Oogenesis

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Oogensis is a type of gametogenesis. It is the production of an ovum or egg cell and occurs in all sexually reproductive species. Oogenesis involves the creation of haploid sex cells through the process of meiosis. Female germ cells undergo extensive mitotic division in the foetal ovary. In the first stage of oogenesis, the oogonium undergoes oocytogenesis, forming the primary oocyte through mitosis. Like the oogonium, the primary oocyte is a diplod cell, containing two sets of chromosomes and these cells are arrested in the first stage of their meiotic division.  Ovaries contain about 1 million primary oocytes and approximately 200-400 of these primary oocytes will ultimately be released as mature ova and be available for fertilisation, while the rest will be degenerated. A small number of these primary oocytes will be surrounded by a layer of granulosa cells to form a primary follicle. These follices will not be able to mature appropriately between birth to puberty. At puberty, single follice matures and releases its oocyte in a process known as ovulation. Proceeding ovulation, a reproductive hormone known as the gonadrotrophin releasing hormone (GnRH), is released from the hypothalamus of the brain. This hormone stimulates the release of two other main reproductive hormones such as follicle stimulating hormone (FSH) and the luteinising hormone (LH) from the anterior lobe of the pituitary gland in the brain. There is a development of secondary follicle when FSH and LH act on primary follicle in the ovaries. At this point in time, oestrogen plays a crucial role in either preparing the female for the possibility of an embryo by thickening the endometrim or it acts on the hypothalamus to switch off the production of GnRK, FSH and LH through negative feedback. This is when meiosis 1 is completed and a secondary oocyte is formed together with a polar body. The secondary  follicle will continue to develop and eventually forms a mature ovarian follicle known as the Graafian follicle. On the 14th day of the menstrual cycle, rapidly rising oestrogen levels triggers LH production which causes ovulation. The cycle now reaches the luteal phase, and ruptured follicle will develop into corpus luteum, which secretes the hormone progesterone. There are two possibilities at the end of menstrual cycle.[1][2][3]

  1. A successful fertilisation of the released ovum will have occurred caused it to implant into the uterine lining.
  2. In the absence of fertilisation, the corpus lutem will cease hormone production, and signals the start of a new cycle.

References:

  1. http://www.ncbi.nlm.nih.gov/books/NBK10008/
  2. http://education-portal.com/academy/lesson/oogenesis-how-the-female-reproductive-system-produces-eggs.html
  3. http://www.fastbleep.com/biology-notes/32/159/859



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