Antenatal Care Module: 4. Hormonal Regulation of the Female Reproductive System: View as single page View larger image Figure 4.1 Hormonal regulation of the female reproductive system. The length of the menstrual cycle is typically 28 days, but it can be highly variable. In some women it may be as short as 21 days or as long as 39 days. The menstrual cycle is best understood if we focus first on events occurring in the ovaries, and then on events occurring in the uterus. We are going to describe each of them in turn. 4.2 The ovarian cycle The ovarian cycle refers to the monthly series of events in the ovaries, associated with the maturation and release of an ovum, and the ‘just in case’ preparation for its fertilisation and implantation in the uterus. You may be wondering why the regulation of the female reproductive system is so complicated, as shown in Figure 4.1. The reason is that the ovarian cycle has to be initiated (switched on) and then suppressed (switched off) in a precisely regulated sequence every month. In this section, we will explain how this is achieved.

The ovarian cycle consists of two consecutive phases, each of about 14 days’ duration. Events are measured from ‘day 1’, which is the first day of the last normal menstrual period (LNMP). 4.2.1 The follicular phase: days 1 to 14 The sequence shown in Figure 4.1 begins with the secretion of GnRH from the hypothalamus, which stimulates the pituitary gland to produce FSH and LH, which in turn act on the ovaries, telling the ovarian follicles to complete the maturation of an ovum. During this period, a few ovarian follicles containing immature ova develop and mature under the stimulation of FSH and LH. Usually by day 14, only one follicle has become fully mature, and the ovum it contains is ready to be released. All other follicles that had begun maturing during this phase of the ovarian cycle degenerate as soon as ovulation has occurred. Note that only one time interval is fairly constant in all females — the time from ovulation to the beginning of menstruation, which is almost always 14 to15 days. However, the time of ovulation is variable and difficult to predict accurately.

4.2.2 The luteal phase: days 15 to 28 This phase is the period of corpus luteum activity, during which the uterus is prepared ‘just in case’ of pregnancy. After ovulation has occurred, the corpus luteum begins to secrete progesterone, as well as a small amount of oestrogen. Progesterone maintains the uterus in a state ready to receive and nourish an embryo. The lining of the uterus (the endometrium) becomes thicker, more richly nourished by blood vessels, and more receptive to the fertilised ovum. Progesterone also inhibits any further release of FSH or LH from the pituitary gland. Thus, ovulation is quickly followed by a rise in the level of progesterone in the circulation, as the corpus luteum takes over the production of this hormone. As the progesterone level rises, it circulates around the body in the blood. When a high concentration of progesterone reaches the hypothalamus in the brain, the effect is to stop the hypothalamus from producing GnRH. What will happen when the hypothalamus stops producing GnRH?

(Look back at Figure 4.1.) The pituitary gland will stop producing FSH and LH. What effect will this have on the ovaries? The maturation of more ova will stop at that point. This type of control system, where the rise in one body chemical (in this case, progesterone) stops the production of another body chemical (in this case, GnRH), is called a negative feedback mechanism. But the corpus luteum has a limited lifespan, and after it stops producing progesterone, the negative feedback on the hypothalamus stops, and this allows it to begin producing GnRH again. So the ovarian cycle begins all over again. 4.3 The uterine cycle Next, we turn our attention to the events occurring in the uterus during the same (typically) 28-day period as the events just described in the ovaries. The uterine cycle indicates the cyclical changes that occur in the uterus in response to the female sex hormones, progesterone and oestrogen.

4.3.1 The menstrual phase: days 1 to 5 If fertilisation does not occur after ovulation, the corpus luteum will degenerate and production of progesterone will decrease, so the stimulus for maintaining the thick endometrium will disappear. The reduction in progesterone causes the shedding of the thick endometrial lining. The muscular wall of the uterus (the myometrium) contracts to help cut off the blood supply to the endometrium, causing it to break away from the uterus. The endometrium was richly supplied with blood vessels to nourish the fetus if a pregnancy occurred, so when it disintegrates and passes down the vagina, some blood is mixed in with it. The mixture of tissue and blood passes out through the vagina as the menstrual fluid (or menstrual flow), usually for a period of about three to five days. Other common names for menstruation are ‘monthly bleeding’ or ‘menstrual period’. Menstruation usually occurs at monthly intervals throughout the reproductive years, except during pregnancy when it is completely suppressed and the woman cannot get pregnant again until after the baby is born.

Breastfeeding a baby also suppresses menstruation, but there is a risk that ovulation and pregnancy may still occur. 4.3.2 The proliferative phase: days 6 to 14 The concentration of oestrogen in the blood is rising during this period, following the end of menstruation, as the ovaries prepare for the next ovulation at around day 14. It is called the proliferative phase (‘proliferate’ means to ‘multiply or increase’), because in this period the endometrium grows thicker and becomes more richly fed by blood vessels in preparation for the possibility of fertilisation and pregnancy. 4.3.3 The secretory phase: days 15 to 28 During this phase, the blood concentration of progesterone increases, which causes even more blood vessels to grow into the endometrium. This makes the endometrium receptive to the fertilised ovum. If the ovum is fertilised and the embryo implants in the endometrium and a placenta develops, it produces a hormone called human chorionic gonadotropin (HCG) throughout pregnancy. The detection of HCG in a woman’s urine is the basis of most pregnancy tests.

