Oogenesis 

The process of formation of ova in the ovary is called as oogenesis.

The germ cells are found in the form of germinal epthelium around the ovary.These cells are diploid and are called P.G.C.

The process of oogenesis is a continuous process but for the convenience or study it is divided into three phases-

1.      MULTIPLICATION PHASE

1. During this phase the germ cells divide reaptedly to form oogonia.
2. The oogonia are diploid and are also called as egg mother cells.
3. Oogonia is diploid only oogonia participate into oogenesis remaning cells convert into follicle cells or nutrative cells.

2.      GROWTH PHASE

1. It is the longest phase of oogenesis which does not involve any division.
2. The oogonia during growth phase enlarge to form primary oocyte which are diploid.
3. The growth in the oogonia is different in different species.
4. It is 20 times in mammals, 200 times in amphibians and 4000 times in birds.
5. In women growth rate is 14 days, in hen 6-14 days and in frog it is of 3 days.
6. The growth phase is further divided into 2 sub-phases viz. &

(a) Pre-vitellogenesis

1. The cytoplasm of the oogonium is called as ooplasm. During this phase the ooplasm becomes more granular & increases in amount. Yolk does not synthesis in this phase.
2. The mitochondria increased in number and they form groups which are called as mictochondrial clouds.
3. In ooplasm increased mitochondrial DNA becomes more than the nuclear DNA.
4. The mitochondrial clouds are also called as yolk nuclei of Balbiani.
5. The golgi body becomes hyperactive & secretes cortical granules. These granuels are full of mucopolysaccha- rides and get arranged in the peripheral part of the ooplasm.
6. At the time of fertilisation these cortical granules form the fertilization membrane.
7. The cortical granules are absent in the eggs of rats, insects, birds & guinea pig.
8. The endoplasmic reticulum is found mainly in the form of annulated lamelale & small vesicles.
9. The annulated lamellae store RNA. According to De Robertis, these lamellae get disintegrated at the end of oogenesis.
10. The size of the nucleus is increased gradually due to increased amount of nucleoplasm.
11. The nucleoli increase in size & particularily in amphibians the number is increaed upto 600 to 1200.
12. Formation of more amount of mRNA, RNA and rRNA from the nucleolar organizer parts of the chromosome. It is called as gene amplification by Epel.
13. Informosomes formed in the cytoplasm by mRNA. These informosomes participate protein synthesis during emergency.
14. In maximum fishes, amiphibians & reptiles lampbrush chromosomes formed in nucleus during growth phase.

(b) Vitellogenesis

1. This sub-phase mainly involves formation & storage of yolk or vitelin.
2. The yolk is stored form of food which consists of proteins, phospholipids & neutral fats.
3. The formation of yolk is extra-gonadal i.e. it takes place outside the ovary. In insects the vitellogenesis takes place in the fat body and in the chordates it occurs in the liver.
4. The yolk in soluble form is transported into the ovarian cells by the help of blood. The golgi body & ER transfer the soluble yolk into the mitochondria clouds which convert it into insoluble form. Hence, the mitochondria cloud help in the storage of the yolk.
5. The yolk of hen's egg contains 32 to 33% phospholipids, 16 to 17% proteins, 10% carbohydrates & 48-49% water.

Chemical Organization of Yolk

1. 48.7% water, 16.8% protein, 32.6% phopholipid and neutral fats] 1% carbohydrate.
2. If yolk have more amount of protein its called protein yolk, if yolk have more amount of fats its called fatty yolk.

3.       MATURATION PHASE

1. The maturation phase of oogenesis involves meiosis.
2. It finaly result in the formation of one haploid ootid & three haploid plocytes or polar bodies.
3. It is furtehr divided into maturation-I and maturation-II.
4. In maturation-I, meiosis-I divides diploid primary oocyte into one large & haploid secondary oocyte and one & small haploid polocyte-I.
5. In maturation-II the meiosis-II divides secondary oocyte into one functional ootid & one polocyte-II.
6. The polycyte-I may also divided by meiosis-II to form polocytes I a & I b.
7. The second maturation division normally takes place outside the ovary. In some animals it takes place before fertilization & in other it occurs after fertilization.
8. In many non-chordates both the maturation-I & II takes place inside the ovary.
9. The process of oogenesis finally forms only one functional ootid which functions as ovum.

                                                   Oogenesis