Oogenesis in Frogs:
Oogenesis is the process of formation of oogonia and its development into mature ova.Oogenesis takes place inside the ovary of a female frog.The ovary epithelium cells are the primordial germ cells which multiply in number to form oogonia.Oogonia further give rise to follicle cells and the primary oocyte.Cells which fail to undergo repetetive mitotic divisions
become follicular cells.
There is no way to determine which oogonium is to be selected for the formation of primary oocyte or which is to become a follicular cell.
The different stages of maturation of oogonium are primary oocyte,secondary oocyte,mature oocyte and fertilized oocyte(ovum)
The development requires 3 stages seasons during which the oogonia passes through 3 phases i.e., Multiplication phase,Growth phase and Retention phase.
MULTIPLICATION PHASE:
This phase of ova starts right from the tadpole stage of the organism and exists upto the older stage i.e., adult stage.During this time the oogonium enters mitosis to form primary oocyte.The primary then enters the prophase I of meiosis I and before completion of this division it enters into the growth phase.GROWTH PHASE:
In this phase the primary oocyte increases tremendously in size.The volume of nuclear sap increases therefore the nucleus becomes inflated and is called as Germinal vesicle.The volume of cytoplasm of oocyte also increases qualitatively and quantitatively.Amount of yolk platelets and glycogen etc increases with tremendous rate and a definite polarity is determined.Yolk tends to accumulate at one pole,i.e.,vegetative pole and occupies 2/3rd space of egg.Germinal vesicle is pushed and move towards the animal pole which is comparatively yolk free.This process of yolk formation is also known as vitellogenesis.There is formation of vitelline membrane around the egg plasma membrane the primary oocyte reach then full growth towards the end of the second season but they are not immediately released.
RETENTION PHASE:
In this phase,the primary oocyte in the ovary are retained until the next breeding season. The 1st maturation division occurs at the time of ovulation.It forms secondary oocyte haploid and the first polar body. These changes occur as the egg leaves the ovary and before it reaches th oviduct. When the egg reaches the oviduct, the second maturation division begins.Spindle of meiosis II is formed and it progresses as far as early stage of metaphase.Again the growth is arrested until and unless the ovum is fertilized.There may be division of 1st polar body to give rise to two polar bodies therefore there may be 2-3 polar bodies attached to the egg surface.After this the egg(female)pronucleas and male pronucleus fuse together to form zygote nucleus which is diploid in nature.
As soon as the sperm touches the vitelline envelope of the egg, the metaphase is completed and there is a formation of matured ovum and the 2nd polar body.
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