenal-derived androgens. Therefore, maternal and fetal adrenal-derived androgens (dehydroepiandrosterone sulfate [DHEAS]) are required for 17β-estradiol and estriol production. Estriol is synthesized through the aromatization of 16α-hydroxyandrostenedione derived from 16α-hydroxyepiandrosterone sulfate produced by the fetal liver and desulfated in the placenta (Figure 9–11); 16α-hydroxyepiandrosterone sulfate is derived from DHEAS produced in the fetal adrenal gland. The enzymes involved are placental sulfatase (DHEAS deconjugation), 3β-hydroxysteroid dehydrogenase (pregnenolone to progesterone conversion), and aromatase. Plasma and urinary levels of estriol increase significantly throughout pregnancy. The principal physiologic effects of estrogen during pregnancy include stimulation of uterine growth, prostaglandin synthesis, thickening of the vaginal epithelium, sensitization to oxytocin effects, and growth and development of the mammary epithelium.

Corticotropin-Releasing Hormone

Corticotropin-releasing hormone (CRH) is produced by the syncytiotrophoblast and trophoblast cells of the placenta. Its structure and function are similar to those of hypothalamus-derived CRH. The CRH concentration increases exponentially throughout pregnancy and peaks during labor. Placental production of CRH has been linked to the length of gestation in humans. CRH is secreted into the maternal circulation in large amounts during the third trimester of pregnancy and may play an important role in the onset of labor. CRH exerts a number of functions within the intrauterine environment, such as induction of prostaglandin production and maintenance of placental blood flow.

Pregnancy and Lactation

Hormonal Control of Parturition

Uterine contractility during pregnancy and parturition can be divided into at least 4 distinct phases.

Phase 0

During pregnancy, the uterus is maintained in a relatively quiescent state, mainly through the effects of progesterone. Additional factors involved in modulation of uterine activity during this period are prostacyclin, relaxin, parathyroid hormone-related peptide, and CRH. In general, their effects are the result of increased intracellular concentrations of cAMP or cyclic guanosine monophosphate and inhibition of intracellular calcium release, resulting in decreased myosin light chain kinase activity. The initiation of parturition results from the transition from the quiescent phase (phase 0) to a phase of activation (phase 1).

Phase 1

This phase of parturition is associated with activation of uterine function and is characterized by release from the inhibitory mechanisms maintaining uterine quiescence throughout pregnancy and activation of factors promoting uterine activity. These factors include uterine stretch and tension caused by the fully grown fetus, activation of the fetal hypothalamic-pituitary-adrenal axis, and increased prostaglandin synthesis. Mechanical stretch or hormonal priming leads to upregulation of gene expression of proteins that facilitate smooth muscle contraction, including connexins (key components of gap junctions), prostaglandin and oxytocin receptors, and ion channel proteins.

Phase 2

This phase of parturition is a period of active uterine contraction and is stimulated by prostaglandins, oxytocin, and CRH. Prostaglandins, particularly those produced in the intrauterine tissues, play a central role in the initiation and progression of labor. They induce myometrial contractility and help produce the changes associated with cervical softening at the onset of labor.

Phase 3

This postpartum phase involves uterine involution after delivery of the fetus and placenta and is mainly caused by the effects of oxytocin.

Mammary Gland Development

Ductal elongation is mediated by estrogen, GH, IGF-1, and epidermal growth factor. At the onset of puberty, estrogen production stimulates the developmental process from the immature breast consisting of a nipple, a few small ductal elements, and an underlying fat pad. The terminal end bud directs ductal growth, elongation, and branching into the fat pad.

Ductal branching and alveolar budding are influenced by progesterone, prolactin, and thyroid hormone. Progesterone stimulates ductal side branching and alveolar development. Prolactin acts directly on mammary epithelium to induce alveolar development. Both progesterone and prolactin synergize to stimulate proliferation of ductal epithelium. In the postpubertal female, during the luteal phase, progesterone stimulates the sprouting of alveolar structures from the sides of the ducts. During pregnancy, prolactin, progesterone, and hPL levels increase, and the terminal duct lobular units undergo expansion and differentiation of the secretory function.

The stage of mammary differentiation into a secretory function in preparation for lactogenesis is called stage I lactogenesis. During pregnancy, progesterone, estrogen, prolactin, hPL, and GH act synergistically to prepare the breast for lactation by promoting lobuloalveolar development. The elevated levels of progesterone prevent milk production during this period. With the progression of pregnancy, the circulating levels of prolactin continue to increase. During the third trimester, prolactin levels increase more than 10-fold. The increase in prolactin levels is the result of estrogen-induced increase in lactotrophs and prolactin synthesis in the anterior pituitary during gestation.

