CBI203 PHYSIOLOGY 2003

Normal Body - physiologyFemale Reproductive System

Female Reproductive System

Mimi Jakoi, PhD

David Schomberg, PhD

Jennifer Carbrey, PhD

The Hypothalamus-Pituitary-Gonadal Axis

The Common Axis

There are many similarities between the hypothalamus-pituitary-gonad axis in males and females. The process of producing sperm or ova is coordinated by the hypothalamus which secretes GnRH (gonadotropin-releasing hormone) in a pulsatile manner. In turn, GnRH stimulates the anterior pituitary to secrete both FSH (follicle-stimulating hormone) and LH (luteinizing hormone) (Fig. 1). In both males and females, androgens (like testosterone) produced in the gonad are needed for development of the ovum or sperm. Production of inhibin B by cells in the testis or ovary decreases FSH secretion while sex hormones like testosterone and estrogen regulate GnRH, LH, and FSH secretion.

The Axis in Females

In the female, the cyclic pattern of hypothalamic secretion controls the menstrual cycle. Starting at puberty, the hypothalamus secretes GnRH which leads to LH and FSH secretion from the anterior pituitary (Fig. 1). LH acts on the theca cells of the ovary to secrete androgen. FSH acts on the follicles in the ovary and specifically on the granulosa cells which surround the ovum. These cells contain the enzyme aromatase which converts local androgen (from the theca cells) to estrogen (Fig. 1). Estrogen exerts a negative feedback blocking secretion of GnRH, LH and FSH. Granulosa cells also secrete inhibin which selectively suppresses FSH secretion (Fig.1).

Kisspeptin

Interestingly, the GnRH producing neurons in the hypothalamus are not sensitive to estrogen or testosterone because they do not express estrogen or androgen receptors. Instead, regulation of GnRH release is through hypothalamic neurons expressing kisspeptin (Fig. 2). Kisspeptin producing neurons express estrogen receptors and as well as the androgen receptor. When kisspeptin neurons are activated, they release kisspeptin which increases the production of GnRH by the GnRH neurons (Fig. 2). In addition, other factors known to affect GnRH secretion, such as leptin levels and circadian signals, may also act through the kisspeptin neurons. Since leptin is produced by adipose, decreased leptin levels

Figure 1. The female hypothalamus-pituitary-gonad axis. Modified from Guyton and Hall, The Textbook of Medical Physiology, 11th ed.

are thought to either directly or indirectly reduce kisspeptin release to prevent GnRH secretion to allow the organism to conserve energy by reducing fertility.

Puberty

During childhood, there are very low levels of GnRH and estrogen (Fig. 3). As a result, the only follicle maturation is gonadotropin-independent (see below) and there is no ovulation. It is thought that the onset of puberty may be due to decreased negative feedback of estrogen on GnRH, allowing increasing GnRH production. The resulting increase in estrogen production causes maturation of the secondary sex characteristics and increased bone growth (Fig. 3). The role of leptin in increasing GnRH levels may explain why the more adipose a girl has, the earlier she tends to start puberty. This may also explain why the age of first menstruation, menarche, has been decreasing as obesity of the population has been increasing (Fig. 2).

Figure 2. The regulation of GnRH neurons and kisspeptin neurons. From Oakley, A. E. et al. Endocr Rev 2009;30:713-743

The Ovary

Steroid Production by the Ovary

The ovary has two primary functions. In oogenesis, the ovum and its surrounding granulosa cells mature into a structure called a follicle. The other role of the ovary is the secretion of estrogens and progestins, two different types of sex hormones.

The primary type of estrogen which is produced by the ovary is -estradiol with only small amounts of estrone being secreted (Fig. 4). Estriol is an oxidative product produced in the liver from estrone and estradiol (Fig. 4). In addition to estradiol being the most prevalent form of estrogen, it is also the most potent form. Both progesterone and testosterone can be converted into estrogens. Estrogen develops and maintains female secondary sex characteristics and causes proliferation of the endometrium after menstruation. Estrogens are formed in the ovary when theca cells produce testosterone that the granulosa cells convert to estrogen.

Figure 3. Levels of FSH and LH in males and females. From Berne and Levy, Principles of Physiology, 4th edition.

The most prevalent and important progestin is progesterone (Fig. 4). One important role of progesterone is to induce the secretory phase of the uterus in preparation for the implantation of an embryo.

Oogenesis

The ovarian germ cell population changes in number over time. The peak stem cell population occurs at about the fifth month of gestation and is followed by a very rapid fall in numbers such that at birth the germ cell population is about 10-20 million cells. During the prepubertal and reproductive periods there is a further reduction in oocyte number such that only about 400-500 oocytes actually mature to ovulation (i.e., leave the ovary). The others are lost to cell death (called atresia). The loss of oocytes is the fundamental basis of menopause.

