Reproductive System, Female
Reproductive System, Female
The female reproductive system is composed of organs that produce female eggs (called female gametes or ova), provide an environment for fertilization of the egg by a male sperm (male gamete), and support the development and expulsion of a fetus in pregnancy and childbirth.
The normal female reproductive system is composed of external and internal genitals (genitalia).
The external genitals (together, they are called the "vulva") are composed of the genital structures visible from outside the body: the greater lips (labia majora), the lesser lips (labia minora), the clitoris, and the opening of the vagina to the outside (the other end of the vagina opens inside the body to the uterus). The labia majora are two large lips that protect the other external genitals. The outer surface of these lips is covered with oil-secreting (sebaceous) glands; their inner surface has hair. The lesser lips (labia minora) are found just inside the greater lips and protect the immediate opening to the vagina (this opening is called the "introitus," Latin for "entrance") and the opening to the urethra (which carries urine from the bladder out of the body). The clitoris is a small structure found at the top of the lesser lips; it is very sensitive to stimulation and may become erect. The perineum is the area between the vagina and the anus in the female (in the male, the perineum is the area between the scrotum and the anus). Two glands, one located on either side of the introitus, are called Bartholin's glands. They secrete a mucus that provides lubrication during sexual intercourse.
The internal genitals are the vagina, the uterus (womb), the fallopian tubes, and the ovaries. The vagina extends approximately 3 to 4 inches (7 to 10 cm) from the outside of the body to the opening of the uterus. The lower third of the vagina (closest to the outside) is encircled by muscles that control its opening and closing. The uterus is the organ found at the top of the vagina and consists of two main parts: the neck (cervix) and the body (corpus). The neck is the opening of the uterus to the vagina that allows sperm to enter the uterus and allows menstrual fluid to exit. The neck is an important means of protecting the body of the uterus from disease-causing germs; a thick mucus normally covers the neck of the uterus but changes in consistency during ovulation to allow sperm to penetrate. The body of the uterus is the main part of the uterus. It can enlarge to hold a developing fetus during pregnancy. The inner lining of the body of the uterus is called the endometrium, which thickens and then sheds menstrual fluid during each menstrual period if fertilization does not occur.
The fallopian tubes (also called the oviducts or uterine tubes) are muscular structures that extend from the upper edges of the uterus to the ovaries. The fallopian tubes facilitate the transfer of a mature egg from one of the two ovaries to the body of the uterus. A fallopian tube is the site of normal fertilization. The ovaries are a pair of small oval-shaped structures and are suspended near the fallopian tubes by ligaments. A human female will not produce any new developing eggs (oocytes) after she is born. Although she is born with approximately two million eggs, only about 300,000 to 400,000 remain at onset of puberty, and only about 300 of these will develop fully and enter a fallopian tube for possible fertilization. The eggs start as oocytes and develop in what are called ovarian or Graafian follicles, small spherical sacs that burst when the mature egg (called an ovum) is ready to be released into a fallopian tube for possible fertilization, or for discharge in the menstrual fluid if fertilization does not take place.
The human egg is a round cell that, when mature, is surrounded by a number of protective layers (the oolemma, zona pellucida, and zona radiata). It contains half the number of chromosomes of a human cell that is not egg or sperm (that is, 23 instead of 46 chromosomes) and is therefore called a haploid (one-fold) cell. When the egg is fertilized by sperm, the resulting cell will have the full number of 46 chromosomes and will be considered a diploid (two-fold) cell.
The menstrual cycle ranges from 21 to 40 days in most women, with an average cycle lasting 28 days. The first time a girl has a period (the onset of menstruation) is called "menarche"; the permanent cessation of menstruation some decades later is called "menopause" and marks the traditional end of a woman's ability to reproduce. In the 1990s, women past menopause have been impregnated with another woman's egg after it has been fertilized by artificial insemination, and these older women have successfully given birth to healthy babies.
Menstruation occurs when the lining of the uterus begins to shed menstrual fluid; the first day of bleeding is the first day of the menstrual cycle. The menstrual cycle has two phases. The follicular phase extends from the first day of the cycle until immediately before a mature egg gets released from the ovary.
In the second phase of the menstrual (ovulatory) cycle, called the "luteal" phase, the mature follicle bursts and releases an egg, a process called ovulation. The second phase of the menstrual cycle lasts approximately fourteen days until the first day of the next period (using as an example the average 28 day menstrual cycle). The ruptured empty follicle collapses to form the corpus luteum.
