Ovulation

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The process of ovulation occurs every month in the organism of the woman. It consists of the egg leaving the ovary and falling into the Fallopian tube. There are two ovaries in the female reproductive system that take turns in releasing a ready for fertilization egg.

As an infant when gonads are formed, primordial germ cells move and find their place in the gonads. Then these cells differentiate into oogonia. Typical of these cells is that they divide using mitosis multiple times and increase in number. Each cell then is surrounded by a circle of epithelium cells that are flat, called follicular cells. The oogonia cell surrounded by a layer of follicular cells forms the primordial follicle. The oogonia cells keep on dividing and some of them begin meiosis and turn into primary oocytes, but they stop the division in the stage of prophase. In this state they are going to be until puberty. The cells keep on dividing and increasing in number until the 5th month of the pregnancy. In this period the number of the cells is seven million. After this, cells begin dying and by the time of birth there are only 400 000 left. The reproductive cells that the female has once born are all primary oocytes stopped in prophase of meiosis I and are surrounded by epithelium cells, forming the primary follicle.

From this moment these cells are inactive. There are no sex hormones released and no ovulation. When entering puberty, hormones are released and the first ovulation occurs. The primary follicles grow and the follicular cells turn from flat to cuboid. Then they turn into secondary ones, when first meiotic division is completed. Secondary follicles consist of secondary occytes. Typical of them is that they start meiosis II and form preovulatory or Graafian follicle. Meiosis II is not complete and it is inhibited at metaphase II. This is when ovulation occurs and secondary oocyte together with surrounding cells leave the ovary and falls into the Fallopian tube. The secondary oocyte will remain in this state until fertilization occurs and meiosis II is completed.

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