02 Reproduction in Lower and Higher Animals - part 06 - Gametogenesis - Spermatogenesis

02 Reproduction in Lower and Higher Animals - part 06 - Gametogenesis - Spermatogenesis


Gametogenesis: 
  • The gametogenesis is the process of formation of gametes in sexually reproducing animals. 
  • The male gamete is sperm and the female gamete is ovum or egg. 
  • The gametes are formed from primordial germ cells of gonads.

Spermatogenesis: 

  • The process of formation of the male gamete (sperm) or spermatozoa from the germinal epithelium of testis is called spermatogenesis.
  • At the onset of puberty, the hypothalamusbegins secretion of gonadotropin releasing hormone (GnRH). 
  • It initiates the significant increase in the secretion of follicle stimulating hormone (FSH)which induces spermatogenesis. 
  • Each seminiferous tubules is lined by a single layer of cuboidal epithelial cells called germinal epithelium. 
  • The cells of germinal epithelium undergo spermatogenesis to produce sperms. 
  • Process of spermatogenesis involves three phases : 
  1. Multiplication phase
  2. Growth phase
  3. Maturation phase


 1. Multiplication phase: 

  • The primordial germ cells (2n) of seminiferous tubules undergo repeated mitotic divisions to produce large number ofspermatogonia (2n). 
  • Each spermatogonium is diploid and with 46 chromsomes. 

2. Growth phase: 

  • Some of the spermatogonia stop dividing and grow in size to develop into primary spermatocytes (2n) due to accumulation of food.


3. Maturation phase: 

  • It involves meiotic or reduction division. 
  • The spermatocyte undergoes the first phase of meiotic division (meiosis I) leading to formation of two haploid cells called secondary spermatocytes (n), which are with 23 chromosomes each. 
  • The secondary spermatocyte undergoes second phase of meiotic division (meiosis II) to produce four haploid spermatids. 
  • The spermatid is nonmotile and non-functional.
  •  It gets transformed into a functional spermatozoa by the process called spermiogenesis. 
  • During this process of change, the spermatids remain held to each other and to the sertoli cells by cytoplasmic bridges. 
  • The sperm heads remain attached to the sertoli cells and their tails hanging in the lumen of seminiferous tubule. 
  • During spermiogenesis, length of spermatid increases. 
  • Centrioles are rearranged as primary and distal centrioles. 
  • Mitochondria become spirally coiled and acrosome is formed from golgi complex.


Structure of sperm: 


  • Sperm is the male gamete. 
  • It is a motile, microscopic elongated cell. 
  • It is divisible into three parts- 
  1. head
  2. middle piece and 
  3. tail.

1. Head: 

  • The sperm head is oval in shape and contains haploid nucleus. 
  • Above the nucleus, there is a cap like structure called acrosome. 
  • It is formed from the golgi body. 
  • Acrosome contains hydrolytic enzymes; hyaluronidase and proteolytic enzymes like zona lysins and corona penetrating enzymes. 
  • Neck: It is a very short region having two centrioles i.e. proximal centriole and distal centriole. 
2. Middle piece: 


  • It has an axial filament surrounded by 10-14 spiral turnsof mitochondria (nebenkern). 
  • It produces energy necessary for the movement of sperm. 
3. Tail: 

  • It is a long, slender and tapering part containing cytoplasm and fine thread- axial filament. 
  • The axial filament arises from the distal centriole and travels through out the length of tail. 
  • It is partly surrounded by plasma membrane (main piece). 
  • The part without plasma membrane is called end piece.

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