02 Reproduction in Lower and Higher Animals - part 07 - Gametogenesis - Oogenesis

02 Reproduction in Lower and Higher Animals - part 07 - Gametogenesis - Oogenesis.....!


Oogenesis: 


  • It is process of formation of the haploid female gamete i.e. egg or ovumfrom the diploid germinal epithelium. 
  • It involves the process of meiosis (and mitosis). 
  • Like spermatogenesis, oogenesis process can be divided into three stages : 
  1. Multiplication phase 
  2. Growth phase 
  3. Maturation phase


 1. Multiplication phase: 

  • In this stage, the primary germinal cells PGCs (2n) of ovary undergo repeated mitotic division to form millions of gamete mother cells or oogonial cells (2n). 
  • This process is completed in the embryonic stage of human females. 
2. Growth phase: 

  • Some of the oogonia stop division and begin to increase in size and form the primary oocytes (2n). 
  • Cellular organelles like ER, golgi appratus and mitochondriaincrease in number. 
3. Maturation phase: 

  • Oogenesis takes place in the ovaries. 
  • The process is initiated prior to birth of the female baby. 
  • The primary oocytes (2n) enter the maturation phase which includes meiotic division (Meiosis I and Meiosis II). 
  • The diploid primary oocytes undergo meiosis I (reduction division) to form 2 haploid daughter cells. 
  • This division is peculiar in females as both the daughter cells are with haploid number of chromosomes i.e. 23 chromosomes. 
  • But due to unequal division of cytoplasm, of the 2 daughter cells produced, one is a large cell called secondary oocyte (n) and another is a small cell called 1st polar body (n). 
  • Normally the 1st polar body does not enter meiosis II. 
  • The secondary oocyte (n) proceeds meiosis II, only upto metaphase II. 
  • It's division is further stopped or arrested at this stage. 
  • The secondary oocyte is shed from the graafian follicle and ovary. 
  • The restart and completion of meiosis II will happen only with entry of the sperm. 
  • This last phase is usually completed in the ampulla of the fallopian tube at the time of fertilization. 
  • In this division also, the two unequal daughter cells are formed- the large cell is ovum (n) and the small cell is 2nd polar body (n). 
  • The ovum (n) so formed functions as the female gamete and is ready for fertilization. (Completion of meiosis II and completion of fertilization go hand in hand. 
  • If the secondary oocyte does not receive the sperm / spermatozoa, it is shed off along with menstrum). 
Structure of secondary oocyte: 


  • In human, unfertilized egg when ovulated i.e. released from the ovary is actually the secondary oocyte. 
  • It is non-cleidoic (without shell) and microlecithal (yolk is present in very small quantity). 
  • It is approximately 0.1mm (100 microns) in size.
  • It is rounded, nonmotile and haploid female gamete. 
  • The nucleus of the egg appears large and is called germinal vesicle. 
  • Typical nucleus or pronucleus is formed at the time of fertilization. 
  • The cytoplasm of egg is also called ooplasm. It is devoid of centrioles. 
  • The egg is surrounded by various coverings.
  • The egg membrane is called vitelline membrane. It secretes a non-cellular glycoproteinous membrane, zona pellucida on its outside.
  •  Adhering to the outer surface of zona pellucida are several radially elongated cells forming the corona radiata. These cells are derived from the innermost layer of granulosa cells. 
  • They are firmly held to the zona pellucida and to each other by hyaluronic acid (mucopolysaccharide). 
  • Between the vitelline membrane and the zona pellucida is a fluid filled perivitelline space. The first polar body lies in this space.
  • The egg shows polarity. 
  • The side having germinal vesicle and first polar body is called animal pole while the side opposite to it is called vegetal pole. 

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