During the ovulatory phase of the menstrual cycle, the mature egg is released from the ovary and swept into the fallopian tube. If sperm cells are present in the fallopian tube, fertilization may occur. Pregnancy begins at the moment of fertilization (also called conception), when the sperm penetrates the egg. The fertilized egg, also called a zygote, then begins to move down the fallopian tube into the uterus, where it implants itself in the thick tissue of the lining of the uterus. In the uterus, this replicating cluster of cells is called a blastocyst; after two weeks of development, it is called an embryo; eight weeks after conception, it is called a fetus.
A complex balance of hormones is required for reproduction. There are two main groups of hormones that are necessary for normal functioning of the female reproductive system.
The first group contains hormones of the central nervous system (CNS). A part of the brain called the hypothalamus is the main area of hormonal control; it secretes so-called releasing hormones that travel to the pituitary gland located at the base of the brain. Gonadotropin-releasing hormone (GnRH) secreted by the hypothalamus triggers the release of gonadotropic hormones from the anterior pituitary gland. Gonadotropin refers to any hormone that stimulates the gonads (the structures capable of producing eggs or sperm, that is, the ovaries or the testicles), regulates their development and their hormone-secreting functions, and contributes to the production of eggs or sperm.
There are two gonadotropic hormones secreted by the anterior pituitary gland: the follicle-stimulating hormone (FSH) and the luteinizing hormone (LH). The development of the ovarian follicles is dependent upon these hormones. FSH (as its name suggests) stimulates the development of several follicles in each cycle. During the first half of the follicular phase, increasing levels of FSH cause maturation of ovarian follicles (only one follicle will mature completely). It is the LH that begins the second phase of the menstrual cycle, when a surge of LH causes the mature follicle to burst and release an egg. FSH and LH also control the production of ovarian hormones (the second group of hormones regulating the female reproductive system).
The ovarian hormones in turn are divided into two groups: ovarian peptide hormones and ovarian steroid hormones.
There are two ovarian peptide hormones, inhibin and relaxin. Inhibin is secreted by the granulosa cells of the follicles. It inhibits the releasing of FSH from the anterior pituitary gland and also inhibits the release of GnRH from the hypothalamus. Thus inhibin has a role in controlling further follicular development. Relaxin is produced near the end of pregnancy by the corpus luteum and promotes relaxation of the birth channel.
There are two biologically extremely active ovarian steroid hormones: estrogen and progesterone. Estrogen is produced by the granulosa cells of developing follicles and by the corpus luteum following ovulation. This production of estrogen is dependent upon luteinizing hormone (LH). The most potent estrogenic hormone in human beings is estradiol. It is synthesized and secreted by ovarian follicles, specifically by the theca interna cells (these cells synthesize androstenedione, which is then converted into estradiol and estrone). Estradiol can also be synthesized by the fetoplacental unit and, perhaps, by the adrenal cortex. It has the following biological functions: to promote the growth and maturation of the female secondary sex characters, to induce estrus in conjunction with progesterone to prepare the endometrium for implantation of a fertilized ovum, and to support pregnancy.
Progesterone is a hormone produced by the corpus luteum. (It can also be secreted by the placenta and by the adrenal cortex.) Together with estrogen, it prepares the endometrium for implantation of the fertilized ovum, it maintains the uteroplacentofetal unit, and it promotes the development of the fetus.
Another important endocrine organ secreting the steroid hormones (estrogen and progesterone) is the placenta. It helps maintain the uterine mucosa during pregnancy. The placenta also produces and secretes chorionic gonadotropic hormone. The actions of human chorionic gonadotropin (hCG) resemble those of LH. The presence of hCG in urine in early pregnancy is the basis of most pregnancy tests. Human chorionic gonadotropic hormone maintains the secretory integrity of the corpus luteum.
Common diseases and disorders
Infertility is diagnosed when a sexually active couple is unable to get the woman pregnant (or she is unable to carry a pregnancy to a successful childbirth) after one year of attempts. There are numerous reasons why infertility may occur:
- low number or lack of sperm cells produced by the male
- lack of ovulation (no eggs released from ovaries)
- abnormal fallopian tubes
- occurrence of what would normally be the lining of the uterus somewhere else than in the uterus (endometriosis), such as in the fallopian tubes
- problems with thick mucus in the neck of the uterus (Hence, sperm are not able to enter the uterus.)
