what is oogenesis?

There are three main stages to reproduction, namely gametogenesis, fertilization and development of embryo. Gametogenesis is the production of gametes. Production of sperm is called spermatogenesis and production of eggs is called 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.

I hope you understood.


 

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Formation of haploid gamete( OVUM) inside the female's body is called OOGENESIS . Oogenesis starts when the female is in feotal stage at the age of 25th week. Oogenesis occurs in three phases                                                                                         1. Multiplication phase                                                                                                                                                                                  2.growth phase                                                                                                                                                                                            3maturation phase                                                                                                                                                         MULTIPLICATION PHASE:-  It occurs during feotal life. in this ohase the premordial germ cell in the germinal epithelium divides and gets converted into oogonia.later on 30 million oogonia shifts from germinal epethelium into the cortex of each ovary .within the cortex oogonia forms groups and each group is called EGG NEST or EGG TUBE OF PLUGGER.                                                                                                                                                                                         2. GROWTH PHASE:- It also occurs during feotal stage. in this phase onwe of the oogonia in each egg nest shows growth and gets converted into PRIMARY OOCYTE ,while the remaining oogonia in each egg nest gets converted  into follicular cells & surrounds the primary oocyte ,the  resulting structure is called PRIMARY FOLLICLE.                        3.MATURATION PHASE:-It begins in feotal stage but ends when the female attains puberty.in the beginning of maturation phase primary oocyte with 2n condition in each primary follicle undergoes meiosis 1  but is hanged at DIAKINESIS of PROPHASE I due to accumulation of cyclins. after that the female is born.                                                    Till puberty no change occur in the primary oocyte. After puberty FSH is released & meiosis I is completed & the primary oocyte is converted into SECONDARY OOCYTE and one POLAR BODY. Secondary oocyte under the influence of FSH undergoes meiosis II but is hanged at METAPHASE II due to accumulation of METAPHASE PROMOTING FACTOR.In this stage ovulation occurs & the secondary oocyte goes into the fallopian tube . The secondary oocyte is suspended at metaphase  II due to presence of metaphase promoting factor which forms SECURINE  which prevents the dissolution of COHESION PROTIENS between sister chromatids , thus preventing their seperation. If the sperm combines with the secondary oocyte , it releases ANAPHASE PROMOTING COMPLEX (APC) which degrades securine into SEPRASE & helps in dissolution of cohesion proteins & seperates the sister chromatids & helps in completion of meiosis II thus converting secondary oocyte into OVUM & 2nd POLAR BODY.    

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