What is the significance of the follicular atresia

   Menstrual bleeding occurs at the end of the secretion phase of the uterus, is caused by falling hormone levels and lasts 3-5 days (day 1 = 1st day of the cycle). Under the influence of hydrolytic proteases - and with the loss of ~ 20-80 ml of blood - 2/3 of the endometrium (stratum functionale) detach as a result of hypoxic atrophy (contraction of spiral arteries), 1/3 remains (stratum basale) and builds up under the effect of estrogen the mucous membrane completely rises again. If pregnancy occurs, day 1 is counted as the beginning of the 1st week of pregnancy (SSW), although the pregnancy in the biological sense only begins with the implantation, usually in the 3rd week of pregnancy
   FSH stimulates the formation of estrogen in the ovaries and the development of several follicles (follicular phase of the ovary), the endometrium is built up by growth factors (proliferation phase of the uterus). Follicle cells express receptors for gonadotropins and LDL (cholesterol for steroid synthesis). Granulosa cells form progesterone, theca cells from progesterone androstenedione, granulosa cells from androstenedione form estradiol (aromatase). In the oocyte, high cAMP levels keep the arrest in prophase I, supported by cGMP, which enters the egg cell via gap junctions from neighboring cells and inhibits the activity of the phosphodiesterase PDE3A and thus the breakdown of cAMP. The meiosis is continued by decreasing cGMP, uninhibited PDE3A and degradation of cAMP
   At the end of the follicular phase, the estrogen concentration rises above a threshold value, which causes the effect on the hypothalamus to tilt: Basophil cells increase the number of their GnRH receptors, the preovulatory LH / FSH peak occurs (hollow path effect). LH induces ovulation, its highest concentration is found from 2 to 3 weeks. pre-ovulatory up to the 1st post-ovulatory day. The LH peak lets the egg cell continue until it arrests in the meiosis 2 metaphase and triggers ovulation, the remaining granulosa and theca cells transform into the corpus luteum. The egg cell can be fertilized for up to 12 hours; it reaches the uterus via the fallopian tubes by means of a flicker current and muscle contractions. The cervical secretion is thin and rich in protein and carbohydrates. The alkaline pH value favors the passage of the sperm
   The second half of the cycle (luteal phase of the ovary) begins with ovulation and lasts 14 ± 2 days. Granulosalutein cells produce progesterone and inhibin, thekalutein cells estrogens and androgens. The progesterone level is high during the second half of the cycle: Progesterone lowers the number of estrogen receptors and promotes the conversion of estradiol to estriol. The endometrium prepares for implantation: it forms glycogen, collagen, laminin, and heparan sulfate (decidualization, secretion phase of the uterus) to build up tissue. The rectal temperature increases by ~ 0.5 ° C 2-3 days after ovulation. The cervical secretion does not allow any further passage of sperm. Maximum progesterone production occurs one week after the LH peak, negative feedback lowers gonadotropin secretion (lower pulse frequency). ~ 11 days after ovulation, luteal function begins to decline (unless hCG occurs). With luteolysis (~ 26th day of the cycle) the production of progesterone, estrogen and inhibin decreases sharply, and the FSH level rises again 2 days before menstruation begins. Spiral arteries contract (lack of estrogen → ischemia → vascular necrosis), desquamation and menstrual bleeding occur
   Menopause is the last menstrual period; it separates the premenopausal from the postmenopausal part of the menopause (amenorrhea is the absence of menstrual bleeding). The falling steroid level (estrogens to ~ 15%, progesterone to ~ 30% of the values ​​of younger women in the follicular phase) increases the release of gonadotropins for several years. Estrogens originate primarily from peripheral conversion of adrenal androgens; Estrone becomes the dominant estrogen. The number of ovarian follicles continues to decrease