Oogenesis: Human Reproduction ||Class 12 Biology

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     The process of formation of a mature female gamete is called oogenesis. It is the production of eggs (ova) that starts in ovaries of female foetus (of 25 weeks). It begins before birth but complete only after fertilization.

     It comprised of the following three main stages:
1. Multiplication Phase
2. Growth Phase
3. Maturation Phase

1. Multiplication Phase: Cells of germinal epithelium undergoes mitotic division producing undifferentiated germ cells called oogonia or egg mother cells within each foetus ovary. These cells start dividing and enter prophase-I of meiotic division and get primarily arrested at this stage, known as primary oocytes.

2. Growth Phase: Each primary oocyte then gets surrounded by a large of granulosa cells and called primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca and are called secondary follicles.

3. Maturation Phase: During this phase, the secondary follicles soon transforms into tertiary follicles. These are characterised by a fluid cavity called antrum. The primary oocyte within this tertiary follicle grows in size. The cytoplasm of fully grown primary oocyte undergoes first meiotic division producing two unequal daughter nuclei in which larger cell is called secondary oocyte and a small cell is known as first polar body.
     The secondary oocyte starts undergoing second meiotic division and form the tertiary follicle which further changes into a mature follicle, called Graafian follicle. The secondary oocyte forms a new membrane called zona pellucida around it and starts its second meiotic division but remains suspended in metaphase-II, till sperms enter into it. At this stage, the Graafian follicle, ruptures to release the secondary oocyte from the ovary. The release of the secondary oocyte from the ovary is called ovulation.

Location: India

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