-
The phases of the menstrual cycle can also be described in terms of uterine events
-
Menstrual phase
- Day 1 is the first day of menstrual flow, and the entire duration of menstruation is known as the menstrual phase (generally about 3 to 5 days in a typical 28-day cycle).
- During this time, the epithelial lining of the uterus—the endometrium—degenerates, resulting in the menstrual flow.
-
Proliferative phase
- The menstrual flow then ceases, and the endometrium begins to thicken as it regenerates under the influence of estrogen.
- This period of growth, the proliferative phase, lasts for the 10 days or so between cessation of menstruation and the occurrence of ovulation.
-
Uterine phase: Secretory phase
- Soon after ovulation, under the influence of progesterone and estrogen from the corpus luteum, the endometrium begins to secrete glycogen in the glandular epithelium, followed by glycoproteins and mucopolysaccharides.
- The part of the menstrual cycle between ovulation and the onset of the next menstruation is called the secretory phase.
- As shown in Figure 17.27, the ovarian follicular phase includes the uterine menstrual and proliferative phases, whereas the ovarian luteal phase is the same as the uterine secretory phase.
-
Causes of uterine changes during a menstrual cycle- proliferative phase
- The uterine changes during a menstrual cycle are caused by changes in the plasma concentrations of estrogen and progesterone secreted by the ovaries (see Figure 17.22).
- During the proliferative phase, an increasing plasma estrogen concentration stimulates growth of both the endometrium and the underlying uterine smooth muscle (called the myometrium).
- In addition, it induces the synthesis of receptors for progesterone in endometrial cells.
-
Causes of uterine changes during a menstrual cycle- secretory phase
- Then, following ovulation and formation of the corpus luteum (during the secretory phase), progesterone acts upon this estrogen-primed endometrium to convert it to an actively secreting tissue.
- The endometrial glands become coiled and filled with glycogen, the blood vessels become more numerous, and enzymes accumulate in the glands and connective tissue.
- These changes are essential to make the endometrium a hospitable environment for implantation and nourishment of the developing embryo.
- Progesterone also inhibits myometrial contractions, in large part by opposing the stimulatory actions of estrogen and locally generated prostaglandins. This is very important to ensure that a fertilized egg can safely implant once it arrives in the uterus.
- Uterine quiescence [inactivity] is maintained by progesterone until the end of the pregnancy and is essential to prevent premature delivery.
- Estrogen and progesterone also have important effects on the secretion of mucus by the cervix.
- Under the influence of estrogen alone, this mucus is abundant, clear, and watery. All of these characteristics are most pronounced at the time of ovulation and allow sperm deposited in the vagina to move easily through the mucus on their way to the uterus and fallopian tubes.
- In contrast, progesterone, present in significant concentrations only after ovulation, causes the mucus to become thick and sticky—in essence, a “plug” that prevents bacteria from entering the uterus from the vagina. The antibacterial blockage protects the uterus and the embryo if fertilization has occurred.
-
Uterine changes- degeneration of corpus luteum
- The decrease in plasma progesterone and estrogen concentrations that results from degeneration of the corpus luteum deprives the highly developed endometrium of its hormonal support and causes menstruation.
- The first event is constriction of the uterine blood vessels, which leads to a diminished supply of oxygen and nutrients to the endometrial cells. Disintegration starts in the entire lining, except for a thin, underlying layer that will regenerate the endometrium in the next cycle.
- Also, the uterine smooth muscle begins to undergo rhythmic contractions.
- Both the vasoconstriction and uterine contractions are mediated by prostaglandins produced by the endometrium in response to the decrease in plasma estrogen and progesterone concentrations.
- The major cause of menstrual cramps, dysmenorrhea, is overproduction of these prostaglandins, leading to excessive uterine contractions.
- The prostaglandins also affect smooth muscle elsewhere in the body, which accounts for some of the systemic symptoms that sometimes accompany the cramps, such as nausea, vomiting, and headache.
- After the initial period of vascular constriction, the endometrial arterioles dilate, resulting in hemorrhage through the weakened capillary walls.
- The menstrual flow consists of this blood mixed with endometrial debris.
- Typical blood loss per menstrual period is about 50 to 150 mL.
-
Dysmenorrhea
- The major cause of menstrual cramps, dysmenorrhea, is overproduction of these prostaglandins, leading to excessive uterine contractions.
- The prostaglandins also affect smooth muscle elsewhere in the body, which accounts for some of the systemic symptoms that sometimes accompany the cramps, such as nausea, vomiting, and headache.
-
Days 1-5
- Major events:
- Estrogen and progesterone are low because the previous corpus luteum is regressing.
- Therefore:
- a. Endometrial lining sloughs.
- b. Secretion of FSH and LH is released from inhibition, and their plasma concentrations increase. Therefore; Several growing follicles are stimulated to mature.
-
Day 7
A single follicle (usually) becomes dominant
-
Day 7-12
- Plasma estrogen increases because of secretion by the dominant follicle. Therefore; Endometrium is stimulated to proliferate.
- LH and FSH decrease due to estrogen and inhibin negative feedback. Therefore; Degeneration (atresia) of nondominant follicles occurs
-
Day 12-13
- LH surge is induced by increasing plasma estrogen secreted by the dominant follicle (positive feedback).
- Therefore:
- a. Oocyte is induced to complete its first meiotic division and undergo cytoplasmic maturation.
- b. Follicle is stimulated to secrete digestive enzymes and prostaglandins.
-
Day 14
Ovulation is mediated by follicular enzymes and prostaglandins.
-
Day15-25
- Corpus luteum forms and, under the influence of low but adequate levels of LH, secretes estrogen and progesterone, increasing plasma concentrations of these hormones.
- Therefore:
- a. Secretory endometrium develops.
- b. Secretion of FSH and LH from the anterior pituitary gland is inhibited, lowering their plasma concentrations. Therefore; No new follicles develop.
-
Day 25-28
- Corpus luteum degenerates (if implantation of the conceptus does not occur).
- Therefore:
- Plasma estrogen and progesterone concentrations decrease. Therefore; Endometrium begins to slough at conclusion of day 28, and a new cycle begins.
|
|
