Reproductive System, Female Health Article

Menstruation

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 womb 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.

Fertilization

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 womb, where it implants itself in the thick tissue of the lining of the womb. In the womb, 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.

Hormones

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.