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Women’s Reproductive Structure
How Women’s Body Works?
The female reproductive organs include the uterus, ovaries, fallopian tubes, and vagina.
The female reproductive organs secrete female hormones, produce eggs necessary for fertilization, and accept sperm from men to become pregnant, thereby nurturing the fetus within the body.
Women't Body Reproductive
Fun Fact 1: When returning a fertilized egg to the uterus during fertility treatment (i.e. embryo transfer), some doctors may request that you come without going to the bathroom (i.e. with urine in your bladder). This is because the uterus is tilted toward the bladder, and by filling the bladder with urine, the uterus will straighten out and may be easier to transfer.
Uterus The uterus is the organ that implants and nurtures a fertilized egg (i.e., a sperm ejaculated by a man arrives at a woman’s egg and combines with it) and acts as a bed for the fertilized egg to develop.
The upper two-thirds of the uterus is called the uterine body, the lower one-third is called the cervix, and the upper part of the uterus is called the uterine fundus.
The endometrium, which is located at the innermost part of the uterine wall, changes with the menstrual cycle (i.e., the period between the onset of a period and the beginning of the next period).
The main hormones involved in this change are estrogen and progesterone.
During the period when follicles are developing in the ovaries, the endometrium grows and thickens due to estrogen.
This is called the “proliferative phase”. After ovulation, the endometrium thickens further due to the effects of progesterone and estrogen, and is ready to receive a fertilized egg. If pregnancy does not occur (i.e., the fertilized egg does not implant), the secretion of progesterone and estrogen decreases, the functional layer becomes necrotic, and the endometrium peels off (i.e., “menstruation” = “menstruation” or “eradication of blood” in technical terms). This is called the ‘menstrual period’.
Ovaries The ovaries, one on each side of the uterus, are reproductive organs that produce, mature, and ovulate eggs, as well as endocrine organs that secrete female hormones (estrogen and progesterone). “Endocrine” refers to the secretion of hormones.
In the ovary, there are many primordial follicles, which are the source of eggs. Some of these follicles develop under the action of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secreted by the pituitary gland, and after developing into mature follicles (Graafian follicles). The developing follicle secretes estrogen (specifically estradiol), which is one of the female hormones.
A follicle is a pouch-like tissue that encloses an egg. If the follicle is not large enough, the egg released will not be fertilized. The number and size of the follicles are very important in fertility treatment.
When a mature follicle protrudes from the surface of the ovary and the egg inside the follicle is expelled from the ovary, this is called ovulation. After ovulation, the remaining follicle becomes the corpus luteum through the action of LH. The corpus luteum secretes female hormones such as progesterone and estrogen to change the uterine lining to a state suitable for implantation. If the fertilized egg does not implant in the uterus at this time (i.e., no pregnancy), the corpus luteum retracts and becomes a white body. On the other hand, if a fertilized egg does implant in the uterus (i.e., pregnancy), the corpus luteum remains as the corpus luteum gestationis until about the 12th week of pregnancy and continues to secrete progesterone.
Before birth, the fetus already has a lifetime supply of primordial follicles in its ovaries, which cannot increase in number through cell division like other cells, but only decrease in number after birth. There are 2 million follicles at birth, 100,000-400,000 at puberty, and after puberty, when menstruation occurs, the number decreases by about 1,000 per month.
Many follicles develop into mature follicles, but usually only one follicle can ovulate at a time. The follicle that can ovulate is the one that is in the right state of development for ovulation and is called the “master follicle”. The rest of the dozens of follicles that have grown to the halfway point will shrink and disappear (these are called closed follicles). Since it takes about 100 days for a primary follicle to develop into a mature follicle, the ovary contains follicles in various stages of development.
Fun Fact 2: Normally, when ovulation occurs naturally, only one ovulation occurs from among a number of follicles. However, in the case of fertility treatments (especially advanced assisted reproductive technologies such as IVF and ICSI), drugs are used to stimulate the ovaries to develop more follicles so that multiple eggs can be retrieved. In fertility treatment, a large number of follicles are developed and retrieved at once, which may make you wonder, “Won’t fertility treatment cause the ovaries to lose their follicles (eggs) too quickly? You may be wondering, “Won’t fertility treatments cause the ovaries to lose their follicles (eggs) too quickly? Normally, when ovulation occurs naturally, multiple follicles are developing, but except for one suitable follicle, the remaining dozens will shrink and disappear. This is called a closed follicle. Fertility treatments do not close the follicles, but only allow more follicles to develop, so fertility treatments do not cause the follicles (eggs) to disappear quickly.
Fallopian Tubes The fallopian tubes are about 7-12 cm long and act like a pipe connecting the ovaries to the uterus, catching the eggs that are ovulated from the ovaries into the abdominal cavity and transporting them to the uterus.
The outer end of the fallopian tube has a funnel-like structure called the oviductal mucosa, where the eggs ovulated in the abdominal cavity are trapped. Fertilization occurs when the egg meets the sperm. The fertilized egg divides and is transported through the fallopian tubes to the uterus, where it implants in the ready uterine lining. In many cases, infertility occurs when the fallopian tubes become narrow or blocked (adhesions around the fallopian tubes), preventing the egg, sperm, or fertilized egg from passing through the fallopian tubes.
Vagina The vagina is located behind the bladder and urethra and in front of the rectum. It is a 7-10 cm long organ that accepts a man’s penis during intercourse and guides sperm to the uterus. It also excretes menstrual blood and mucus from the uterus, and serves as a birth canal during childbirth. The vaginal mucosa is kept acidic by a lactic acid bacterium called Dederline bacillus, which prevents the entry of bacteria from the outside.