Chorionic is pronounced ‘korr ee onn ik’. Gonadotropin is pronounced ‘gonn add oh troh pinn’. HCG signals the corpus luteum to continue to supply progesterone to maintain the thick, nourishing endometrium throughout the pregnancy. Continuous levels of progesterone act as a negative feedback mechanism on the hypothalamus and pituitary gland, preventing the release of FSH and LH, and hence further ovulation ceases. What happens if fertilisation of the ovum does not occur? The corpus luteum degenerates and the level of progesterone declines; the endometrium disintegrates and the woman menstruates — a signal that she did not become pregnant during that menstrual cycle. 4.4 The menarche, puberty and the menopause You probably know that menarche (the first menstruation) starts on average between the ages of 12 to15 years in Ethiopia. But in some cases it can start as late as 17 to 20 years, or as early as 8 to 9 years. Some of the factors that affect the age of menarche are biological, and some are cultural.

Menarche is pronounced ‘menn ark’. Menarche begins when the hypothalamus in the brain is sensitised to begin producing GnRH at around the age of 12 to15 years. But evidence suggests that GnRH may begin at an earlier age in girls who are well nourished and exposed to sexual motivating factors, such as watching sexual films and talking about sex. In malnourished girls, who have little exposure to sexual motivating factors, menarche may be delayed until the age of 17 to 20 years. Disease conditions that affect the hypothalamus and pituitary gland, or the ovaries and uterus, can also affect the age of first menarche. Around the age of the menarche, the female sex hormones, oestrogen and progesterone, are responsible for the development of secondary sexual characteristics in the female. These include: the development of the breasts the broadening of the pelvis increased activity of sweat glands and sebaceous glands (oil glands in the skin) the growth of pubic and armpit hair.

Together with the menarche, the appearance of the secondary sexual characteristics marks the period known as puberty — the period of life (typically between the ages of 10 to15 years) during which the reproductive organs grow to adult size and become functional. The secondary sexual characteristics are termed ‘secondary’ because they develop after the primary sexual characteristics, which distinguish females from males. Name some of the primary sexual characteristics of females. You already learned about them in Study Session 3 — they are the anatomical structures of the external female genitalia (e.g. the labia minora and clitoris), and the internal female reproductive organs (e.g. the ovaries, uterus and vagina). Menstruation continues every month, except during pregnancy, until the woman reaches the menopause at around the age of 48 to 50 years, when menstruation ceases. You may recall from Study Session 3 that at birth a female baby’s ovaries already contain about 60,000 immature eggs, and she cannot produce any more in her lifetime. By the time she reaches the menopause, her ability to bring ova to maturity has come to an end.

In Study Session 5, we describe what happens when an ovum is fertilised and it implants in the uterus and develops into a fetus. Summary of Study Session 4 In Study Session 4, you have learned that: The hormones controlling the female reproductive system include gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH) and leutenizing hormone (LH), all of which are produced in the brain; oestrogen and progesterone produced by the ovaries and the corpus luteum; and human chorionic gonadotropin (HCG), produced by the placenta during pregnancy. The menstrual cycle typically lasts 28 days, but it can be highly variable. It is characterised by menstruation from days 1 to 5 and ovulation at around day 14, but the date of ovulation is difficult to predict accurately. Menstruation is the monthly shedding from the uterus of the endometrium with some blood, which emerges through the vagina, typically for a period of three to five days. Menstruation continues from menarche to the menopause, except during pregnancy. It may also be suppressed by breastfeeding.

The ovarian cycle refers to the regular, repeating events occurring in the ovaries during the menstrual cycle, characterised by the development of a few ovarian follicles; the maturation and release (ovulation) of a single ovum; and the formation and subsequent degeneration of the corpus luteum if pregnancy does not occur. The uterine cycle refers to the regular, repeating events occurring in the uterus during the menstrual cycle, characterised by the thickening of the endometrium and an increase in its blood supply, followed by its degeneration and shedding as the menstrual flow if pregnancy does not occur. The menarche and the development of secondary sexual characteristics signal the period known as puberty, when the internal reproductive organs grow to adult size and a girl becomes fertile and capable of becoming pregnant. Self-Assessment Questions (SAQs) for Study Session 4 Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions. Write your answers in your Study Diary and discuss them with your Tutor at the next Study Support Meeting. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.

SAQ 4.1 (tests Learning Outcomes 4.1 and 4.2) The left-hand column of Table 4.1 gives the names of the various phases in the menstrual cycle. Complete the right-hand column by entering the correct period in days measured from the last normal menstrual period (LNMP). Table 4.1 for SAQ 4.1. Phase of the menstrual cycle Days (1 = first day of the LNMP) Follicular phase of the ovarian cycle Luteal phase of the ovarian cycle Menstrual phase of the uterine cycle Proliferative phase of the uterine cycle Secretory phase of the uterine cycle Answer Antenatal Care Module: 4. Hormonal Regulation of the Female Reproductive System Except for third party materials and/or otherwise stated (see terms and conditions ) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Sharealike 2.0 licence . In short this allows you to use the content throughout the world without payment for non-commercial purposes in accordance with the Creative Commons non commercial sharealike licence.

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Unauthorised use of any of the content may constitute a breach of the terms and conditions and/or intellectual property laws. We reserve the right to alter, amend or bring to an end any terms and conditions provided here without notice. All rights falling outside the terms of the Creative Commons licence are retained or controlled by The Open University. Head of Intellectual Property, The Open University Hormonal Regulation of the Female Reproductive Cycle About Watch and Favorite Watch Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made. Favorite Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students. Hormonal Regulation of the Female Reproductive Cycle The menstrual cycle is controlled by a series of changes in hormone levels, primarily estrogen and progesterone. Learning Objective Differentiate among the phases of the menstrual cycle Key Points The follicular phase begins with an increase in follicle -stimulation hormone ( FSH ), which causes increases in luteinizing hormone ( LH ) and gonadotropin-releasing hormone ( GnRH ).