The second stage (stage II lactogenesis) is initiated after termination of pregnancy. The sudden decrease in circulating progesterone that accompanies parturition, in association with the concurrent increase in prolactin secretion mark the onset of lactation. The removal of the inhibition of synthesis of α-lactalbumin, and β-casein by the decrease in progesterone action allow prolactin, insulin, and glucocorticoids to stimulate the synthesis of the milk proteins. Continuous milk production is maintained by prolactin secretion from the anterior pituitary (see Chapter 3) throughout the period of lactation. Prolactin is the main regulator of milk protein synthesis; a process that requires the presence of glucocorticoids and insulin. Pharmacologic analogs of dopamine such as bromocriptine inhibit lactogenesis through the inhibition of prolactin release. Weaning, or cessation of the lactation period, is followed by involution of the terminal duct lobular units mediated by alveolar cell apoptosis and gland remodeling, returning the breast to its mature quiescent state.

Hormonal Control of Milk Secretion and Ejection

The onset of adequate milk production during the postpartum period requires developed mammary epithelium, persistent elevation in plasma prolactin (approximately 200 ng/mL), and a decrease in circulating levels of progesterone. Milk secretion from the mammary glands is triggered by stimulation of tactile receptors in the nipples by sucking (see Chapter 2). Oxytocin produces contraction of the myoepithelial cells of the lactiferous ducts, sinuses, and breast tissue alveoli. The immediate process (30 seconds to 1 minute) of milk flow in response to sucking is called the milk let-down reflex, and persists throughout the lactation period. Milk production increases during the first 36 hours postpartum from a volume of less than 100 mL/d to an average of 500 mL/d at approximately day 4 postpartum. The composition of breast milk changes considerably during this period as well.

Puberty

Female puberty is initiated by low-amplitude nocturnal pulses of gonadotropin release, which increase serum estradiol concentrations. The increased synthesis and secretion of estrogen by the ovary cause the progressive skeletal maturation that eventually leads to epiphysial fusion and the termination of linear growth. The onset of puberty causes a rapid increase in bone mass that correlates with bone age. The initial stage of puberty (at age 8–13 years) in girls involves breast development, accompanied by ovarian and follicular growth. This is followed by androgen- plus estrogen-induced pubic and axillary hair growth and the onset of menses (approximately at age 13 years), indicating sufficient estrogen production to stimulate endometrial proliferation. The first cycles are usually anovulatory, becoming fully ovulatory after 2–3 years. In girls, leptin serum concentrations increase dramatically as pubertal development progresses, and this increase in leptin levels parallels the increase in body fat mass. As mentioned in Chapter 8, leptin is considered to play a permissive role in the initiation of puberty.

Menopause

Menopause is the permanent cessation of menstruation resulting from loss of ovarian follicular activity. Menopause is preceded by a perimenopausal period, starting when the first features of impending menopause begin (ie, irregular menstrual bleeding and cycle frequency) and lasting until at least 1 year after the final menstrual period. During the menopausal transition, gonadotropins, estradiol, and inhibin show a marked degree of variability in their circulating levels. Within 1–2 years after the final menstrual period or the onset of menopause, FSH levels are markedly elevated, LH levels are moderately high, and estradiol and inhibin levels are low or undetectable. The resultant ovarian changes include short follicular phases with early ovulation and luteal insufficiency characterized by lower levels of progesterone secreted for shorter periods of time compared with the luteal phase of younger women. Postmenopausally, adrenal androstenedione is the major source of estrogen, and serum testosterone levels decrease moderately.

Starting from age 36 years, ovarian follicular apoptosis accelerates, leading to a steady decline in ovarian estradiol production (Figure 9–13). This loss of ovarian function results in a 90% loss of circulating estradiol; serum estradiol concentrations are often lower than 20 pg/mL. However, extragonadal estrogen synthesis increases as a function of age and body weight, and most of the estradiol is formed by extragonadal conversion of testosterone. The predominant estrogen in menopausal women is the weak estrogen estrone, produced through aromatase conversion of androstenedione.

