The reproductive system of women, unlike the male system, shows regular cyclical changes that can be considered periodic preparations for fertilization and pregnancy. In humans and other primates, the cycle is called the menstrual cycle. Its most obvious feature is vaginal bleeding that occurs with peeling of the uterus’ lining (menstruation). The cycle’s length is notoriously variable, but the average figure is 28 days from the beginning of one menstrual cycle to the beginning of another. In practice, the days of the cycle are marked with a number, where the countdown begins on the first day of menstruation.
At birth, there are many primordial follicles under the ovarian capsule in women. Each contains an immature ovum. At the beginning of each cycle, several of these follicles enlarge, and a cavity is created around this (creation of an antrum). This cavity is filled with follicular fluid. In humans, usually, one of these ovarian follicles begins to grow rapidly, and around the sixth day, it becomes the dominant follicle, while the others undergo regression and become atretic follicles. The atretic process involves apoptosis.
It is not clear the mechanism by which one of the follicles is selected to become the dominant follicle in this follicular phase of the menstrual cycle, but this appears to be related to the follicle’s ability to secrete estrogen, needed for final maturation. Many follicles develop simultaneously when women are given highly purified human pituitary gonadotropins by injection.
The primary source of circulating estrogen in ovarian granulosa cells; Theca cells of the inner layer of the follicle are necessary for estrogen production as they secrete androgens that are aromatized into estrogens in granulosa cells.
At about 14. on the day of the cycle, the follicle ruptures, and the egg is released into the abdominal cavity – this is called the ovulation process. The ovum is “collected” by the fallopian tubes’ hairy ends. It is transmitted to the uterus and, unless fertilization occurs, further out through the vagina.
At the time of ovulation, the ruptured follicle quickly fills with blood, forming something sometimes called the corpus hemorrhagic. Minor bleeding from the follicles into the abdominal cavity can irritate the lower abdomen’s peritoneum and dull pain. Granulosa and theca follicle cells multiply rapidly, and clotted blood is rapidly replaced by yellowish, lipid-rich luteal cells, forming the corpus luteum. This begins the menstrual cycle’s luteal cycle, during which luteal cells secrete estrogen and progesterone. The growth of the corpus luteum depends on developing an adequate blood supply. There is evidence that vascular endothelial growth factor (VEGF) is likely essential for this process.
If pregnancy occurs, the corpus luteum remains preserved, and there is usually no menstruation until after delivery. If pregnancy does not occur, the corpus luteum begins to degenerate about four days before the next menstruation (day 24 of the cycle) and is eventually replaced by scar tissue, forming the corpus Albicans.
New eggs are not formed after birth. During fetal development, the ovaries contain about 7 million primordial follicles. Yet many involute before birth, and others are lost after birth. There are about 2 million eggs at the time of birth, but 50% are atretic. A million normal ones then go through the first part of the meiotic division and enter stagnant prophase in which those who survive to remain in the prophase until adulthood. Atresia continues during development, and the number of eggs in both ovaries at puberty decreases to less than 300,000. Only one of these eggs per cycle (about 500 during normal reproductive life) normally reaches maturity – the rest decays.
Just before ovulation, the first meiotic division is completed. One of the daughter cells, the secondary oocyte, takes up the most cytoplasm, while the other, the first polar body, fragments and disappears. The secondary oocyte immediately begins a second meiotic division, but this division stops in metaphase and is completed only if the egg is fertilized. At that time, the second polar body is discarded, and the fertilized egg begins to form a new individual.
At the end of menstruation, all layers, except the endometrium’s deep layers, are peeled off. The new endometrium then grows again under estrogen’s influence from developing ovarian follicles. The endometrium thickens rapidly from 5. to 14—days of the menstrual cycle. As the thickness increases, the uterine glands elongate but have no secretion. These uterine endometrium changes are called the proliferative phase, or sometimes the preovulatory or follicular phase of the cycle. After ovulation, the endometrium becomes more vascularized and somewhat edematous under the influence of estrogen and progesterone from the corpus luteum. The glands become more convoluted and begin to secrete clear fluid. Consequently, this phase of the cycle is called the secretory or luteal phase. Late in the luteal phase, the endometrium, an anterior pituitary gland, produces prolactin, but its role is unknown.
The endometrium is supplied with blood by two arteries. The superficial two-thirds of the endometrium that exfoliates during menstruation (stratum functional) is perfused by long, convoluted spiral arteries. Short and straight basilar arteries perfuse the deeper layer that is not exfoliated (stratum basale).
When the corpus luteum involutes, hormones’ flow to the endometrium stops. The endometrium thins, which contributes to further twisting of the spiral arteries. Foci of necrosis appear on the endometrium. Also, degeneration and spasm of the spiral arteries’ walls occur, leading to punctate hemorrhages that produce menstrual blood.
