Oogenesis

oogenesis is process of formation of ovum in ovary.

1. OOGENESIS Dr. R.K. Upadhyay PGT, Biology KV, 39 GTC, Varanasi cantt

2. Oogenesis  is the  process  by which the female gametes (ovum) are created.  Site - Ovary Ploidy – Haploid (n) Number- one egg in a month OOGENESIS

3. Oogenesis is initiated in the EMBRYONIC STAGE. Oogenesis starts   in fetus 6-7 week- start oogenesis 20 weeks – 6/7 million oogonia Birth – about 2million ogonia Puberty- 60,000- 80,000 oogonia in the form of oocytes OOGENESIS

4. PRE NATAL PUBERTY FERTILIZATION STAGES

5. oogonia Germinal epithelium in ovary divide by mitosis, producing a couple of million oogonia ( 2n) The oogonia multiply by mitotic divisions forming egg nest. One egg from egg nest grows and become the primary oocytes . No more oogonia are formed or added after birth.

6. Meiotic arrest I Oogonium develops into priamry oocyte ( oocytogenesis ) These primary oocyte remains arrested at Meiosis I ( at prophase I/ Diplotene ) The number of primary oocyte remains 60000- 80000 at puberty. Out of these only about 400 primary oocyte will enter in the menstrual cycle. And out of these 400 only one will form ovum during each menstrual cycle. Meiosis I completed about 36- 48 hr prior to ovulation.

7. MEIOTIC ARREST II After puberty Meiosis I continues One polar body and one secondary oocyte formed Secondary oocyte again enters in Meiotic arrest II (at methaphase II state) It remains arrested till fertilization with sperm.

8. Reason of Meiotic A rrest In menstrual cycle ovary releases secondary oocyte It reamin arrested at this stage as it does not have centriole . When sperm fuses with egg it transfer its centriole from the tail region to the egg. After getting centriole Meiotic II begins. In this way second polar body and ovum/ ootid is formed.

9. Growth phase .This phase of the primary oocyte is very long. It may extend over many years. The oogonium grows into a large primary oocyte by taking food from the surrounding follicle cells . It happens after puberty.

10. Development of follicles Primary follicle Primary oocyte surrounded by Granulosa cell (Theca) Small cavities nay observed

11. Development of follicles Secondary follicle Secondary oocyte surrounded by two layers cell (Theca externa and Theca enterna ) Cavity becomes large (ANTRUM)

12. Tertiary follicle ( G raffian follicle) Contains secondary oocyte Large Antrum filled with fluid Graffian follicle rupture due to LH surge and seconadry oocyte released. Development of follicles

13. Ootid / ovum

14. ell type ploidy / chromosomes chromatids Process Time of completion Oogonium diploid/46(2N) 2C Oocytogenesis  ( mitosis ) third trimester primary Oocyte diploid/46(2N) 4C Ootidogenesis  ( meiosis  I) ( Folliculogenesis ) Dictyate  in prophase I for up to 50 years secondary Oocyte haploid/23(1N) 2C Ootidogenesis  ( meiosis  II) Halted in metaphase II until fertilization Ootid haploid/23(1N) 1C Ootidogenesis  ( meiosis  II) Minutes after fertilization

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