explain the steps in the formation of an ovum from an oogonium in humans
Stages before birth (25 weeks old foetus)
- Germinal layer of ovary forms many Premordial Germ Cell/ Egg Mother Cell.
- Premordial Germ Cells divide by mitosis to form many oogonia.
- These oogonia arranged in a chain called Egg Tube of Pfluger/ Vigerous Cords.
- This forms a circular structure called Egg Nest, attached to germinal layer of ovary.
- A cell of egg nest grows larger and called Central Cell/Egg Mother Cell(2n) while other cells arround this central cell nourish it.
- MULTIPLICATION PHASE- Egg Mother Cell divides many times by mitosis to form manyOogonium(2n).
- GROWTH PHASE- Oogonium divides by mitosis to form Primary Oocyte.
Stages after birth (Attaining puberty)
- MATURATION PHASE- Primary Oocyte(2n) enters in meiosis I and forms a large Secondary Oocyte(n) and a small Ist Polar Body(n) .Scondary Oocyte releases from the ovary during menstrual cycle. If sperm reaches to the oviduct and gets touch with secondary oocyte thensecondary oocyte enters in meiosis II and forms a large Ootid/ Ovum and a small 2ndPolar Body. During this process 1st polar body divides by mitosis to form two polar body. After this process ovum pronuclus fuses with sperm pronucleus to form zygote(2n).
- If Sperm is not present in oviduct then secondary oocyte degenerates and menatrual flow starts.So, in menstrual flow there is seconday oocyte is present not ovum.