Oogenesis

Oogenesis is the process of maturation of primordial germ cells from primary oocyte to mature ovum.Oogenesis is the  process of gametogenesis in female.

Introduction to process of oogenesis

All the primordial female germ cell are contained within the cortex of the ovaries, which are the paired reproductive glands of female .These germ cells appear in the embryo in the 3rd week of development .These primordial cells multiply rapidly by mitotic division and give rise to numerous oogonia .By the end of 3rd month some of the oogonia increase in cell size and differentiate into primary oocyte.By the 7th month all oogonia cease to divide and are converted to primary oocyte.Immediately after its formation all the primary oocyte enters into pro-phase of the first meiotic division in the process of maturation.Each primary oocyte is surrounds by a layer of flattened cells called follicular cells .The primary oocyte together with the follicular cells form the primary ovarian or primordial follicle.
At birth each ovary contains about 2 million primary oocyte.The primary oocyte completes the pro-phase of first meiotic division at birth and enters into a prolonged resting phase,dictyotene stage , in which they remain shortly before ovulation.It is estimated that about 40,000 follicles persist at puberty and the rest undergo degeneration to form interstitial cells of ovary..

Process of Oogenesis

• Maturation of primary oocyte is affected by a process of meiosis which consists of meiosis 1 and meiosis 2.At puberty the primary oocyte from dictyotene stage completes the first meiotic division which started during prenatal life .The nuclear membrane disappears and the chromosomes arrange themselves in homologous pairs at the equator of the achromatic spindle.After crossing over ,the chromosomes separate and one member of each pair passes centrally ,while the other member passes outwards and separates from oocyte to form first polar body.This result in the formation of two daughter cells each with 23 chromosomes.One cell is large and has abundant cytoplasm and is known as secondary oocyte .The small cell with scanty cytoplasm ,is the first polar body which appears in the pervitelline space.

• The secondary oocyte immediately enters into the 2nd meiotic division..The Graafian follicle fully matures and ruptures on the surface of ovary to release the secondary oocyte.This process is called ovulation.The secondary oocyte enters the uterine tube.The entrance of oocyte into the uterine tube is encouraged by movement of ciliated epithelium and peristaltic contraction of the musculature of the tube.

• If fertilization takes place in the uterine tube by the union of male and female gametes ,then secondary oocyte completes the 2nd meiotic division and results in formation od two unequal daughter cells each possessing 23 chromosomes .The large cell presenting abundant cytoplasm ,is calledmature ovum and the small cell forms the second polar body which is accommodated in perivitelline space..Thus the process of oogenesis starts in the ovary in intrauterine life and is completed during the process of fertilization.

• In the absence of fertilization secondary oocyte does not complete the second meiotic division and degenerates within 24 hours of ovulation and is expelled during menstrual bleeding.

Process of Oogenesis