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.
**I was doubtful at one point but now my doubt is clear.
Actually, central cell does not divide by mitosis but takes nurishment and grows into a primary oocyte. primary oocyte divides by meiosis as i explained previously.
My spelling Mistakes-
Primordial Germ Cell
(a) Multiplication phase (b) Growth phase
(c) Maturation phase
(a) Multiplication phase : In this stage primordial germ cells or ovum mother cells are repeatedly divided by mitosis to form large number of diploid oogonia. This process completes in embryo stage of female in most higher animals.
(b) Growth phase : In this process oogonia grow in size and form primary oocytes. The growth phase is the longest phase oogenesis (except humans). During growth phase size of egg increases many times.
(c) Maturation phase : Oogenesis takes place in the ovaries. In contrast to males the initial steps in egg production occur prior to birth. By the time the foetus is 25 weeks old, all the oogonia that she will ever produce, are already formed by mitosis. Hundreds of these diploid cells develop into primary oocytes, begin the first steps of the first meiotic division, proceed up to diakinesis, and then stop any further development. The oocytes grows much larger and complete the meiosis I, forming a large secondary oocyte and a small polar body that receives very little amount of cytoplasm but one full set of chromosomes.
Hope it helps