Anatomy & Physiology
Hormones and the Cycle
OsteoporosisAnatomy & Physiology
Anatomically, the female reproductive system consists of essential and accessory organs. The ovaries are essential to the production of eggs and hormones that initiate female secondary sexual characteristics and maintain normal reproductive function. The Fallopian tubes conduct the egg or (fertilized egg, the zygote) from the ovary to the uterus that is monthly changed into a habitable place for a fertilized egg. The cervix (narrowest portion of the uterus) serves as a gatekeeper to the body of the uterus. The vagina opens to the exterior in association with the external genitalia. Accessory glands participate in normal reproductive function. These include glands that produce mucus to lubricate the vagina and urethral opening.
These small oval-shaped glands are located on either side of the uterus supported by several ligaments. The ovary consists of 3 areas: 1) cortex, 2) medulla, 3) hilum. The cortex contains supportive cells, blood vessels, and developing follicles. The medulla contains connective tissue, smooth muscle, blood and lymph vessels and nerves. Nerves, blood vessels and connective tissue are found in the innermost portion, the hilum. The ovaries produce eggs(ova) and hormones.
The pear-shaped uterus opens to the vagina at the cervix and then widens toward the top where the Fallopian tubes enter the uterus. The uterus is a very muscular organ containing 3 layers of tissues. The interior layer, the endometrium, changes in thickness and secretory capability due to the influence of ovarian hormones over the course of the menstrual cycle. The myometrium, or muscle, is composed of 4 poorly defined layers of smooth muscle that is thickest at the top of the uterus. This makes for greater force during labor and delivery. The exterior of the uterus is covered with connective tissue. During pregnancy the baby (fetus) develops inside the uterus causing it to expand tremendously.
These narrow muscular tubes are attached to the upper outer angles of the uterus and serve as tunnels for the egg (ova) to travel from the ovaries to the uterus. Ova are captured by the infundibulum which has a wide webbed finger-like appearance, called fimbriae, near the ovary. Wave-like contractions create a current that moves the ovulated egg towards the tubular opening. Conception normally occurs in the tubes, with the fertilized egg then propelled to the uterus by the peristaltic contractions of the tubes and ciliary beating of the tubular epithlium to the uterus for implantation. Sometimes implantation will occur in the Fallopian tubes. Such an ectopic pregnancy is undesirable and must be treated immediately before the growing embryo causes rupture of the tube.
This muscular canal extends from the midpoint of the cervix to its opening located between the urethra and rectum. The mucous membrane lining the vagina and musculature are continuous with the uterus. The epithelium lining the vagina thickens and produces lubricating substances in response to estrogen. These secretions aid in sexual intercourse.
The breasts are milk producing glands located over the pectoral muscles consisting of a nipple, lobes, ducts and fibrous and fatty tissue. The nipple is surrounded by a pigmented, circular area (areola) and contains ductal openings. Nipple erection is produced with stimulation. The 15 to 25 lobes of each breast are further divided in lobules that are separated and supported by fibrous tissue. Each lobule contains small saclike aveoli surrounded by milk producing cells and small muscular cells. The muscular cells contract to express the milk during lactation. The lobules are drained by ducts that empty into a larger reservoir that lies just below the nipple. Reproductive hormones are important in the development of the breast in puberty and in lactation. Estrogen promotes the growth of the gland and ducts while progesterone stimulates the development of milk producing cells. Prolactin, released from the anterior pituitary, stimulates milk production. Oxytocin, released from the posterior pituitary in response to suckling, causes milk ejection from the lactating breast.
The first change to herald the coming of reproductive capability in females is the development of breasts. This is followed by the growth of axillary (underarm) and pubic (groin) hair and finally by the first menstrual period. Intitial periods are usually anovulatory (i.e. no egg released) with regular ovulation occurring within a year. The age at the time of puberty is variable. In the U.S. puberty occurs in girls around the age of 8 to age 13. Because of the individual variability in the onset of puberty, a delay cannot be considered pathological until menstruation has not begun sometime before the age of 17. Sometimes the delay is called primary amenorrhea and can be due to emotional stress, poor nutrition, weight loss or intensive athletic training.
Hormones & The Cycle
Females have four major hormones involved in the menstrual cycle: follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen (estradiol) and progesterone. FSH and LH are protein hormones produced by cells of the anterior pituitary within the brain, in response to small peptide hormones from the hypothalamus (hypothalamic releasing factors). These pituitary hormones travel in the blood to the ovary where they stimulate the development of one or more eggs, each within a follicle. A follicle consists of an ovum surrounded by cells responsible for the growth and nurturing of the ovum. As the cycle progresses, one follicle becomes dominant and all others regress. Estrogen, and progesterone to a lesser degree, are steroid hormones produced by cells of the developing follicle. Estrogen causes the endometrium to increase in thickness and vascularization (i.e.blood supply).
After ovulation (at the midpoint of the cycle), under the influence of LH, these same follicular cells shift to the production of progesterone. Progesterone causes the endometrial lining to become secretory and nutritive in anticipation of implantation of a fertilized egg. These four hormones are in a constant balance that shifts during progress through the menstrual cycle. The average menstrual cycle is 28 days, however only a very small percentage of cycles are exactly 28 days, most cycles range from 25-36 days.
The menstrual cycle can be divided into three phases: the follicular phase, the ovulatory phase, and the luteal phase. The follicular phase begins with the first day of menses (menstrual flow) and continues to approximately day 13 or 14 when ovulation takes place. During the follicular phase, FSH and LH are slowly rising in preparation for the LH surge (very high level of LH) at the time of ovulation. FSH is stimulating the growth of follicles in the ovary. Estrogen and progesterone are relatively low throughout this time but slowly begin to rise toward the end of this phase.
LH surges and peaks during the ovulatory phase (around day 14) and estrogen peaks at the same time. These peaks trigger ovulation. The ovum lives about 72 hours after ovulation, but it is fertilizable for only about 36 hours. Just before ovulation, progesterone levels begin to rise rapidly. Changes in cervical mucus accompany ovulation. The amount of mucus increases and it becomes clear and thin. This facilitates conception by aiding the passage of sperm through the cervical canal. Sperm can live for up to 72 hours in the female reproductive system. Therefore, the fertile period during a 28-day cycle is only about 4-5 days.
After the egg is released, the remainder of the follicle stays intact in the ovary and produces both estrogen and progesterone. This is called the corpus luteum (hence the luteal phase). The corpus luteum remains intact for the remainder of the cycle. The breast swelling, tenderness and pain experience by some is most likely due to the effects of progesterone on breast tissue.
Right after ovulation, the luteal phase begins and during this phase, progesterone levels are very high–progesterone is important during this phase because if the egg is fertilized, and implanted in the uterus, progesterone keeps the uterus intact so that the pregnancy is maintained. The continued health of the corpus luteum (progesterone secretion) is assured by the production of human chorionic gonadotropin (hCG) by the implanted embryo, until the placenta develops and can take over. The detection of hCG in urine is the basis of laboratory and home pregnancy tests.
If fertilization and implantation have occurred, than the corpus luteum will be stimulated by hCG to continue its production of estrogen and progesterone to maintain the pregnancy. This is important because the corpus luteum dies 14-22 days after ovulation if fertilization and implantation do not occur. With no progesterone to keep it intact, the lining of the uterus (the endometrium) is then shed, resulting in the monthly menstrual flow that normally lasts about 5 days. A variety of feminine products are available to help women during menses, including absorptive pads and tampons, deoderants, and vaginal cleansers.