Follicle growth

The growth of each follicle begins with a gonadotropin-independent phase. During this phase primordial follicles are converted to follicles with a zona pellucida and several layers of granulosa cells. This process is regulated by growth factors within the ovary and occurs throughout infancy, childhood, and adulthood. In infancy and childhood, without significant levels of LH and FSH, the follicles undergo atresia and do not become fully mature.

Figure 5. Coordination of uterine and ovarian cycles by hormones.

Figure 4. The predominant female sex hormones. From Guyton and Hall, The Textbook of Medical Physiology, 11th ed.

The Menstrual Cycle

During the menstrual cycle the hypothalamus, pituitary, ovary, and uterus interact to cause a follicle to mature, egg to be released, and provide a uterus suitable for pregnancy. This is accomplished by the hypothalamus and pituitary exerting control over the ovary while the steroids of the ovary feedback to regulate the hypothalamus and pituitary. The steroids of the ovary are also responsible for initiating and controlling changes that occur in the endometrium of the uterus.

The Ovary

The menstrual cycle in the ovary can be divided into follicular and luteal phases (Fig 5). The follicular phase extends from day 1 (beginning of menses) to day 14. At the beginning of this period, many follicles that had begun to develop during the gonadotropin-independent phase continue to mature in response to FSH binding to granulosa cells. This initiates the gonadotropin-dependent phase of follicle growth (Fig. 5). FSH stimulates the granulosa cells to produce aromatase, which converts the androgens produced by the theca cells to estrogen. LH stimulates the theca cells surrounding the follicle to produce androgens (Fig. 6). The estrogen produced further stimulates the granulosa cells.

Figure 6. The regulation of the follicle selection process.

Estrogen and FSH increase the expression of LH and FSH receptors on granulosa cells (Fig 6). In addition, estrogen increases expression of LH receptors on theca cells. Granulosa cells of the follicle continue to secrete increasing amounts of estrogen which feeds back in a negative manner to regulate FSH. Plasma FSH levels decline (Fig. 5). FSH levels are also regulated by a second hormone, inhibin B, secreted by the granulosa cells. Inhibin B also negatively regulates FSH secretion from the pituitary (Fig 1).

Each month 6-12 follicles start to mature but only one is chosen to be ovulated (i.e., expelled from the ovary) for transport to the uterus. This chosen follicle expresses the highest number of FSH and LH receptors and is therefore more sensitive to those hormones. It survives and the others die (atresia) due to falling FSH levels.

Between days 12-14, there is a rapid rise in estrogen secretion by the granulosa cells of the chosen follicle. Now estrogen acts in a positive manner resulting in an increase in kisspeptin within the hypothalamus. This local rise in kisspeptin increases GnRH secretion and consequently a surge in FSH and LH secretion (Fig 5). Thirty six to forty eight hours after the LH surge, ovulation of the chosen ovum occurs.

The surge of LH production in response to increased estrogen is unique to females. Male brains develop in the presence of testosterone which is converted to estrogen. It is thought that increased levels of estrogen in the male brain prevent development of a certain group of neurons in the hypothalamus. However, under conditions of low estrogen during prenatal development as is seen in females, neurons are thought to gather in this second site and synapse with the cell body of GnRH producing neurons and stimulate GnRH release under increased estrogen levels as occurs during follicular development (Fig. 7).

Ovulation

Ovulation is dependent on the LH surge which initiates several processes. LH causes the oocyte to finish the 1st meiotic division and become arrested at metaphase II. LH causes granulosa cells to produce progesterone and the progesterone receptor. The newly formed progesterone acts on the progesterone receptors of the granulosa cells in an autocrine manner to cause expression of proteolytic enzymes that weaken the wall of the follicle. Increased prostaglandin production initiates an inflammatory process, vasodilation, as well as contraction of cells that help the ovum to escape the follicle during ovulation. Ovulation occurs about 16-20 hours after the peak LH concentration is reached. Both progesterone and prostaglandin production are necessary for ovulation. Ovulation can be blocked through use of NSAIDs that inhibit prostaglandin synthesis. The egg leaves the ovary surrounded by a layer of cumulus cells, with the inner cells contacting the egg called the corona radiata.

Figure 7. The role of kisspeptin neurons in the regulation of the H-P-G axis in males and females. From de Tassigny and Colledge, Physiology, 25:207-217, 2010.

