Explain the process of oogenesis.

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  • The ovum is formed by the process of oogenesis.

  • It starts during embryonic growth and millions of gamete mother cells (oogonia) are formed in the foetal ovary.

  • These cells undergo meiosis, but get temporarily arrested at the prophase and are called primary oocytes.

  • Before reaching puberty, a large number of primary oocytes degenerate and the remaining ones get surrounded by layers of granulosa cells and new theca and are called secondary follicles.

  • The secondary follicles are then converted into tertiary follicles that have characteristic fluid-filled cavity called antrum. At this stage, the primary oocyte present within the tertiary follicle completes meiosis, which results in the formation of haploid secondary oocyte and a tiny polar body.

  • This tertiary follicle further changes into the Graafian follicle. The secondary oocyte is surrounded by the zone pellucida.

  • Then the Graafian follicle ruptures to release the ovum by ovulation.

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 oogenisis starts with the mitotic division of germinal epithelial cells of the ovary to form 2 primary oogonia which further undergo second mitotic division to form 4 secondary oogonia.This is called  as multiplicative phase. Then the accumulation of biomolecules in the secondary oogonia occurs. Its size also increases and primary oocyte is formed. This is called growth phase. Then primary oocyte undergoes first meotic division and 1 secondary oocyte and 1 polar body is formed . Polar body degenerates and secondary oocyte undergoes second meotic division to form ovum. This is called maturation phase. Oogenisis starts at embryonic stage and primary oocyte is formed till the time of birth of the girl child. Secondaty oocyte is formed at puberty and ovum is released at the time of fertilisation.

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Oogenesis involves the formation of haploid cells from an original diploid cell, called a primary oocyte, through meiosis. The female ovaries contain the primary oocytes. There are two major differences between the male and female production of gametes. First of all, oogenesis only leads to the production of one final ovum, or egg cell, from each primary oocyte (in contrast to the four sperm that are generated from every spermatogonium). Of the four daughter cells that are produced when the primary oocyte divides meiotically, three come out much smaller than the fourth. These smaller cells, called polar bodies, eventually disintegrate, leaving only the larger ovum as the final product of oogenesis.

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