Concurrent increases in estrogen levels cause increases in progesterone , stimulating proliferation of the endometrium . A spike in LH and FSH ("LH surge") causes ovulation, following a suppression of GnRH. Estrogen levels continue to rise following ovulation and the corpus luteum forms, which secretes progesterone in significant levels and causes decreases in LH and FSH levels. Without implantation , estrogen and progesterone levels will fall and the corpus luteum will degrade. Terms Register for FREE to remove ads and unlock more features! Learn more Full Text The menstrual cycle is the physiological change that occurs under the control of the endocrine system in fertile women for the purposes of sexual reproduction and fertilization . Phases of the Menstrual Cycle The menstrual cycle is divided into three stages: follicular phase, ovulation, and the luteal phase . Follicular Phase During the follicular phase (or proliferative phase), follicles in the ovary mature under the control of estradiol . Follicle-stimulating hormone (FSH) is secreted by the anterior pituitary gland beginning in the last few days of the previous menstrual cycle.

Levels of FSH peak during the first week of the follicular phase. The rise in FSH recruits tertiary-stage ovarian follicles (antral follicles) for entry into the menstrual cycle. Follicle-stimulating hormone induces the proliferation of granulosa cells in the developing follicles and the expression of luteinizing hormone (LH) receptors on these cells. Under the influence of FSH, granulosa cells begin estrogen secretion. This increased level of estrogen stimulates production of gonadotropin-releasing hormone (GnRH), which increases production of LH. LH induces androgen synthesis by theca cells , stimulates proliferation and differentiation , and increases LH receptor expression on granulosa cells. Throughout the entire follicular phase, rising estrogen levels in the blood stimulate growth of the endometrium and myometrium of the uterus . This also causes endometrial cells to produce receptors for progesterone, which helps prime the endometrium to the late proliferative phase and the luteal phase. Two or three days before LH levels begin to increase, one or occasionally two of the recruited follicles emerge as dominant.

Many endocrinologists believe that the estrogen secretion of the dominant follicle lowers the levels of LH and FSH, leading to the atresia (death) of most of the other recruited follicles. Estrogen levels will continue to increase for several days. High estrogen levels initiate the formation of a new layer of endometrium in the uterus, the proliferative endometrium. Crypts in the cervix are stimulated to produce fertile cervical mucus that reduces the acidity of the vagina , creating a more hospitable environment for sperm. In addition, basal body temperature may lower slightly under the influence of high estrogen levels. Ovulation Estrogen levels are highest right before the LH surge begins. The short-term drop in steroid hormones between the beginning of the LH surge and ovulation may cause mid-cycle spotting or bleeding. Under the influence of the preovulatory LH surge, the first meiotic division of the oocytes is completed. The surge also initiates luteinization of theca and granulosa cells. Ovulation normally occurs 30 (± 2) hours after the beginning of the LH surge.

Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (oocyte). The time immediately surrounding ovulation is referred to as the ovulatory phase or the periovulatory period. In the preovulatory phase of the menstrual cycle, the ovarian follicle undergoes cumulus expansion stimulated by FSH. The ovum then leaves the follicle through the formed stigma. Ovulation is triggered by a spike in the amount of FSH and LH released from the pituitary gland. Luteal Phase The luteal phase begins with the formation of the corpus luteum stimulated by FSH and LH and ends in either pregnancy or luteolysis. The main hormone associated with this stage is progesterone, which is produced by the growing corpus luteum and is significantly higher during the luteal phase than other phases of the cycle. Progesterone plays a vital role in making the endometrium receptive to implantation of the blastocyst and supportive of the early pregnancy. It also raises the woman's basal body temperature.

Several days after ovulation, the increasing amount of estrogen produced by the corpus luteum may cause one or two days of fertile cervical mucus, lower basal body temperatures, or both. This is known as a secondary estrogen surge. The hormones produced by the corpus luteum suppress production of the FSH and LH, which leads to its atrophy . The death of the corpus luteum results in falling levels of progesterone and estrogen, which triggers the end of the luteal phase. Increased levels of FSH start recruiting follicles for the next cycle. Alternatively, the loss of the corpus luteum can be prevented by implantation of an embryo: after implantation, human embryos produce human chorionic gonadotropin (hCG). Human chorionic gonadotropin is structurally similar to LH and can preserve the corpus luteum. Because the hormone is unique to the embryo, most pregnancy tests look for the presence of hCG. If implantation occurs, the corpus luteum will continue to produce progesterone (and maintain high basal body temperatures) for eight to 12 weeks, after which the placenta takes over this function.

SEER Training:Female Sexual Response & Hormone Control Female Sexual Response & Hormone Control Updates Female Sexual Response & Hormone Control The female sexual response includes arousal and orgasm, but there is no ejaculation. A woman may become pregnant without having an orgasm. Follicle-stimulating hormone, luteinizing hormone, estrogen, and progesterone have major roles in regulating the functions of the female reproductive system. At puberty, when the ovaries and uterus are mature enough to respond to hormonal stimulation, certain stimuli cause the hypothalamus to start secreting gonadotropin-releasing hormone. This hormone enters the blood and goes to the anterior pituitary gland where it stimulates the secretion of follicle-stimulating hormone and luteinizing hormone. These hormones, in turn, affect the ovaries and uterus and the monthly cycles begin. A woman's reproductive cycles last from menarche to menopause. The monthly ovarian cycle begins with the follicle development during the follicular phase, continues with ovulation during the ovulatory phase, and concludes with the development and regression of the corpus luteum during the luteal phase. The uterine cycle takes place simultaneously with the ovarian cycle.

