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  • The menstrual cycle is a term used to describe monthly events that occur within a woman's

  • body in preparation for the possibility of pregnancy. Each month, an egg is released

  • from an ovary in a process called ovulation. At the same time, the lining of the uterus

  • thickens, ready for pregnancy. If fertilization does not take place, the lining of the uterus

  • is shed in menstrual bleeding and the cycle starts over.

  • An ovary contains hundreds of thousands of primary oocytes - immature eggs, or ova. Each

  • of these is enclosed in a structure called a follicle, and at this stage -- a primordial

  • follicle. The menstrual cycle is under control of hormones

  • secreted by the pituitary gland and the ovaries. The pituitary itself is under control of the

  • hypothalamus. The hypothalamus produces a hormone called

  • the gonadotropin-releasing hormone or GnRH. GnRH stimulates the anterior lobe of the pituitary

  • to secrete follicle-stimulating hormone (FSH). FSH travels in the bloodstream to the ovaries

  • and stimulates a group of follicles to grow. These primordial follicles develop into primary

  • follicles and then secondary follicles. These produce a hormone named estrogen which acts

  • to stimulate the growth of the endometrium -- the inner lining of the uterus. The secondary

  • follicles compete with each other and only one of them will survive and become a mature

  • follicle, the rest atrophy and die. The increasing level of estrogen also acts

  • on the hypothalamus and the anterior pituitary to increase the level of GnRH and induce the

  • production of another hormone -- luteinizing hormone (LH). A surge in LH secretion triggers

  • ovulation - the release of the egg from the follicle and the ovary. The egg is then swept

  • up by the fimbriae and taken into the uterine tube.

  • Fertilization by a spermatozoon, when it occurs, usually takes place in the ampulla, the widest

  • section of the fallopian tube. The fertilized egg immediately begins the process of development

  • while travelling toward the uterus. After 6 days it becomes a blastocyst and is implanted

  • into the endometrium of the uterus. Meanwhile, the left-over of the ruptured follicle

  • has become a corpus luteum which secretes progesterone. Progesterone further stimulates

  • uterine development making it a nutritious bed for the embryo in the event of pregnancy.

  • In the absence of pregnancy, the corpus luteum atrophies and progesterone level falls. This

  • leads to the breakdown of the endometrium, menstruation begins and the cycle starts over.

The menstrual cycle is a term used to describe monthly events that occur within a woman's

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月經週期的荷爾蒙控制,動畫。 (Hormonal Control of the Menstrual Cycle, Animation.)

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    Lizzy 發佈於 2021 年 01 月 14 日
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