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Oogenesis

 What is oogenesis ?

Gametogenesis in females ,which causes formation and maturation of  female gametes (ovum ) is called oogenesis. 
Oogenesis occurs in the ovary of females and begins before birth in female ( spermatogenesis occurs at puberty).

Oogenesis is divided into three stages, these are 1) multiplication phase
                  2) growth phase 
                  3) maturation phase 

Multiplication phase-

During fetal stage of development,  primodial germ cell originated in endoderm of yolk sac migrates to ovaries.  Some cells are divided mitotically to form oogonia also known as egg mother cells.
Approximately 20 lakh oogonia are present in each ovary. It enters growth phase 

Growth phase-

In this phase growth of oogonia occurs . Oogonia drives nutrients from surrounding follicular cells and converted in to primary oocyte. 
In growth phase no division occurs. 




Maturation phase-

The primary oocyte from growth phase under goes two successive meiotic division in this phase.
Primary oocyte undergoes first meiotic division (meiosis I) and arrested in diplotene stage.
At the onset of puberty primary oocyte from diplotene stage starts diving and gives two unequal daughter cells. The larger cell is called secondary oocyte ( n) and the smaller one is called first polar body( n).
The ovulation of secondary oocyte occurs and it enters in to the ampula of the fallopian tube. 
In this time the secondary oocyte faces another arrest ( second resting stage) and this stage continues until the entry of sperm for fertilization. 
Meiosis II  starts when exposure to sperm,which further divides the secondary oocyte in to a large ovum and a small second polar body.
The two polar bodies formed are soon degenerate. 


Structure of ovum 





Ovum is a rounded, non motile structure of extremely small in size. 
A human ovum is alecithal ,which have cytoplasm called ooplasm and other cytoplasmic organelles. The nucleus present in ovum is large in structure which present in the center. 
The outer thin membrane of the ovum is called vitelline membrane. Immediate to this membrane their is a gap present known as perivitelline space. The zonapellucida layer surrounded over the perivitelline space.
The outer most layer made up of several layers of epithelial cells called corona radiata adhere to zonapellucida layer. 
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