Luteal phase

The luteal phase of the ovary extends from days 14-28 (Fig. 5). In the ovary, the now empty chosen follicle converts to a progesterone secreting tissue called the corpus luteum. The same LH surge that causes ovulation initiates conversion of the granulosa cells to become granulosa lutein cells and the theca cells to become theca lutein cells. The corpus luteum secretes both estrogen and progesterone with the theca lutein cells producing androgens and the granulosa lutein cells converting the androgens to estrogen and progesterone. The progesterone initiates the secretory phase of the endometrium. In the luteal phase, estrogen and inhibin from the corpus luteum inhibits FSH and LH secretion from the pituitary, as well as GnRH. Estrogen inhibition of LH and FSH is stronger in the presence of progesterone. If fertilization does not occur, then 12-14 days after ovulation, the corpus luteum degenerates by apoptosis and ceases estrogen and progesterone production. The decrease in progesterone allows for menstruation and the decrease in estrogen allows for increased GnRH production to begin the cycle all over again (Fig. 5).

The Uterus

The menstrual cycle of the ovary causes the uterus to be either proliferative or secretory

The uterus proliferative phase (growth phase) is regulated by estrogen from the developing follicle during days 1-14 of the menstrual cycle. During this time estrogen causes the uterine wall to thicken and uterine glands to develop (Fig. 5). In addition, under the control of estrogen, the cervix produces clear mucous which facilitates movement of sperm into the uterus. In contrast, later, during the secretory phase when progesterone is abundant, the mucous becomes thick to produce a plug that protects the uterus (and possibly a forming embryo) from bacteria from the vagina.

The secretory phase of the uterus is regulated by progesterone produced by the corpus luteum during days 14-28 of the menstrual cycle. Progesterone prepares the uterus for the possibility of embryo implantation by causing the endometrial glands to produce glycogen. In addition, the progesterone acts to decrease the number of estrogen receptors which stops the proliferative effects of estrogen. In the absence of fertilization and implantation, the corpus luteum degenerates and progesterone levels decline late in the menstrual cycle. The decreased levels of progesterone and estrogen cause prostaglandins to be produced in the endometrium. The prostaglandins lead to vascular constriction and enhanced uterine smooth muscle activity. Menses, sloughing off of the endometrium, ensues. Overproduction of these prostaglandins can lead to menstrual cramps due to the uterine smooth muscle contractions as well as systemic effects such as vomiting and headaches.

Successful implantation and pregnancy require a continuous production of progesterone. If fertilization and pregnancy occurs, then the newly formed embryo (blastocyst) secretes the hormone, human chorionic gonadotrophin (hCG), which stimulates the corpus luteum to produce progesterone and estrogen during the first trimester of pregnancy. The maintenance of estrogen levels prevents FSH and LH levels from increasing to start follicular maturation and the next cycle. hCG is used to assess the viability of the fetus and is the basis of the at home pregnancy test. If there is not successful implantation, there is no hCG produced and the corpus luteum degenerates and ceases estrogen and progesterone production.

Mechanisms of action for estrogen

Steroid hormones act as transcription factors in their target tissues. In the uterus and breast, estrogen up-regulates estrogen receptors. In addition, estrogen primes the tissue for progesterone action by increasing the numbers of progesterone receptors expressed. The effects of estrogen allow it to amplify its own effects and prepare tissues for an efficient response to progesterone. Continual exposure to unopposed estrogen can result in pathological stimulation of these target tissues (endometrial and/or breast cancer). There are two different estrogen receptors, ER and ERwhich can have different tissue distributions. Since the two estrogen receptors have different affinities for synthetic estrogens, the two receptors must be considered when developing and using selective estrogen receptor modulators (SERMs) such as tamoxifen.

In contrast, progesterone-dependent transcription down-regulates the estrogen receptor (decreases its numbers) thereby facilitating differentiation instead of proliferation.

The role of androgens in females

Androgens are produced by the adrenal cortex and the ovaries. In females they play a role in sex drive, growth of skeletal muscle and pubic and axillary hair.

Fertilization

Once ovulated, an egg survives for about 24 hours before degenerating. During this time the egg travels down the oviduct towards the uterus.

Sperm that have entered the female tract must go through capacitation by contacting the surface of the epithelium of the isthmus portion of the oviduct. During capacitation, cholesterol is lost from the sperm plasma membrane which causes it to become more fluid. Capacitation is necessary for sperm to be able to bind the zona pellucida and undergo the acrosome reaction to penetrate the egg. Capacitation is also important for inducing a hyperactive state of sperm motility which is necessary for penetrating the layers of the egg.

Figure 8. Hormone levels during pregnancy. From Vander’s Human Physiology, 11th ed.