The uterine cycle begins with menstruation during the menstrual phase, continues with repair of the endometrium during the proliferative phase, and ends with the growth of glands and blood vessels during the secretory phase. Menopause occurs when a woman's reproductive cycles stop. This period is marked by decreased levels of ovarian hormones and increased levels of pituitary follicle-stimulating hormone and luteinizing hormone. The changing hormone levels are responsible for the symptoms associated with menopause. Menstrual Cycle - Women's Health Issues - Merck Manuals Consumer Version Luteal (after egg release) Changes During the Menstrual Cycle The menstrual cycle is regulated by the complex interaction of hormones: luteinizing hormone, follicle-stimulating hormone, and the female sex hormones estrogen and progesterone. The menstrual cycle has three phases: Follicular (before release of the egg) Ovulatory (egg release) Luteal (after egg release) The menstrual cycle begins with menstrual bleeding (menstruation), which marks the first day of the follicular phase. When the follicular phasebegins, levels of estrogen and progesterone are low.

As a result, the top layers of the thickened lining of the uterus (endometrium) break down and are shed, and menstrual bleeding occurs. About this time, the follicle-stimulating hormone level increases slightly, stimulating the development of several follicles in the ovaries. Each follicle contains an egg. Later in this phase, as the follicle-stimulating hormone level decreases, only one follicle continues to develop. This follicle produces estrogen. The ovulatory phase begins with a surge in luteinizing hormone and follicle-stimulating hormone levels. Luteinizing hormone stimulates egg release (ovulation), which usually occurs 16 to 32 hours after the surge begins. The estrogen level peaks during the surge, and the progesteronelevel starts to increase. During the luteal phase,luteinizing hormone and follicle-stimulating hormone levels decrease. The ruptured follicle closes after releasing the egg and forms a corpus luteum, which produces progesterone. During most of this phase, the estrogen level is high. Progesterone and estrogencause the lining of the uterus to thicken more, to prepare for possible fertilization.

If the egg is not fertilized, the corpus luteum degenerates and no longer produces progesterone, the estrogenlevel decreases, the top layers of the lining break down and are shed, and menstrual bleeding occurs (the start of a new menstrual cycle). Follicular phase This phase begins on the first day of menstrual bleeding (day 1). But the main event in this phase is the development of follicles in the ovaries. At the beginning of the follicular phase, the lining of the uterus (endometrium) is thick with fluids and nutrients designed to nourish an embryo. If no egg has been fertilized, estrogen and progesterone levels are low. As a result, the top layers of the endometrium are shed, and menstrual bleeding occurs. About this time, the pituitary gland slightly increases its production of follicle-stimulating hormone. This hormone then stimulates the growth of 3 to 30 follicles. Each follicle contains an egg. Later in the phase, as the level of this hormone decreases, only one of these follicles (called the dominant follicle) continues to grow. It soon begins to produce estrogen, and the other stimulated follicles begin to break down.

The increasing estrogen also begins to prepare the uterus and stimulates the luteinizing hormone surge. On average, the follicular phase lasts about 13 or 14 days. Of the three phases, this phase varies the most in length. It tends to become shorter near menopause. This phase ends when the level of luteinizing hormone increases dramatically (surges). The surge results in release of the egg (ovulation) and marks the beginning of the next phase. Ovulatory phase This phase begins when the level of luteinizing hormone surges. Luteinizing hormone stimulates the dominant follicle to bulge from the surface of the ovary and finally rupture, releasing the egg. The level of follicle-stimulating hormone increases to a lesser degree. The function of the increase in follicle-stimulating hormone is not understood. The ovulatory phase usually lasts 16 to 32 hours. It ends when the egg is released, about 10 to 12 hours after the surge in the level of luteinizing hormone. The egg can be fertilized for only up to about 12 hours after its release.

The surge in luteinizing hormone can be detected by measuring the level of this hormone in urine. This measurement can be used to determine when women are fertile. Fertilization is more likely when sperm are present in the reproductive tract before the egg is released. Most pregnancies occur when intercourse occurs within 3 days before ovulation. Around the time of ovulation, some women feel a dull pain on one side of the lower abdomen. This pain is known as mittelschmerz (literally, middle pain). The pain may last for a few minutes to a few hours. The pain is usually felt on the same side as the ovary that released the egg, but the precise cause of the pain is unknown. The pain may precede or follow the rupture of the follicle and may not occur in all cycles. Egg release does not alternate between the two ovaries and appears to be random. If one ovary is removed, the remaining ovary releases an egg every month. Luteal phase This phase begins after ovulation. It lasts about 14 days (unless fertilization occurs) and ends just before a menstrual period.

In this phase, the ruptured follicle closes after releasing the egg and forms a structure called a corpus luteum, which produces increasing quantities of progesterone. The progesterone produced by the corpus luteum prepares the uterus in case an embryo is implanted. The progesterone causes the endometrium to thicken, filling with fluids and nutrients to nourish a potential embryo. Progesterone causes the mucus in the cervix to thicken, so that sperm or bacteria are less likely to enter the uterus. Progesterone also causes body temperature to increase slightly during the luteal phase and remain elevated until a menstrual period begins. This increase in temperature can be used to estimate whether ovulation has occurred . During most of the luteal phase, the estrogen level is high. Estrogen also stimulates the endometrium to thicken. The increase in estrogen and progesterone levels causes milk ducts in the breasts to widen (dilate). As a result, the breasts may swell and become tender. If the egg is not fertilized or if the fertilized egg does not implant, the corpus luteum degenerates after 14 days, levels of estrogen and progesterone decrease, and a new menstrual cycle begins.

