NB: Sperm with morphologic abnormalities lack normal motility and probably cannot fertilize oocyte

Unlike gametogenesis, Oogenesis is associated with several unique features that include:

1. 
   An early embryonic commencement (3rd month) with the first meiotic division of the primary oocytes.
   

2. 
   Termination of the first meiotic division at the prophase stage at birth.
   

3. 
   Graduation atrophy/degeneration of oocytes during child hood and adulthood.
   

4. 
   Resumption of first meiotic division at puberty and subsequent completion with the release of the first polar body.
   

5. 
   Commencement of the 2^nd meiotic division after the completion of the 1^st but terminates also to resume at fertilization. As such 2^nd meiotic division is never completed if fertilization does not occur.
   

6. 
   At fertilization however, the 2^nd meiotic division resumes and then completed in the presence of the spermatozoon. It is believed that the spermatozoa play an inductive role.
   

7. 
   It is associated with the female reproductive cycles (i.e. ovarian and menstrual cycles).
   

8. 
   It ceases at the attainment of menopause.
   

Other facts associated with Oogenesis include:

1. 
   By the third Month, Oogonia undergo a number of mitotic divisions. By the end of the third month, they are arranged in clusters surrounded by a layer of flat epithelia cells called Follicular cells. The follicular cells are believed to originate from the surface epithelium of the ovary. The majority of the Oogonia continue to divide by mitosis, but some of them arrest their cell division in prophase of meiosis 1 and form Primary Oocytes. During the next few months, Oogonia increase rapidly in number.
   

2. 
   It is observed that by the fifth month of prenatal development, the total number of germ cells in the ovary reaches its maximum, estimated at about 7 million. At this point cell deaths begin, and many Oogonia as well as primary Oocytes become atretic.
   

3. 
   By the seventh Month, majority of the Oogonia have degenerated except for a few near the surface. All the surviving primary Oocytes have entered prophase of meiosis 1, and most of them are individually, surrounded by a layer of flat epithelia cells.
   

4. 
   At birth, the total number of primary oocytes is estimated to vary from 700, 000 to 2 million.
   

5. 
   During childhood, most of the oocytes become atretic; only approximately 400,000 are present by the beginning of puberty, and fewer than 500 will be ovulated.
   

6. 
   Some oocytes that reach maturity late in life have been dormant in the diplotene stage of the first meiotic division for 40years or more before ovulation.
   

7. 
   The fact that the risk of having children with chromosomal abnormalities increases with maternal age (particularly in mothers who are 35 and older) indicates that primary oocytes are vulnerable to damage as they age.
   

8. 
   As it is known, fetal oocytes are arrested in the first prophase of meiosis shortly after chromosome crossing over. At this time, the nuclear membrane is still visible and is called germinal vesicle. Some oocytes remain at this "dictyate" stage until the beginning of the menstrual cycle. With each cycle, a cohort of these follicles begins to develop and resume meiosis. One dominant follicle complete maturation and release an egg.
   

9. 
   At ovulation, the oocyte completes the first meiotic division and extrudes the first polar body and proceeds to metaphase of the second meiotic division, where it again arrests pending arrival of the sperm.
   

10. 
    It is only after activation by the process of sperm-egg fusion, that meiosis II is completed with extrusion of the second polar body and formation of a female pronucleus.
    

11. 
    Like spermatogenesis, abnormal oocytes can be encountered e.g.
    

1. 
   An ovarian follicle may contain two or three clearly distinguishable primary oocytes but they usually degenerate before reaching maturity.
   

2. 
   One primary oocyte may contain two or even three nuclei but usually die before reaching maturity.
   

3. 
   Similarly, as described for the sperm cells, non-disjunction of chromosomes may occur and some ova may have 22 or 24 chromosomes instead of 23; and such has been attributed to some genetic disorders. E.g. Monosomy and Trisomy.
   

ii. Proliferating follicular cells are also termed granulosa cells.

iii. Terms describing follicular stages during oogenesis include:

1. 
   Primordial Follicle
   
2. 
   Primary Follicle
   
3. 
   Secondary follicle
   
4. 
   Tertiary or Grafian Follicle
   

• 
  Primordial follicles appear in the ovaries during the third month of development.
  

• 
  Primary follicles consist of primary oocyte surrounded by the Zona Pellucida (a glycoprotein coat) and a layer of follicular cells.
  

• 
  Primary follicle becomes the secondary follicle with rapid follicular cell proliferation and the development of the follicular anthrum. It is otherwise called the antral follicle
  

• 
  The mature, vesicular or graafian follicle is characterized by an enlarged antrum with defined granulosa cells in contact with the zona pellucida (called corona radiate) and those that bulge into the antrum called the cumulus oophorus.