Many women know that menstruation is the vaginal bleeding that occurs once a month. The first menstrual period is a sign of initial maturation during puberty. Vaginal bleeding is caused by the periodic exfoliation of the endometrium. Why does the endometrium exfoliate and why does it do so periodically, in cycles of about 28 days, is exactly what is described in this chapter. 1. What is menstruation? What is the first menstruation? Menstruation is the periodic shedding and bleeding of the endometrium that accompanies the periodic ovulation of the ovaries and is one of the signs of maturity of reproductive function. The first menstrual period is called menarche. The age of menarche is mostly between 13 and 15 years old, later in rural areas than in cities, and with the development of the economic level, the age of menarche has advanced. Menstrual blood is generally dark red. 70% of the blood comes from vascular bleeding, 5% from cellular exudation, and 25% from venous rupture reflux. In addition to blood, there is endometrial debris, cervical mucus and shed vaginal epithelial cells. The main characteristic of menstrual blood is that it does not clot, but there are occasionally small clots under normal circumstances. Symptoms during menstruation: Generally, there are no special symptoms during menstruation, but due to the increased blood stasis in the pelvic cavity and uterine blood flow during menstruation, some women may have a feeling of lower abdominal and lumbosacral cramping. Individuals with bladder irritation symptoms (such as frequent urination), mild neurological instability symptoms (such as headache, insomnia, mental depression, easily agitated), gastrointestinal disorders (such as loss of appetite, nausea, vomiting, constipation or diarrhea) and nasal mucous membrane bleeding, skin acne, etc., these symptoms are not serious. It does not affect work and study. 2, menstrual cycle characteristics The typical menstrual cycle characteristics of women is regular 28 days, but also often shows irregularity, can fluctuate between 25-35 days. After menarche or before menopause, anovulatory cycles often occur, making the length of menstruation vary and the bleeding date can fluctuate from 3-7 days. 3. The cyclical changes in ovarian function during the menstrual cycle The characteristics of the menstrual cycle are the result of cyclical changes in ovarian sex hormone secretion. When it comes to ovarian sex hormone secretion, ovulation must be mentioned. Ovulation Each month during the fertile period, a number of follicles develop, only one of which is recruited and selected to mature and expel the oocyte. The rest of the follicles develop to a certain point and degenerate on their own through apoptosis, which is called follicular atresia. There are approximately 400-500 follicles that develop and ovulate during a woman’s lifetime. Luteal formation and degeneration After ovulation, the follicular wall collapses and many folds are formed. Follicular granulosa cells and follicular lining cells invade inward and are surrounded by a connective tissue follicular epithelium, which together form the corpus luteum. Follicular granulosa cells and follicular lining cells are further luteinized by the LH ovulation peak to form granulosa luteal cells and follicular membrane luteal cells, respectively. The corpus luteum reaches its peak volume and function 7-8 days after ovulation (corresponding to about day 22 of the menstrual cycle). The corpus luteum begins to degenerate 9~10 days after ovulation and also kicks function limited to the 14th day. Menstruation follows luteal regression. During sexual maturation, except during pregnancy or postpartum lactation, the ovaries are constantly repeating the above cyclic changes. Hormonal cyclicity As the ovarian follicles develop cyclically, estrogen and progesterone also undergo cyclic changes. During the follicular phase, the follicles mainly secrete estrogen and the secretion of progesterone is negligible. As the follicles continue to grow, the amount of estrogen secreted by the ovaries increases, reaching a peak just before ovulation when the follicles mature. After ovulation, the amount of estrogen secretion decreases. After ovulation, the corpus luteum forms and the corpus luteum secretes a large amount of progesterone and a certain amount of estrogen at the same time. Before menstruation, the corpus luteum atrophies and fails to function, the ability to secrete progesterone decreases and the blood progesterone level drops to the early follicular phase level. FSH and LH also undergo cyclic changes in response to changes in estrogen and progesterone levels. During the follicular phase, FSH and LH levels gradually decrease as estrogen levels rise. Before ovulation, the positive feedback of estrogen, the elevated estrogen will in turn contribute to the rise of FSH and LH, forming FSH peaks and LH peaks to induce ovulation. After the formation of the corpus luteum, the negative feedback effect of estrogen and progesterone will in turn cause the FSH and LH levels to fall. 4. Cyclic changes of the endometrium With the cyclic changes of the ovaries, other parts of the genitalia also produce corresponding cyclic changes, among which the changes of the endometrium are the most significant and important. After ovulation, the ovaries secrete progesterone and estrogen, and the proliferative endometrium undergoes secretion phase changes. In the late luteal phase, the corpus luteum atrophies, estrogen and progesterone secretion decreases, the endometrium loses support, necrosis and exfoliation occur, which manifests as menstrual flow, at which time the endometrium is called menstrual endometrium. Structure of the endometrium The endometrium has 3 main tissue structures, namely epithelium, mesenchyme and blood vessels. As a whole, the endometrium is divided into two parts, namely the functional layer and the basal layer. The functional layer is located on the surface and is divided into a dense layer and a spongy layer. The dense layer is near the surface of the uterine cavity and is formed by the stroma immediately adjacent to the luminal epithelium. The spongy layer is predominantly glandular with little interstitium, which is loose and rich in blood supply. The functional layer responds to the hormones secreted by the ovaries, changes with the ovarian cycle, and is necrotic and shed during menstruation. The basal layer is located between the cavernous layer and the muscular layer and contains the base of the uterine glands and supporting vessels. The basal layer is insensitive to hormones secreted by the ovaries, and the cyclic