Female Reproductive System

secondary sex characteristics. You should know that, in women, testosterone is also secreted by the female ovaries but in a smaller amount than is seen in males.

The endocrine system controls the production of testosterone. It all starts with the release of gonadotropin releasing hormone or GnRH from the hypothalamus. This happens steadily in males but is cyclical in females. The GnRH causes the pituitary gland to make follicle stimulating hormone or FSH and luteinizing hormone or LH. The FSH is responsible for Sertoli cell function and spermatogenesis. Sertoli cells make inhibin, which is a peptide hormone that feeds back to block FSH release. The LH triggers the Leydig cells to make testosterone.

The entire process is a type of negative feedback system. Low testosterone concentrations will increase GnRH production by the hypothalamus, which sets the process in motion. When testosterone levels are considered high enough, the testosterone will block both the GnRH release and the LH release. The same thing happens to the feedback loop involved in FSH secretion.

FEMALE REPRODUCTIVE SYSTEM

The female reproductive system is responsible for making the female gamete, which is called an oocyte. There are internal and external structures in the female reproductive system. Figure 7 shows the external genitalia in the female:

Figure 7.

The vulva is the female external genitalia. There is the mons pubis, which is the fat pad that covers the pubic bone in front. This will be covered with pubic air after puberty. The labia minora or larger lips are hair-covered folds on either side of the vagina. Just inside these are the labia minor or minor lips. Each of these protect the urethra and vagina.

Coming together in the front or anterior of the labia minora is the clitoris. This is embryologically the same tissue as the glans penis in men and is extremely sensitive with nerve endings. The clitoris is erectile and will swell with stimulation. It has a tissue

fold covering, called the prepuce. The clitoris is the most important structure in aiding female orgasm. There are corpora cavernosa, which are also erectile, extending on either side of the vulva.

The hymen is a membrane that covers, at least in part, the vaginal opening until it is disrupted, usually when the woman first has intercourse. The urethra is where urine exits. It is located in the anterior part of the vaginal introitus or vaginal opening. On either side of the vaginal opening are lubricating glands called Bartholin glands, which are not visible under normal circumstances. There are lesser mucus-secreting glands near the clitoris. The perineum is the tissue space between the vulva and the anus. This is the part that is cut when a woman has an episiotomy at the time of birth.

The vagina opens at the vaginal introitus. It is a ten-centimeter muscular tube that becomes the entrance to the reproductive tract. It is where the fetus and menstrual blood exit the body. There are rugae or transverse folds that make the surface rough in texture. The fornix is the very top of the vagina. This is where the cervix protrudes into the vagina. The cervix is smooth rather than rough, making it easier to identify. The muscles and rugae allow the vagina to expand during sexual excitement and childbirth.

The vagina is not a sterile environment but has a collection of normal bacteria and other organisms that prevent other, more serious organisms from gaining traction in the vagina. Most of these bacteria are called Lactobacillus, which secrete lactic acid as part of their metabolism. This has the effect of lowering the pH of the vagina. This acidity prevents the alternative growth of pathogenic bacteria. Douching will disrupt this milieu and can lead to an increase in infections.

The female gonads are called the ovaries. These are oval and located on either side of the uterus, being about two to three centimeters in length. There is part of the abdominal peritoneum called the mesovarium; this will help to support the position of the ovaries. There is also a suspensory ligament that helps to connect the ovary to its blood and lymph vessels. Third, there is an ovarian ligament that attaches the ovary to the uterus.

There is an outer epithelial layer on the ovarian surface, underneath which is the tunica albuginea, which is a tough layer of connective tissue around the ovary. Beneath this is the cortex, which is the framework for the germ cells and supportive cells. A combination of the oocyte plus its supportive cells is referred to as a follicle. Figure 8 is what an ovarian follicle looks like:

Figure 8.

