01-26-2011, 11:50 AM
It all starts from germs cell precursors migrating from yolk sac to the developing gonadal ridge...then>>>
1. When they reach the gonadal ridge, they undergo several rounds of mitosis and finally differentiate into Oogonia (46,2n)...this means they have 46 numbers of single chromatids and NOT 23 pairs !!
2. The Oogonia now start entering the Meiosis. When they enter meiosis, they are termed Primary ooCYTES.
3. In the Interphase of meiosis 1, the primary oocytes double the amount of DNA and now have 46 chromosomes (46,4n) instead of 46 chromatids (46,2n) as in oogonia...
4. These oocytes are then arrested in prophase 1 (diplotene) and remain in this phase up to the puberty...remain as 46,4n !!
5. At puberty, under the influence of FSH, an oocyte completes the meiosis 1 and is now termed a secondary oocyte (now 23,2n) (this occurs monthly) + a polar body is also formed which degenerates...
6. The secondary oocyte immediately enters meiosis 2 but progresses only to halt in the metaphase 2 (metaphase of meiosis 2)
7. It is at this stage (metaphase 2) that an egg is OVULATED...
8. When a sperm fertilizes a secondary oocyte, it completes meiosis 2 and becomes a mature oocyte (ovum) (now 23,n) + a polar body that degenerates...
9. The genetic meterial of a sperm combines with an egg (23,n) and zygote is formed...
Summary:
OOGONIA are derived from germ cells from yolk sac...they are called OOCYTES when they start meiosis 1...they remain in meiosis 1 until puberty when under the influence of FSH they complete meiosis 1 and start meiosis 2. They get arrested in metaphase of meiosis 2 and are ovulated as such...they complete meiosis when fertilized by a sperm... the overall result of meiosis is 1 mature oocyte and 3 polar bodies that degenerate...
see this diagrams >>
http://en.wikipedia.org/wiki/File:Gray5.svg
http://upload.wikimedia.org/wikipedia/co...iagram.jpg
Hope this clears things up
1. When they reach the gonadal ridge, they undergo several rounds of mitosis and finally differentiate into Oogonia (46,2n)...this means they have 46 numbers of single chromatids and NOT 23 pairs !!
2. The Oogonia now start entering the Meiosis. When they enter meiosis, they are termed Primary ooCYTES.
3. In the Interphase of meiosis 1, the primary oocytes double the amount of DNA and now have 46 chromosomes (46,4n) instead of 46 chromatids (46,2n) as in oogonia...
4. These oocytes are then arrested in prophase 1 (diplotene) and remain in this phase up to the puberty...remain as 46,4n !!
5. At puberty, under the influence of FSH, an oocyte completes the meiosis 1 and is now termed a secondary oocyte (now 23,2n) (this occurs monthly) + a polar body is also formed which degenerates...
6. The secondary oocyte immediately enters meiosis 2 but progresses only to halt in the metaphase 2 (metaphase of meiosis 2)
7. It is at this stage (metaphase 2) that an egg is OVULATED...
8. When a sperm fertilizes a secondary oocyte, it completes meiosis 2 and becomes a mature oocyte (ovum) (now 23,n) + a polar body that degenerates...
9. The genetic meterial of a sperm combines with an egg (23,n) and zygote is formed...
Summary:
OOGONIA are derived from germ cells from yolk sac...they are called OOCYTES when they start meiosis 1...they remain in meiosis 1 until puberty when under the influence of FSH they complete meiosis 1 and start meiosis 2. They get arrested in metaphase of meiosis 2 and are ovulated as such...they complete meiosis when fertilized by a sperm... the overall result of meiosis is 1 mature oocyte and 3 polar bodies that degenerate...
see this diagrams >>
http://en.wikipedia.org/wiki/File:Gray5.svg
http://upload.wikimedia.org/wikipedia/co...iagram.jpg
Hope this clears things up