Gametogenesis

Gametogenesis:

• Gametogenesis is the process of formation of gametes i.e., sperms and ovary from the primary sex organs (i.e., the testis in the males and the ovaries in the females) in all sexually reproducing organisms.
• Meiosis plays the most significant role in the process of gametogenesis.
• Gametogenesis includes spermatogenesis and oogenesis.

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 hypothalamus begins secretion of gonadotropin releasing hormone (GnRH).
• It initiates a significant increase in the secretion of follicle stimulating hormone (FSH) which induces spermatogenesis. 

Spermatogenesis

Process of spermatogenesis involves three phases: 

   1) Multiplication phase 

   2) Growth phase 

   3) Maturation phase 

Structure of sperm- Sperm is the male gamete. It is a motile, microscopic, elongated cell. It is divisible into three parts- head, middle piece and tail. 

Structure of Sperm 

Oogenesis- It is process of formation of the haploid female gamete i.e. egg or ovum from the diploid germinal epithelium. It involves the process of meiosis (and mitosis). Oogenesis process can be divided into three stages: 

1) Multiplication phase 

2) Growth phase 

3) Maturation phase 

Oogenesis 

Structure of secondary oocyte 

• In humans, an 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.  

• The cytoplasm of egg is also called ooplasm. 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. 

Unfertilized egg/ Ovum