Locally released prostaglandins probably create vasospasm. Large amounts of prostaglandins are present in the secretory endometrium and menstrual blood, and infusions of prostaglandin PGF2-alpha create endometrial necrosis and bleeding.
From the perspective of endometrial function, the menstrual cycle’s proliferative phase represents the renewal of the epithelium from the previous menstruation. The secretory phase represents the uterus’s preparation for implantation and fertilization of the ovum. The length of the secretory phase is constant (about 14 days), and the variations seen with the length of the menstrual cycle are mainly the result of the variation in the length of the proliferative phase. If fertilization does not occur during the secretory phase, the endometrium peels off, and a new cycle begins.
Menstrual blood is mostly arterial – only 25% of blood is of venous origin. It contains tissue debris, prostaglandins, and relatively large fibrinolysin from endometrial tissue. Fibrinolysin lyses clots, so menstrual blood does not contain clots unless the amount of blood is excessive.
The usual duration of menstruation is 3 to 5 days, but it can be 1 day to 8 days, which is not considered abnormal. The amount of blood lost can vary from a few drops to 80 mL; the average amount lost is 30 mL. Loss of more than 80 mL is abnormal. The amount of blood lost can be influenced by various factors, including endometrial thickness, medications, and diseases that affect the blood clotting system.
Cyclic changes of the cervix of the uterus and vagina
Although the cervix continues directly to the uterus’ body, the cervical mucosa does not undergo cyclic desquamation but only through changes in cervical mucus. Estrogen makes mucus less viscous and more alkaline – changes that help sperm survive and transport. Progesterone, on the other hand, makes mucus more viscous and cellular. Mucus is the least viscous at the time of ovulation.
Under estrogen’s influence, the vaginal epithelium becomes cornified, and you can identify the cornified cells in the vaginal smear. Under the influence of progesterone, viscous mucus is secreted, and the epithelium proliferates and is infiltrated by leukocytes.
Cyclic breast changes
Although lactation does not normally occur until the end of pregnancy, cyclical changes occur in the breasts during the menstrual cycle. Estrogens cause duct proliferation, while progesterone causes the growth of lobules and alveoli. The swelling of the chest and the pain that many women feel ten days before menstruation are probably the results of ductal distension, hyperemia, and edema of the interstitial breast tissue. All of these changes regress, along with symptoms, during menstruation.
Changes during sexual intercourse
During sexual arousal in women, fluid is secreted into the vaginal walls, probably due to the release of VIP (gastrointestinal polypeptide) from the nerves of the vagina. The vestibular glands also secrete a lubricating fluid. The upper part of the vagina is sensitive to stretching, while tactile stimulation of the labia minora and clitoris helps with sexual arousal. These stimuli are amplified by tactile stimulants from the breast and, as in men, by visual, auditory, and olfactory stimuli that can all together lead to a crescendo known as orgasm. During orgasm, autonomically mediated rhythmic contractions of the vaginal walls occur. Impulses also travel to the pudendal nerves and create rhythmic contractions of the bulbocavernosus and ischiocavernosus muscles. Vaginal contractions can help transport sperm but are not essential for this, as egg fertilization is not dependent on orgasm.
Knowing when ovulation occurs during the menstrual cycle is essential for increasing fertility or, conversely, family planning. A simple and fairly safe indicator of ovulation time is a change – usually an increase – in basal body temperature. The increase in body temperature begins one to two days after ovulation. Women interested in getting the correct body temperature should use a digital thermometer and measure the temperature (oral and rectal) in the morning before getting out of bed. The cause of the change in body temperature at the time of ovulation is probably an increase in progesterone secretion, as progesterone is thermogenic.
A sudden increase in LH secretion triggers ovulation, and ovulation normally occurs about 9 hours after LH concentration peak. The egg lives for approximately 72 hours after being excreted from the follicle but can be fertilized in a much shorter period of time. In a study examining the association of isolated intercourse with pregnancy, 36% of women were pregnant after intercourse on the day of ovulation. Still, with intercourse on the days after ovulation, the percentage was zero.
Isolated sexual intercourse on the first and second days before ovulation also led to pregnancy in about 36% of women. Several pregnancies resulted from sexual intercourse 3., 4. or Day 5 before ovulation, although this percentage was much lower, for example, 8% 5th day before ovulation. Therefore, some sperm can survive in the female genital tract and fertilize the egg up to 120 hours before ovulation, but the most fertile period is apparently 48 hours before ovulation. However, for those interested in contraception methods by counting “fertile days,” it should be noted that there are rare cases of pregnancy due to sexual intercourse for each day of the cycle.