A number of techniques may be used to assist a couple in getting the woman pregnant. These include fertilization in a dish (in vitro fertilization, IVF; in vitro is Latin for "in glass"). Eggs are removed from the woman, placed in a culture dish, and fertilized by sperm, then inserted into the uterus for implantation. An alternate technique is gamete intrafallopian transfer, or GIFT. Male and female reproductive cells are removed from the man and woman and then transferred to the fallopian tube where fertilization may take place naturally.
Cancer (uncontrolled and abnormal new growth of cells) may occur in any of the structures of the reproductive system, male or female. Common types of cancer in women include the following:
- Cancer of the uterus (uterine carcinoma). It is the most common cancer of the female reproductive system.
- Cancer of the neck of the uterus (cervical carcinoma). It may be caused by the sexually-transmitted human papillomavirus or HPV.
- Cancer of the ovaries (ovarian carcinoma). It has the highest death rate of all cancers of the female reproductive system.
- Cancer of the external genitals (vulvar carcinoma). It is usually a type of skin cancer.
- Cancer of the vagina (vaginal carcinoma). It may be caused by the sexually-transmitted human papillomavirus (HPV).
- Cancer of the fallopian tubes. It is the rarest cancer of the female reproductive system.
- Tumors that form in the uterus during or after pregnancy (hydatidiform moles).
Amenorrhea is the absence or abnormal stopping of menstrual periods. A number of factors may abnormally stop menstruation. They include abnormal production of LH and FSH, excessive exercise, extreme stress, and near starvation.
Painful menstruation, that is, menstruation with severe cramps or aches is called dysmenorrhea. It may be caused by excessive production of prostaglandins (the hormones that cause the uterus to contract forcefully at childbirth, thus squeezing the fetus into the vagina) or by diseased genitals.
Amenorrhea— Abnormal absence or stopping of menstrual cycles.
Dysmenorrhea— Painful menstruation.
Endometrium— The inner lining of the uterus.
External genitals— The greater lips (labia majora), the lesser lips (labia minora), the clitoris, and the opening of the vagina.
Follicle— A small spherical sac located in an ovary in which an oocyte develops and matures; when the follicle bursts, the mature egg (ovum) is released into the fallopian tube. Only about 300 follicles burst during a woman's lifetime.
Gamete— A one-fold (haploid, that is, having 23 instead of 46 chromosomes) cell involved in sexual reproduction; the male gamete is the sperm; the female gamete is the egg.
Internal genitals— The vagina, uterus, fallopian tubes, and ovaries.
Menarche— The first menstrual cycle in a girl's life.
Menopause— The permanent stopping of menstrual cycles, traditionally marking the end of a woman's ability to reproduce.
Menstruation— The discharge of the lining of the uterus (endometrium) as it sheds during the menstrual cycle when pregnancy does not take place.
Zygote— A two-fold (diploid, that is having 46 chromosomes) cell resulting from fertilization of the female egg by a sperm.
Premenstrual syndrome (PMS) occurs during the luteal phase of the menstrual cycle and is characterized by numerous symptoms. These include changes in mood and behavior, cramps, headaches, fluid retention, and fatigue. Approximately 40% of menstruating women complain of some sort of PMS.
Toxic shock syndrome (TSS) is a rare but devastating disease associated with tampon use. Although the exact cause of the disease is not known, it has been linked to infection by Staphylococcus aureus. If S. aureus enters the vagina, it is possible that tampon use could promote the growth of these deadly bacteria. S. aureus may then secrete poisons (toxins) that enter the bloodstream and lead to TSS. Symptoms start with fever, vomiting, diarrhea, and low blood pressure, but may eventually involve multiple organ systems and result in death.
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Hart, David M., and Jane Norman. Gynaecology Illustrated. London: Harcourt Publishers, 2000.
Shephard, Bruce D., and Carroll A. Shephard. The Complete Guide to Women's Health. New York: Penguin Books, 1997.
American College of Obstetricians and Gynecologists. 409 12th Street SW, P.O. Box 96920, Washington, DC, 20090-6920. 〈http://www.acog.org〉.
National Association for Women's Health. 300 West Adams Street, Suite 328, Chicago, IL 60606-5101. (312) 786-1468. 〈http://www.nawh.org〉.
Berkow, Robert, Mark H. Beers, Andrew J. Fletcher, and Robert M. Bogin, eds. "Female Reproductive System." The Merck Manual of Medical Information: Home Edition. 2001. 〈http://www.merck.com/pubs/mmanual_home/sec22/231.htm〉.