If the embryo is implanted, the cells around the developing embryo begin to produce a hormone called human chorionic gonadotropin. This hormone maintains the corpus luteum, which continues to produce progesterone, until the growing fetus can produce its own hormones. Pregnancy tests are based on detecting an increase in the human chorionic gonadotropin level. Resources In This Article Female Reproductive System: Organs, Function, and More What Parts Make up the Female Anatomy? continued... The internal reproductive organs in the female include: Vagina : The vagina is a canal that joins the cervix (the lower part of uterus) to the outside of the body. It also is known as the birth canal. Uterus (womb): The uterus is a hollow, pear-shaped organ that is the home to a developing fetus . The uterus is divided into two parts: the cervix, which is the lower part that opens into the vagina, and the main body of the uterus, called the corpus. The corpus can easily expand to hold a developing baby . A channel through the cervix allows sperm to enter and menstrual blood to exit.

Ovaries: The ovaries are small, oval-shaped glands that are located on either side of the uterus. The ovaries produce eggs and hormones. Fallopian tubes: These are narrow tubes that are attached to the upper part of the uterus and serve as tunnels for the ova (egg cells) to travel from the ovaries to the uterus. Conception , the fertilization of an egg by a sperm, normally occurs in the fallopian tubes. The fertilized egg then moves to the uterus, where it implants into the lining of the uterine wall. What Happens During the Menstrual Cycle? Females of reproductive age experience cycles of hormonal activity that repeat at about one-month intervals. With every cycle, a woman's body prepares for a potential pregnancy , whether or not that is the woman's intention. The term menstruation refers to the periodic shedding of the uterine lining. (Menstru means "monthly.'') The average menstrual cycle takes about 28 days and occurs in phases: the follicular phase, the ovulatory phase ( ovulation ), and the luteal phase.

There are four major hormones (chemicals that stimulate or regulate the activity of cells or organs) involved in the menstrual cycle: follicle-stimulating hormone, luteinizing hormone , estrogen , and progesterone . Female Hormonal Cycle ABSTRACT The female reproductive cycle is an example of a real biological oscillating system where the periodic nature of the system may be described by a set of difference equations. The model is developed using blood-level of three different ovarian and gonadotropic hormones, estrogen, progesterone, and luteinizing hormone. We construct this model using actual sample data describing the levels of these hormones present in the body throughout the month, then discuss the applications of such research. BACKGROUND The female reproductive cycle is an intricate series of chemical secretions and reactions, and delicate changes in physical anatomy, which produce the ultimate potential for fertility and birth. These chemicals, known as hormones, are human proteins with the capacity to affect and regulate human growth and function. The female reproductive cycle is a general term which encompasses the ovarian cycle, the uterine cycle, the hormonal changes that regulate them, and cyclical changes in the breasts and the cervix.

The ovarian cycle is the term used to describe the series of events associated with a developing oocyte, or, egg cell, within the ovaries, while the uterine cycle is the preparation of the environment of the uterus for the reception of a fertilized ovum. The regulation of both the ovarian and uterine cycles is the function of several gonadotropic and ovarian hormones. Among the most important of the gonadotropins are luteinizing hormone (LH) and follicle stimulating hormone (FSH), while the most crucial ovarian hormones are the androgens, estrogen and progesterone. The hypothalamus-derived hormone in control of the gonadotropins, and indirectly, all female sexual function, is gonadotropin releasing hormone (GNRH). The release of GNRH in an adolescent female marks the commencement of that female's reproductive capability, and ultimately, the onset of menstruation (Tartora 938-40). Among the most important aspects of the female cycle is the periodic nature in which it occurs. From the time a young woman begins to menstruate, until the time that woman reaches menopause, hormonal pulses and secretions occur extremely regularly, varying little from month to month.

This periodicity is the characteristic on which this research capitalizes, since it is the periodicity which we may describe using several mathematical difference equations. These equations are functions of three of the key hormones, luteinizing hormone, estrogen, and progesterone. These hormones were chosen over the rest of the involved hormones for their paramount roles in the regularity of the cycle. It is necessary to understand the biological proceedings which these equations describe. The duration of the female reproductive cycle is typically 24-35 days. This research will assume the average 28-day cycle, in the absence of fertilization and pregnancy, divided into three phases; the menstrual phase, the preovulatory phase and the postovulatory phase. It is the preovulatory phase that is the most variable phase lasting anywhere from 6-13 days and accounts for most of the time difference when cycles are either shorter or longer than the 28-day average, while the postovulatory phase is almost always 14 days in length (Tartora 938). These proceedings begin within the ovaries of a developing female. The ovaries contain oocytes in various stages of development.

In the preliminary stages of development an oocyte contains only a single layer of surrounding cells. These cells are referred to as follicle cells. As the egg matures further, and multiple layers of surrounding cells form, the cells are then referred to as granulosa cells. The surrounding cells are responsible for nourishing the developing oocyte, and, as the follicle grows larger, for the production and secretion of estrogen. Estrogen, then, is produced at regular levels throughout childhood. It is during a girl's childhood that GNRH is inhibited from being secreted by the negative feedback effect that estrogen has on the midbrain gland responsible for the output of GNRH, the hypothalamus. As a girl matures toward puberty, the hypothalamus undergoes significant changes in its childhood sensitivity to the ovarian hormone and is stimulated toward the release of GNRH, in a rhythmic, pulse-like manner. As the pulses become stronger and stronger, the release of GNRH into the brain positively effects the anterior pituitary gland to commence the production and secretion of the human gonadotropins, LH and FSH.

