The process of development and maturation of ovum is known as oogenesis . Gametogenesis in female is also called oogenesis .
It is consists of three phases- multiplication , growth and maturation phase .
Multiplication phase
During multiplication phase , the germinal epithelium cells divide and detach to produce oogonia , which multiply mitotically and project into the stroma as a cord , the egg tube of Pfluger , which later becomes a round mass , the egg nest .
One cell in the egg nest grows to form diploid primary oocyte cells having the same number of chromosomes as in parent somatic cells , which cease to divide and enter the growth phase.
Growth phase
This phase of the primary oocyte is very long extending over many years .
There is accumulation of food materials and other resources for nourishment of the oocyte .
Meiosis begins in the primary oocytes soon after their formation . However , the oocytes are arrested in the early part of meiotic prophase I ( diplotene stage ) , the first resting stage . They undergo a round of DNA synthesis and chromosome pairing takes place , but meiosis does not proceed further until years later.
Maturation phase
After primary oocyte has finished its growth, there are two specialised nuclear divisions , first one is the reductional division .
Beginning of puberty , a small number of primary oocytes are activated each month . However , only one continues meiosis I , producing two haploid cells of dissimilar size , smaller cell is called first polar body and larger cell is called secondary oocyte.
The secondary oocyte proceeds with meiosis II but the division gets arrested in metaphase II stage , the second resting stage. This is due to an activity called cytostatic factor which maintains arrest through preventing loss of Maturation Promoting Factor ( MPF ) .
MPF is a protein in cell cycle which stimulates M - phase of cell cycle .
It is in this stage of oocyte that the ovum is shed during ovulation . It passes into oviduct, where in the ampulla part , cell cycle will resume only after the entry of sperm .
Ca2 + rise initiated by fertilising sperm results in degradation of regulatory unit of MPF through Anaphase Promoting Complex ( APC ) thus promoting completion of cell cycle .
The first polar body may divide to form two - second polar bodies . Thus from one oogonium , one ovum and three polar bodies are formed .
The ovum , is the actual female gamete . The polar bodies take no part in reproduction and soon degenerate due to lack cytoplasm and food . The formation of non - functional polar bodies enables the egg to get rid of excess chromosomes.
MANISH MEVADA
M.Sc, M.Phil, B.Ed
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