The decline in ovarian function associated with the perimenopausal period is also responsible for an early decline in the release of inhibin B (inhibitor of FSH secretion), leading to an increase in follicular phase FSH. The decrease in serum inhibin B is believed to reflect the age-related decrease in ovarian follicle reserve, which is the primary source of serum inhibin B. The later increase in serum LH during the menopausal transition is caused by the cessation of ovarian follicle development. Despite a 30% decrease in GnRH pulse frequency with aging, there is an increase in the overall amount of GnRH secreted. FSH levels gradually increase with age in women who continue to cycle regularly. The consequences of loss of ovarian function during reproductive life may be severe. Symptoms include hot flashes, night sweats, vaginal dryness and dyspareunia (painful intercourse), loss of libido, loss of bone mass with subsequent osteoporosis, and abnormalities of cardiovascular function, including a substantial increase in the risk of ischemic heart disease. As already mentioned, estrogens (like androgens) have general metabolic roles that are not directly involved in reproductive processes. These include actions on vascular function, lipid and carbohydrate metabolism, bone, and brain. Within these sites, aromatase action can generate high levels of estradiol locally without significantly affecting circulating levels. Circulating C19 steroid precursors are essential substrates for extragonadal estrogen synthesis. The levels of these androgenic precursors decline markedly with advancing age in women. This is thought to contribute to the greater risk of bone mineral loss and fracture, and possibly the decline in cognitive function, in women as compared with men.

The multiple steps involved in the regulation of ovarian hormone production, the consequent modifications of the endometrium, and the regulation of uterine motility are all under tight control, ensuring ovulation, fertilization, implantation, and maintenance of pregnancy. Multiple approaches have been implemented for contraception. Some of the principal interventions are summarized in Table 9–2.

Combined oral contraceptive pills contain synthetic estrogen (ethinyl estradiol or mestranol) together with one of several synthetic progestogens and produce a range of effects on the female reproductive tract. Their main mode of action is the inhibition of ovulation; both estrogen and progestogen inhibit the ability of estrogen to produce a preovulatory surge of LH. The degree of follicular activity occurring during oral contraceptive use depends on the type and dose of steroid. The dose of estrogen used in modern contraceptive preparations is likely to be the minimal amount (20 μg ethinyl estradiol) that will reliably suppress FSH enough to prevent the growth of an ovulatory follicle.

Natural methods, such as the rhythm method, are based on the identification of a period for intercourse with less likelihood of fertilization that relies on knowledge of the life span of an ovum (24 hours) and of sperm (3 days) and of the interval between ovulation and menstruation, which is generally constant (14 days). The time of ovulation is determined by charting basal core temperature or using sensors for urinary LH levels throughout the cycle. The likelihood of conception is greatest during the follicular or preovulatory phase, during ovulation, and during the immediate postovulatory phase. Thus, for a typical 28- to 30-day cycle, the abstinence period would fall at about days 10–19 of the menstrual cycle.

Alterations in female reproductive endocrine function are of multiple etiologies and produce manifestations that range from precocious puberty to infertility, depending on the age at presentation. The most frequent are abnormalities in the menstrual cycle, consisting of either absent menstruation (amenorrhea) or excess bleeding (metrorrhagia), and infertility. Abnormalities of ovarian development and function are usually caused by gonadal dysgenesis and rarely by defects in the synthesis of ovarian steroids. In general, increased ovarian hormone production can be caused by increased gonadotropin release (hypergonadotropic hypergonadism) related to tumors, brain inflammatory diseases, and head injury, among other causes, or it can result from excess hormone production by ovarian tumors. Hypogonadotropic hypergonadism may be caused by congenital adrenal hyperplasia, activating mutations of the α-subunit of the Gs protein, and excess aromatase (the last enzymatic step in estrogen synthesis) activity. Decreased ovarian hormone production can be genetic (eg, FSH and LH receptor gene mutations, mutation of the β-subunit of FSH, enzymatic deficiencies) or acquired (eg, radiation) despite adequate gonadotropin release (hypergonadotropic hypogonadism). Decreased ovarian hormone production caused by impaired gonadotropin release (hypogonadotropic hypogonadism) is rare and may result from GnRH receptor gene mutations, lesions in the hypothalamic area, and other causes.

• Gonadotropin release is under negative and positive feedback regulation by ovarian steroid and peptide hormones.
• Estrogen synthesis requires LH and FSH regulation of coordinated metabolism by granulosa and theca cells of the ovarian follicle.
• FSH stimulates estrogen synthesis and ovarian follicle growth and maturation.
• LH stimulates ovulation and corpus luteum progesterone and estrogen synthesis.
• The corpus luteum is a temporary endocrine organ that plays a central role during the initial stages of pregnancy.
• The ovarian cycle produces cyclic changes in steroid hormone production, which in parallel produce marked morphologic and functional changes in the endometrium, preparing it for embryo implantation.
• Estrogen has important systemic effects affecting the risk of cardiovascular disease, osteoporosis, and endometrial and breast cancer.
• Progesterone ensures uterine quiescence and prevents lactogenesis during pregnancy.
• Mammary gland morphologic development occurs during puberty and is functionally modified during pregnancy by prolactin and hPL, ensuring lactogenesis.