Once a sperm comes into contact with the egg, usually in the ampulla of the oviduct, it must penetrate a layer of cumulus cells to reach the zona pellucida. Contact with the zona pellucida triggers a regulated exocytosis process in the sperm, the acrosome reaction. Fusion of the acrosome with the sperm plasma membrane releases enzymes that digest the zona pellucida and allow the sperm access to the egg plasma membrane. The sperm plasma membrane fuses with the oocyte plasma membrane and the sperm (with the exception of the tail plasma membrane) enters the egg.

The sperm entering the egg triggers several processes which prevent polyspermy. One is rapid depolarization of the egg plasma membrane which prevents other sperm from entering. Another is the cortical reaction where the egg is stimulated by an increase in intracellular calcium to release granules that digest the zona pellucida and cross-link proteins to prevent other sperm from gaining access to the egg plasma membrane. The increase in intracellular calcium also causes the egg nucleus to finish its 2nd meiotic division. Once the diploid zygote is formed, it contains male centrosomes and female mitochondria.

Pregnancy

If fertilization and implantation are successful, the synctiotrophoblast of the embryo will produce human chorionic gonadotropin (hCG) which acts on the corpus luteum to prevent it from degenerating and to stimulate estrogen and progesterone secretion. hCG levels increase and peak at 10-12 weeks of pregnancy (Fig. 8). Once the hCG levels decrease, 3 months into the pregnancy, the corpus luteum regresses. However, estrogen and progesterone levels continue to increase throughout the pregnancy (Fig. 8). Once the corpus luteum regresses, the placenta produces the progesterone and estrogen necessary to maintain the pregnancy. In order to make estrogen, the placenta must convert androgens received from tissues such as the fetal adrenal glands.

Figure 9. Changes in the mammary gland in response to hormones. From Netter’s Atlas of Human Physiology.

Lactation

Starting in puberty, when estrogen levels increase, secretion of prolactin increases. During puberty, estrogen stimulates mammary gland duct branching and growth while progesterone and prolactin stimulate alveolar growth (Fig. 9). Only very high levels of prolactin stimulate production of milk. The mammary gland does not become fully developed until prolactin (from the maternal pituitary) and estrogen, progesterone, and placental lactogen (all 3 from the placenta) act on the mammary gland during pregnancy (Fig. 9). During pregnancy when estrogen is very high, the stimulation of milk production by prolactin is inhibited until delivery when the source of the estrogen, the placenta, is delivered (Fig. 9). After several months of breastfeeding, due to decreased levels of estrogen since the mother is no longer pregnant, basal levels of prolactin decrease. However during each episode of nursing, prolactin levels are high to induce milk production.

Also during nursing, suckling of the baby causes oxytocin to be released from the pituitary which causes contraction of the myoepithelial cells of the mammary gland and ejection of milk (Fig. 9). Suckling of the baby also can prevent ovulation, possibly through effects on the kisspeptin neurons. This effect can prevent pregnancy for years, as long as nursing continues. However, it is not a reliable method of birth control. Due to the inhibitory effects of estrogen on prolactin release, nursing mothers are often prescribed progesterone-only birth control pills.

Menopause

At around age 50, a woman who stops having menstrual cycles reaches menopause. Menopause occurs due to a lack of viable follicles. Without follicles to mature and produce estrogen and inhibin, there is a lack of negative feedback on the hypothalamus and pituitary. As a result, LH and FSH levels are high in peri- and post-menopausal women (Fig. 10). Post-menopausal women do produce some estrogen due to the conversion of adrenal androgens by aromatase. However, levels are low enough that estrogen-dependent tissues such as the breasts and genitalia atrophy. In addition, the bone-protecting effects of estrogen are lost. The loss of the effects of estrogen on the vasculature causes hot flashes, when the arterioles in the skin dilate and there is a feeling of warmth, and sweating. In addition, protective effects of estrogen on the cardiovascular system are lost.

Oral Contraceptives

Oral contraceptive pills are usually a combination of synthetic estrogen and progesterone. The doses are lower than would be normally found in the body due to a prolonged half-life (4-8 hours versus 20 min). This is accomplished by adding groups such as acetyl or methyl groups which slow metabolism of the synthetic hormones by the liver. They act by inhibiting pituitary secretion of LH (negative feedback) which often prevents ovulation. Some pills, such as progesterone-only pills, are not as effective at preventing ovulation. However, they prevent pregnancy by affecting cervical mucus which prevents sperm from entering the uterus or by making the endometrium unable to facilitate implantation. Periodic withdrawal of steroids is needed to allow for a sloughing of the uterine wall (menstrual flow).

Figure 10. Levels of FSH and LH in the urine of males and females. From Guyton and Hall, The Textbook of Medical Physiology, 11th ed.

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