Menstrual cycle phases
The menstrual cycle is the process of discharging blood and other things from a woman’s vagina every month from puberty to menopause, excluding pregnancy. It is a natural periodic process that brings changes in the female reproductive system responsible for the pregnancy.
–It includes changes that occur in the ovary and uterine walls simultaneously as a result of changes in the level of hormones in the blood.
–Two significant events occur within the female reproductive organs:
- First is the release of a single egg from one of the ovaries and,
- the second is that the uterine endothelium is prepared for the planting of a fertilized egg.
–If the ovum is not fertilized, the lining is released, resulting in menstruation.
–The average cycle length is approximately 28 days. However, the period that may differ for different women can range from 20 days to 45 days. The difference in duration is associated with decreased fertility.
Hormones involved in the menstrual cycle
–The Hypothalamus secretes a luteinizing hormone-secreting hormone that stimulates secretion from the anterior part of the pituitary:
- Follicle-stimulating hormone (FSH) causes maturation of the ovarian follicles and stimulates the release of estrogen, which is responsible for ovulation.
- Luteinizing hormone (LH) that triggers ovulation and the formation of the corpus luteum and the release of progesterone.
–The Hormones released in the cycle are stimulated by a negative feedback mechanism. The hypothalamus is stimulated when the level of estrogen and progesterone is low in the blood and turns off when its concentration is high.
–The menstrual cycle is described by ovarian and uterine cycles. The ovarian cycle involves the formation and maturation of follicular cells in the ovary, while the uterine cycle describes changes in the endothelial layer of the uterus.
The menstrual cycle phases/ Ovarian Cycle
The ovarian cycle involves the formation and maturation of follicular cells in the ovary. This cycle is divided into three phases:
- After puberty, as a result of the release of a large amount of FSH and LH by the pituitary, the ovaries with the follicles begin to grow.
- During the first stage, the target follicle cells enlarge up to two to three times in diameter. These enlarged follicle cells are called primordial follicles.
- After a few days, the FSH level exceeds LH, accelerating the growth of 6-10 primary follicles competing for dominance. Under the influence of FSH, these cells develop layers of granulosa cells and express LH receptors on granulosa cells.
- The second layer of cells called theca develops around these follicles, which can produce other sex hormones such as estrogen and progesterone.
- LH from the pituitary and estrogen within the follicles cause accelerated growth of the primary follicles into the vesicular follicles.
- The ovum present within the follicles develops aggressively and increases in size three to four times. After a week or more of growth, one of the follicles begins to grow to other follicles (a process called atresia). The reason behind the atresia process is not yet known; however, it is essential since it prevents more than one child during pregnancy.
- The individual follicle increases further in size and forms the mature follicle.
- Ovulation in women with regular 28-day sexual cycles occurs 14 days after the onset of menstruation. A few days before ovulation, the follicle begins to swell with a protruding center called a stigma.
- The intake of the LH hormone and the release of estrogen from the follicle breaks down the cells in the stigma and produces a hole. The secondary oocyte leaves the follicle through the hole and reaches the peritoneal cavity. The secondary oocyte then reaches the fallopian tube through the fimbriae. If there is left-right coordination between the ovaries, it is not yet known. However, occasionally, both ovaries release one egg, resulting in the formation of fraternal twins.
- If a sperm fertilizes the oocyte, it becomes a mature egg. If fertilization does not occur, the secondary oocyte degenerates within the fallopian tube.
- The luteal phase is the last phase of the ovarian cycle and corresponds to the secretory phase of the uterine cycles.
- During the first few hours of ovulation, the remaining follicular cells, including granulosa and theca, develop into lutein cells. This cell is filled with lipid components that give them a yellow appearance. The total mass of the cell is called the corpus luteum.
- The corpus luteum produces progesterone that inhibits the release of FSH and LH by the pituitary. Consequently, the concentration of FSH and LH decreases over time and the corpus luteum degenerates.
- Falling progesterone levels trigger menstruation. The process from the start of ovulation to the withdrawal of progesterone takes about two weeks. Although the sexual cycle in all women is not the same; The duration of the luteal phase remains more or less the same in all women.
- In the case of a successful pregnancy, the release of human chorionic gonadotropin (hCG) from the placenta prevents the degeneration of the corpus luteum. The corpus luteum is essential for producing progesterone that maintains the new pregnancy.
The uterine cycle includes changes in the endothelial layer of the uterus. It is divided into three phases.
- Menstruation, also called menstruation, menstrual bleeding, or period is the first phase of the uterine cycle. This occurs as a result of the degeneration of the corpus luteum that inhibits the release of FSH and LH from the pituitary and, therefore, prevents the proliferation of other follicular cells.
- Menstrual flow often serves as a sign to indicate that the woman is not pregnant; however, bleeding can also occur during pregnancy due to various reasons.
- The menstrual flow consists of blood from broken capillaries, secretions from the endometrial glands, endometrial cells, and an unfertilized egg.
- This phase generally lasts 3 to 5 days but can vary from 2 to 7 days in some women. On average, 35 milliliters of blood is lost during menstruation, but 10 ml to 80 ml is considered normal.
- A protein called plasmin is responsible for preventing clotting during menstruation.
- Pain in the back, stomach, and upper thigh is common during the first days of menstruation, and severe pain is commonly seen in adolescents (67% of women observe severe cramps).
- This is the second phase of uterine cycles where estrogen causes the proliferation of the endometrial layer in the uterus.
- After maturation of the follicles in the ovary, they cause the release of estrogens, causing the growth of a new layer of endometrium called the proliferative endometrium. The endometrium becomes thick with rapid cell multiplication and increases mucus-producing cells and blood capillaries.
- Estrogen also causes crypts to form on the cervix that facilitate the secretion of vaginal discharge.
- This phase ends when ovulation occurs and the level of estrogen decreases.
- The final phase of the uterine cycle corresponds to the luteal phase of the ovarian cycle, which occurs after ovulation.
- The corpus luteum releases the hormone progesterone, which is particularly essential for the uterus to be receptive to implantation of the fertilized egg. The endometrium becomes edematous, and the secretory glands produce a large amount of aqueous fluid to aid the passage of sperm.
- If fertilization occurs, the fertilized egg travels to the uterus through the uterine tube and becomes embedded.
- However, if fertilization does not occur, menstruation occurs and a new cycle begins.
Menstrual cycle phases, Hormones, Uterine Cycle