changes are not obvious. Only the functional layer is shed during menstruation, but not the basal layer. The endometrium is divided into 3 phases, the proliferative phase, the secretory phase, and the menstrual phase. There are two estrogen receptors on the endometrium during the proliferative phase, and estrogen causes the glandular and interstitial cells of the endometrium to be in a proliferative state. The duration of the endometrial proliferative phase is variable and depends on the timing of follicular growth. For a woman with a 28-day menstrual cycle, ovulation occurs approximately on day 14 of the menstrual cycle, then the endometrial proliferation period is days 4-14 of the menstrual cycle. The secretory phase is the change in the endometrium during the secretory phase after the formation of the corpus luteum and under the action of progesterone. The physiological changes of the endometrium during the secretory phase are the result of the action of progesterone on the basis of the proliferative phase. The main effects of progesterone are: down-regulation of estrogen receptors, so that the effect of estrogen is weakened; acceleration of estrogen metabolism and decrease of local estrogen level in the endometrium; pre-melanization changes of endometrial interstitial cells and secretory changes of the glandular epithelium. Ovulation is a sign of the beginning of the secretory phase. 1-5 days after ovulation is the early secretory phase, in which the endometrium thickens and the glands enlarge and bend; 6-10 days after ovulation is the middle secretory phase, in which the endometrium shows high secretory activity, the previous bending and expansion reaches its peak, the thickness of the endometrium increases, and the endometrium is thick and soft, rich in nutrients, which is conducive to the development of fertilized eggs. Late secretion, also called premenstruation, occurs 4-24 hours before the start of menstruation when the small spiral arteries of the endometrium contract in local spasms, the endometrium distal to the spasm becomes necrotic due to ischemia, the permeability of the vessel wall increases, followed by vasodilation and blood flowing from the broken vessels. Menstruation Due to the decrease of estrogen and progesterone levels, the endothelial tissue degenerates and badly exfoliates, and the degenerated and necrotic endothelium is mixed with blood and discharged, and menstruation forms. During the menstrual period, the basal layer of the endothelium begins to proliferate and a new endothelium is formed. Therefore, the menstrual period is both the end of the previous cycle and the beginning of the next menstrual cycle. Changes in the vascularity of the endometrium Previously, more attention was paid to the spiral arteries of the endometrium, and the cyclic changes of the spiral arteries were more intensively studied. Angiogenesis is always present in the endometrium throughout the menstrual cycle, and is manifested by the growth of blood vessels during the proliferative phase, the growth of spiral arteries during the secretory phase, the degeneration of blood vessels before menstruation, and the repair of ruptured blood vessels during menstruation. Angiogenesis is regulated by factors such as vascular endothelial factor, fibroblast growth factor, angiopoietin, and angiogenin. Many studies have found that these growth factors also change cyclically with the menstrual cycle, which is related to the cyclical changes of angiogenesis. 5. Cyclic changes in other organs During the menstrual cycle, the fallopian tubes and the cervix are also undergoing significant cyclic changes. Cyclic changes in the fallopian tubes The role of the fallopian tubes is to facilitate the transport of eggs, to provide a site for fertilization and to transport fertilized eggs. The fallopian tubes are divided into four parts: (i) the umbilical end, where the cilia of the ciliated cells swing towards the uterine cavity and have the function of egg collection. (2) The abdomen, which is the site of fertilization. The isthmus, which is thicker in muscle and thinner in mucosa. (iv) The interstitial part, which is within the muscular wall of the uterus and surrounded by thicker muscular layers. The fallopian tube can be divided into a mucosal layer and a muscular layer from the inside out. The mucosal layer consists of epithelial cells, including ciliated cells and secretory cells. The oviduct picks up eggs by muscle contraction and ciliary oscillation, and the transport of eggs and fertilized eggs is mainly achieved by contraction of the oviductal muscles. Muscle contraction and ciliary oscillation are mediated by estrogen and progesterone secreted by the ovary. Estrogen promotes cilia production and progesterone causes cilia shedding. Cyclic changes in cervical mucus The cervical mucus consists mainly of secretions from the endocervical glands, in addition to small amounts of fluid from the endometrium and fallopian tubes, as well as debris and white blood cells from the uterine cavity and cervix. The secretion of uterine glandular mucus is influenced by estrogen and progesterone and varies cyclically with the menstrual cycle. Before ovulation, the secretion of cervical mucus increases, with good lacrimation and low viscosity, which facilitates sperm penetration. After ovulation, the secretion of cervical mucus decreases, with increased viscosity and poor lacrimation. After pregnancy, the mucus becomes thicker and can form a mucus plug to block the cervical opening, preventing the passage of sperm and bacteria. Changes in the vaginal mucosa during the menstrual cycle The epithelial cells of the vaginal mucosa are also affected by estrogen and progesterone and also undergo cyclic changes. Estrogen causes proliferation of the underlying cells of the vaginal epithelium. The underlying cells proliferate and gradually evolve into middle and superficial cells, causing the superficial cells to become keratinized, and the cells are rich in glycogen, which is broken down into lactic acid by the action of lactobacilli. After ovulation, progesterone causes the epidermal cells of the vaginal mucosa to shed, so the vaginal exfoliated cells can be used to understand the female reproductive endocrine status. Basal body temperature Progesterone can up-regulate the questionable set point of the temperature center. Usually, the basal body temperature is <36.5℃ before ovulation and rises 0.3-0.5℃ after ovulation, which is maintained for 12-16 days. Basal body temperature can be used clinically to determine whether there is ovulation and if so, whether the corpus luteum is functioning well.