The ovary goes through a natural ovarian cycle, which leads to a mature egg. The cycle happens only during the reproductive years and completes itself every twenty-eight days. This is not exactly the same thing as the menstrual cycle. The two parts of this cycle are related to oogenesis or egg-making and folliculogenesis or follicle-making.

As in the male testes, there are stem cells or germ cells called oogonia, which are formed in fetal life. They divide normally in the process of mitosis but become primary oocytes before birth. This is where they stop developing and remain until the process of meiosis resumes after puberty. Most of the egg cells are present at birth but decline to be about 400,000 in total at the time of puberty. This number becomes zero at the time of menopause.

Ovulation is the release of an oocyte from the ovary. This happens for the first time at puberty and again happens about every 28 days until menopause is reached. Luteinizing hormone triggers meiosis to continue in the primary oocyte to make the secondary oocyte. The main difference between men and women is that, while men create four equal sperm cells, the process of oogenesis gives rise to just one mature egg cell and three polar bodies, which are inactive cells that disintegrate.

Interestingly, the secondary oocyte does not complete meiosis until the sperm cell has penetrated the egg cell. These leads to a haploid ovum plus a polar body. The haploid ovum combines with the haploid sperm cell to become a zygote. As you can imagine, this haploid ovum state is very brief.

The female gamete or ovum is very large. It contains all the mitochondria and organelles not found in the sperm cell. Mitochondria have their own DNA, which is passed on to the offspring. In fact, mitochondrial DNA analysis can look specifically at the ancestry of a child along the maternal lines.

Folliculogenesis is the development and maturation of the oocytes and the supportive cells. One follicle matures every 28 days in women of reproductive age. There are other follicles developing at the same time but most of these undergo atresia, which is regression and disintegration. There are primordial follicles, primary follicles, secondary follicles, and tertiary follicles. The oocyte stays inside the follicle until the time of ovulation.

Primordial follicles are found in newborn babies and in adults. There is a single layer of supportive cells called granulosa cells around the oocyte. Primordial follicles often remain this way for years. After the time of puberty, some of the primordial follicles will be recruited to make primary follicles. The granulosa cells thicken and proliferate. These follicles become secondary follicles and a new layer of estrogen-producing cells called theca cells begin to do their job.

There is a thin membrane around the primary oocyte called the zona pellucida. This is the layer that gets broken down by the enzymes in the acrosome of the head of the sperm cell. Follicular fluid is made to create a space called the antrum, which enlarges the follicle. This tertiary follicle is called an antral follicle. There are several antral

follicles made per cycle but must will undergo atresia, leading to a dominant follicle. Only one percent of follicles will completely mature. Atresia can happen at any time in folliculogenesis.

There is a great deal of hormonal control over this ovarian cycle. It takes about two months from the time of the primordial follicle to the time of the tertiary follicle. It is the secondary oocyte that gets ovulated every month. There is GnRH, exactly as in men but it is released in a cyclical fashion by the hypothalamus. This leads to FSH and LH to be released by the anterior pituitary gland. FSH contributes to folliculogenesis, while LH stimulates the release of female sex hormones. There is a similar feedback loop regarding these hormones in females as there is in males. The feedback mechanism contributes to the survival of just one follicle per cycle.

The surviving follicle is called the dominant follicle, which continues to secrete estrogen. It creates a great deal of estrogen that overrides the feedback loop and actually becomes a positive feedback loop, leading to a surge in luteinizing hormone, called the LH surge. It is the LH surge that triggers ovulation. It also triggers protein-degrading enzymes to break down the follicle wall so the oocyte can be released in the process of ovulation.

After the oocyte has been released, there is a change in the granulosa cells and theca cells, which undergo luteinization, causing them to secrete progesterone as a dominant hormone. Then the follicle remnants are called the corpus luteum. Progesterone increases the woman’s body temperature slightly and will support the beginnings of a pregnancy. If no pregnancy happens, the corpus luteum stops making progesterone and the menstrual flow begins. Progesterone also suppresses the development of more dominant follicles. The corpus luteum ultimately degrades into a nonfunctional corpus albicans.