Continued production of FSH and LH stimulates growth and maturation of the follicles inside the ovaries, and the first preovulatory phase of the female menstrual cycle begins (Marieb 979-81). Since the follicular cells are responsible for the output of estrogen, LH and FSH, in turn, promote estrogen secretion in preparation for ovulation. At this point in the cycle, there are changes that take place within the ovaries. A single primary oocyte is targeted to resume the halted stages of meiosis and develop into a mature follicle, a fluid-filled follicle that will soon rupture and expel a haploid secondary oocyte in the process of ovulation. Maturation of the follicle means increased production of estrogen. This increase in estrogen serum levels exhibits a negative feedback on the anterior pituitary inhibiting the release, but stimulating the production and storage of the gonadotropins. With the cessation of gonadotropin output, estrogen levels continue to rise; and, while the initial small rise in estrogen inhibits gonadotropin release, high estrogen levels have the opposite effect.

By the time the ovarian estrogen production reaches its maximum before day 14, the negative feedback effect on the anterior pituitary is reversed to a dramatically positive one and a burst-like surge of stored LH is released (Marieb 982). The LH surge is actually responsible for affecting the final development, and subsequent rupture of the mature follicle. LH also induces the follicle cells to secrete digestive enzymes thereby enabling the secondary oocyte to pass through the ovary wall marking ovulation at day 14 of the cycle (Tartora 940). As the secondary oocyte actually moves through the ovary wall, the outer layer of cells -- the cells responsible for estrogen secretion -- are damaged by the rough contact. Estrogen output is then halted and serum levels drop slowly for a short period immediately following the ovulatory process, and the beginning of the postovulatory phase commences (Marieb 982). The extraordinarily high levels of LH now have another function. Once the secondary oocyte and its remaining surrounding cells are swept into the fallopian tube, luteinizing hormone works to establish the corpus luteum from the remnants of the ruptured mature follicle inside the ovary.

The corpus luteum, sometimes referred to as the yellow body, actually stems from a blood clot that develops as the mature follicle ruptures in ovulation. The remaining follicular cells that then enlarge and change character absorb the clot. Under the influence of LH, these enlarged cells become the corpus luteum. The corpus luteum is a gland which maintains the responsibility of both the production and secretion of estrogen and progesterone. As estrogen and progesterone levels again rise, their combined effect on the anterior pituitary's production of the gonadotropins is extremely negative and LH levels decline significantly. Since it is LH that is responsible for the maintenance of the corpus luteum, the marked decline in LH levels also (assuming that the oocyte has not, in the meantime, been fertilized) heralds the degeneration of the corpus luteum into a scar called the corpus albicans. With the corpus luteum no longer producing progesterone and estrogen, the serum levels of the ovarian hormones decline sharply and the menstrual phase may then begin (Tartora 940-1).

At this time, the uterine endometrial lining, which had been developing anticipating a pregnancy, is shed in the form of 50-150 ml of blood, mucus, tissue fluid, and epithelial cells. The menstrual flow moves out the uterine cavity to the cervix, then through the vagina to the exterior. The marked decline of the ovarian hormones at the end of the cycle also ends their blockade of the hypothalamic-pituitary axis, thus allowing once again for the release of the gonadotropins (Tartora 939). The cycle then begins anew with the initial climb of LH stimulating the follicles within the ovary to develop toward maturation. After the first menstrual cycle a young woman experiences, the menstrual phase, by convention, marks the beginning of each new cycle. The events of the female reproductive cycle may be easily understood if associated with a timeline. The menstrual cycle lasts roughly five days -- days one through five the average cycle. By day six, the follicle within the ovary is actively secreting estrogen, until estrogen serum levels reach their threshold value at around day 11.

Between days 11 and 14 the LH surge is actively promoting oocyte division and follicular rupture so that ovulation may occur at day 14 of the cycle. The corpus luteum is generally established by day 15, and LH levels have already declined by the end of the 16th day. It then takes about 10 days for the corpus luteum to degenerate and estrogen and progesterone to begin their decline at around day 26 of the cycle. In the next two days LH levels are elevated and follicles stimulated to mature so that by the beginning of the new cycle, estrogen levels are once again on the rise (Marieb 982). In summary then, the hormonal cycle may be viewed as the following series of events: 1. Luteinizing hormone rises and stimulates the follicle to produce estrogen 2. As estrogen is produced by the follicle, estrogen levels rise inhibiting the output of LH 3. Estrogen levels continue to rise to the threshold level at which point the negative feedback is reversed and LH surges 4. Ovulation occurs just after the LH surge damaging the estrogen-producing cells which results in a decline in estrogen 5.

The LH surge results in the establishment of the corpus luteum -- an estrogen and progesterone secreting gland 6. Estrogen and progesterone serum levels climb steadily, supressing LH output 7. Lack of LH promotes the degeneration of the corpus luteum 8. Cessation of corpus luteum activity means a decline in estrogen and progesterone output 9. The decline in the ovarian hormones ends their negative effect on the secretion of LH 10. LH is secreted and the cycle begins again (Marieb 982). Knowing these basic trends is paramount in translating the biological activities associated with the hormonal cycle into purely mathematical statements ï¿½ a system of difference equations. After careful analysis of hormone serum levels found in a compilation of experimental graphs, the decision to use difference equations to describe the cycle stems from the obvious dependence of each particular daily hormone serum level on the level of that hormone the previous day.

