Oogenesis

The process of formation of mature female gamete or ovum is called oogenesis.

Spermatogenesis(process of formation of male gametes) begins at puberty in male. But oogenesis is initiated in female fetus in the embryonic development stage.

million of gamete mother cell which is called oogonia are formed within each fetal ovary.

After the birth of a female newborn, there is no formation of gamete occur.

Oogonia start division and enter into prophase -1 of the meiotic division and get arrested here in this early phase.

Then each primary oocyte get enclosed by a layer of granulosa cells and is called primary follicle.

a large number of these follicle degenerates from birth to puberty. therefore at puberty only 60,000 to 80,000 primary follicles are left in each ovary.

Then the primary follicle get enclosed by more layers of granulosa cells and a new theca are called secondary follicles

Next secondary follicles develop into tertiary follicles in which fluid-filled cavity appear called antrum..

Primary oocyte grows in size in tertiary follicle and complete it first meiotic division.

Resulting formation of a large haploid secondary oocyte and a tiny first polar body.

In the second maturation division the first polar body divide to form two second polar body.

Then the secondary oocyte again devides into unequal daughter cell, a large ootid and and very small second polar body.

The ootid grows into a functional haploid ovum.

Thus from one oogonium three polar bodies and one ovum are formed.

Ovum released from the ovary in the secondary oocyte stage and this secondary oocyte get mature in the fallopian tube when the sperm entered the secondary oocyte for fertilisation.

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