Figure 9 shows the internal structure of the female reproductive system:

Figure 9.

The uterine tubes are also called the oviducts or fallopian tubes. These are passageways for the egg from the ovary to the uterus and is where fertilization takes place. There are several parts to the fallopian tubes. The fimbriae are on the far outer side. They are fingerlike projections that embrace the ovary to sweep in the oocyte as it is being released. The oocyte enters the wider infundibulum and then the ampulla in the middle, where fertilization occurs. The isthmus is the connecting piece to the uterus.

There are cilia and contractions of the fallopian tubes that coordinate to push the otherwise immotile oocyte down the tube. You should know that the oocyte is actually ovulated into the peritoneal cavity. It is only because of the fimbriae activity that the egg is swept into the fallopian tube itself. The sperm travel up into the tube to fertilize the moving egg.

Once the oocyte is fertilized, the zygote successively divides into twos, fours, eights, etcetera. It will ultimately implant in the uterus. The egg that does not get fertilized degrades within one to two days after ovulation. Sperm cells survive longer than egg cells before disintegrating.

The uterus is where the embryo and fetus develop. It is about the size of a fist in the nonpregnant state and is highly muscular. There are three sections to it. The fundus is the top part above where the tubes enter the uterus. The body is the main, central part of

the uterus. The cervix is narrow and protrudes into the vagina. It produces secretions that change in character, depending on the stage of the woman’s menstrual cycle. When it becomes clear and stretchy, the woman can get pregnant as this fluid best supports the passage of sperm cells.

There are three layers in the uterus. The perimetrium is the outermost layer, which is basically a thin, serous membrane. The middle layer is the myometrium, which contains all the muscles of the uterus. These muscles stretch greatly in pregnancy and contract forcefully in labor and during the menstrual period. The endometrium or inner lining has two layers itself. The stratum basalis is near the myometrium and doesn’t shed each menstrual period but the stratum functionalis will shed each time the woman has her period. This inner layer changes greatly in character in different parts of the menstrual cycle.

The stratum functionalis thickens in the follicular or beginning part of the menstrual cycle. Once progesterone is made by the corpus luteum, the stratum functionalis changes in character to become secretory and more mature for implantation. If the corpus luteum degrades, the endometrium thins out and the spiral arteries feeding it constrict and break open. Prostaglandins, which are what cause menstrual cramping, contribute to this process. The stratum functionalis dies off and gets shed during menses. The first period a girl has is called menarche.

The menstrual cycle is considered different from the ovarian cycle but, of course, these are related. It refers to the changes in the endometrial lining and pelvic structures over the course of a month. The first day of bleeding is called day 1 of the menstrual cycle. A typical menstrual cycle is 28 days but this can be longer or shorter than that, depending on the women and specific circumstances. These difference in cycle length are generally due to shortening or lengthening of the first half of the cycle and not the second half, which is almost always 12 to 14 days in total length.

The menses phase is when a woman bleeds. This often lasts between 2 and 7 days, with an average of five days. The uterine lining is shed and the LH, FSH, and progesterone levels are very low. In fact, it is the sharp decline in progesterone secretion that triggers menses. Figure 10 shows the different phases of the menstrual cycle:

Figure 10.

The proliferative phase starts next. The granulosa cells and theca cells make more estrogen, which thickens the uterine lining. This continues to grow until the LH surge, when the ovulation phase takes place. When ovulation happens, the proliferative phase is over.

After ovulation is the secretory phase of the menstrual cycle. The vagina becomes more hospitable to sperm about the time of ovulation and then the corpus luteum begins to make progesterone. The secretory phase is what prepares the uterine lining for implantation. Glycogen is secreted by the uterine glands that will ultimately nourish the zygote after implantation. The spiral arteries develop to supply blood to this tissue.