Also of notable importance is the apparent postovulatory exponential growth of the progesterone curve, the mid-cycle luteinizing hormone spike, and the seemingly exponential growth of estrogen in the beginning of the cycle, followed by a second postovualtory rise concurrent with the rise in progesterone. These general trends served as a necessary guideline, while care was taken to match equation-generated values to those determined experimentally. The following set of difference equations is the product of that quest. The equations, which approximate the periodicity of the cycle, represent general hormonal trends in terms of estrogen (E), luteinizing hormone (L), and progesterone (P), where "n" represents cycle day (Ln = luteinizing hormone level on day "n"). Women's Reproductive Health Information | Hormone Health Network strenuous exercise that burns more calories than are taken in through food some medical conditions or illnesses Amenorrhea can also be caused by benign tumors in the pituitary gland, obesity, polycystic ovary syndrome, and adrenal gland disorders.

(The adrenals are two small glands located above each kidney that produce some sex hormones.) Polycystic ovary syndrome (PCOS) is a common reproductive problem that sometimes causes amenorrhea. However, most women with PCOS have irregular menstrual periods, not amenorrhea. Treatment Treatment of amenorrhea varies depending on the cause. It could involve changes in lifestyle such as gaining or losing weight, exercising less intensely, or reducing stress, medication, surgery to correct abnormalities in reproductive organs, or a combination of approaches. Some women take birth control pills to regulate their periods. These medications, which combine estrogen and progesterone, maintain balanced levels of the hormones in the body. Prolonged amenorrhea may increase the risk of bone loss, so taking calcium and vitamin D supplements is recommended. Women with hypothalamic amenorrhea also need to gain weight or correct other underlying problems to keep their bones strong. Female infertility Infertility is the inability of a sexually active couple, not using birth control, to get pregnant after one year of trying. About one quarter of female infertility is caused by a problem with ovulation.

Normally, each month an egg matures within its own follicle (a sort of bubble) in the ovary and is released into the fallopian tube. This process of ovulation is the fertile time of the menstrual cycle. Reasons for infertility Lack of ovulation or infrequent ovulation: Absent or irregular ovulation is a common cause of infertility, and can be caused by polycystic ovary syndrome (PCOS) hypothalamic amenorrhea problems that cause the pituitary to release too much prolactin, a hormone that affects estrogen levels primary ovarian insufficiency Age: There is a gradual decrease in egg quantity and quality beginning when a woman is in her mid 30s, or about 10 years before menopause. Other hormone-related conditions: An overactive or underactive thyroid gland, diabetes, obesity, and, occasionally, Cushing’s syndrome (a disorder of the adrenal glands) can affect fertility. Other causes of infertility include tubal blockages, endometriosis, and male factor infertility. Treatment of infertility Infertility due to ovulation problems is treated with fertility drugs. These drugs, in general, work like natural hormones, such as FSH and LH, to bring about ovulation.

Some commonly used fertility drugs are: Clomiphene: this oral medication causes the pituitary gland to release more FSH and LH. Gonadotropin therapy: these injected medications contain either FSH or both FSH and LH. These drugs stimulate the ovaries to develop egg-containing follicles. Most women need FSH only, but women with hypothalamic amenorrhea need both LH and FSH. Human chorionic gonadotropin (hCG): used in combination with clomiphene and gonadotropin therapy, this drug, which is also given by injection, stimulates the follicle to release its egg (i.e., to ovulate). Bromocriptine and cabergoline: these oral medications are used to treat infertility caused by the body’s production of too much prolactin. Too much prolactin can cause amenorrhea. Polycystic ovary syndrome (PCOS) and infertility PCOS is a leading cause of infertility in women (for a detailed discussion, see our Polycystic Ovary Syndrome overview).

Although not all women with PCOS have all these features, common signs and symptoms include irregular menstrual cycles excess hair growth on the face and body infertility (due primarily to lack of ovulation) overweight or obesity (especially at the waist) On ultrasound, the ovaries appear to have a ring of small cysts. These are painless and do not become large cysts. Normally, a woman’s ovaries and adrenals produce small amounts of male hormones (androgens), but women with PCOS have increased androgen levels. Higher levels of androgens lead to an imbalance in the hormones that regulate ovulation and normal egg development. Obesity, which is common in women with PCOS, contributes to infertility. Thus, weight loss, if needed, and treatment for PCOS with medications can restore ovulation. Primary ovarian insufficiency (POI) and infertility POI (previously known as premature ovarian failure (POF), or early menopause), occurs when the ovaries stop working normally before a woman turns age 40. When this happens, women don’t ovulate on a regular basis or produce normal amounts of estrogen. The most common signs of POI are irregular or missed periods, along with hot flashes.

Other symptoms may include night sweats irritability difficulty concentrating In most cases, the cause of POI is unknown. It is sometimes linked to genetic disorders such as Turner syndrome and Fragile X syndrome, conditions that are due to changes to the X chromosome. The risk of developing POI is greater in women with a family history of the disorder. Although some women with POI become pregnant, the chance of success is very low. Research is ongoing to identify treatments for restoring fertility in women with POI. Hormone therapy may be used to relieve menopause symptoms (such as hot flashes, night sweats, vaginal dryness) or to prevent osteoporosis (brittle bones). Menopause Menopause is the time in a woman’s life when her menstrual cycles end and she can no longer get pregnant naturally. As a woman nears age 50 or so, the ovaries are close to running out of eggs, estrogen levels begin to drop, and the first signs of menopause may appear. The menopausal transition takes an average of 4 to 5 years, and during that time hormone levels are likely to fluctuate widely. Periods may become irregular, but a woman may still be able to get pregnant.

Once the ovaries have stopped producing eggs, however, fertility ends. After menopause, women may experience sexual dysfunction as a result of lower levels of hormones. For a detailed discussion of symptoms, treatment options, and the pros and cons of treatment, see our menopause section. Female sexual dysfunction: hormone-related causes Female sexual dysfunction is defined as a lack of interest in or enjoyment of sexual activity that is distressing to a woman. It can result from a loss of sex drive (libido), an inability to become aroused or to reach an orgasm, or painful intercourse. A combination of many personal, interpersonal, and medical factors may contribute to sexual dysfunction. Physical causes may include health issues like diabetes, heart disease, nerve disorders, or hormone problems. Estrogen results in increased blood flow to the genitals and vaginal lubrication during arousal, and, because of its impact on mood and sleep, indirectly affects sexual interest. With menopause or menopause-like conditions, estrogen levels drop significantly, leading to physical changes (such as thinning vaginal tissue, dryness), that can dampen a woman’s sex drive. Androgens (male hormones) are also believed, but not proven, to be important for libido in women.

A hormonal imbalance may or may not contribute to any one woman’s low sex drive. But if a woman does have a hormonal cause for sexual dysfunction, treatment might include hormone therapy. Vaginal forms of estrogen— a cream, tablet, or vaginal ring—can improve vaginal tone, elasticity, and lubrication. Estrogen applied directly to the vagina is more effective at a lower dose than estrogen in pill or skin patch form. Also, the small amount of estrogen in these products is believed to pose less risk for heart attack, stroke, and breast cancer than pill or patch hormone therapy, which affects the whole body. Androgen therapy for female sexual dysfunction is an emerging area of research. Androgens are male hormones, such as testosterone. Women produce testosterone, although in much smaller amounts than men. Testosterone production drops gradually after about age 30, and blood levels are considerably lower in postmenopausal women. Surgically menopausal women (who have had their ovaries removed) have even lower testosterone levels than women who go through natural menopause and are thus more likely to experience sexual dysfunction. To date, however, the U.S. Food and Drug Administration (FDA) has not yet approved testosterone-containing medications for women with sexual dysfunction.

Loss of libido, called hypoactive sexual desire disorder (HSDD), describes low or absent sexual interest or desire that causes marked distress or relationship problems and is not caused by a medical condition or drug. Currently, there is no FDA-approved treatment for HSDD but several products are being studied. These a testosterone gel absorbed through the skin and DHEA vaginal suppositories. (DHEA, or dehydroepiandrosterone, is a hormone produced by the adrenal glands that is converted to testosterone and estrogen.) Although not approved in the U.S., a testosterone patch is available in other countries as a treatment for HSDD. Editors: Hormones of the Female Reproductive System | LIVESTRONG.COM Hormones of the Female Reproductive System by CAROLE WEGNER Last Updated: May 20, 2010 Carole Wegner Carole Wegner is a Ph.D. scientist and in-vitro fertilization lab director in the Midwest. For more than 20 years, she has published scientific findings in peer-reviewed journals such as "Endocrinology" and "Fertility & Sterility" and also written on the topic of ethics in reproductive medicine. Female hormones regulate all phases of female life.

Photo Credit Yellow Florial Female image by Vojsek from <a href="">Fotolia.com</a> The female reproductive system is primarily regulated by five hormones including estrogen, progesterone, gonadotropin releasing hormone, follicle stimulating hormone, and luteinizing hormone. These hormones play a role in one or more stages of development and function of the female reproductive system. Estrogen Ovaries produce three varieties of estrogen called estradiol, estrone, and estriol. Estrogens play a role in development of secondary sex characteristics that appear at puberty. Estradiol plays a role in breast development and promotes fat distribution to breasts, hips and legs. Estrogens are produced by follicles within the ovary and help regulate the menstrual cycle. Estrogen promotes the rapid growth of cells lining the uterus called the endometrium in preparation for possible implantation and pregnancy. At the end of pregnancy, high levels of estrogen induce the ovaries to produce oxytocin which stimulates uterine contractions. Estrogen also has non-reproductive functions. Estrogen prevents abnormal blood clotting which maintains heart health. In addition, estrogen helps bones retain calcium which keeps them strong.

The natural decline of estrogen with aging often requires medical supplementation with exogenous estrogen to prevent declining heart health and osteoporosis. Progesterone Progesterone is also produced by the ovaries and levels fluctuate during the menstrual cycle. After ovulation, the follicle collapses and reorganizes itself into a new steroid secreting gland called the corpus luteum. The corpus luteum produces progesterone. Progesterone maintains the health of the uterine lining during pregnancy. If pregnancy occurs, the implanting embryo produces human chorionic gonadotropin, or hCG, which keeps the corpus luteum functional so that it can continue secreting progesterone to support the pregnancy until the placenta is developed enough to take over progesterone production. If there is no pregnancy, the corpus luteum dies, progesterone levels fall and the uterine lining is shed during menses. Gonadotropin Releasing Hormone The monthly hormonal changes and development of a mature egg that occur with the monthly cycle are initiated in the brain by secretion of gonadotropin releasing hormone, or GnRH.

GnRH in turn stimulates the production of follicle stimulating hormone, or FSH and luteinizing hormone, or LH which locally regulate follicular function in the ovary. FSH and LH FSH stimulates the follicles within the ovaries to produce increasing amounts of estrogen and small amounts of progesterone during the first two weeks of the menstrual cycle. Under the influence of FSH, multiple follicles grow until one or two follicles become dominant, secreting the most estrogen. As estrogen levels rise in the ovary, they are also released back into the blood system and return to the brain where estrogen signals the brain to stop producing FSH. The rising estrogen levels also signal another gland in the brain, the pituitary gland, to release luteinizing horomone which signals the dominant follicle to release its egg for possible fertilization. Related Searches Lose Weight.

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Question: Which hormone controls the female reproductive cycle?
Answer: Oestrogen