Introduction to Human General Embryology Developmental Genetics

This is the series of events leading to the fusion of the male and female gametes. It also refers to the process in which the spermatozoon penetrates into the ovum to form fertilized ovum. It occurs in the ampulla of oviduct. The events include:

1. 
   Sperm Capacitation
   

2. 
   Sperm-Zona Pellucida Binding
   

3. 
   The Acrosome Reaction
   

4. 
   Penetration of the Zona Pellucida
   

5. 
   Sperm-Oocyte Binding
   

6. 
   Oocyte Activation and the Cortical Reaction
   

7. 
   The Zona Reaction
   

1. 
   Sperm penetrates the Corona Radiata
   
2. 
   Sperm attaches to Zona Pellucida (Sperm Zona Pellucida Binding)
   
3. 
   Acrosome of sperm releases trypsin like enzyme (Acrosome Reaction)
   
4. 
   Sperm penetrates the Zona Pellucida
   
5. 
   Sperm binds with Oocyte (Sperm-Oocyte Binding)
   
6. 
   Egg membrane depolarizes (Na diffuses in, Ca out) (Oocyte activation).
   
7. 
   Cortical granules released (Cortical Reaction)
   
8. 
   Zona hardens (Zona Reaction)
   
9. 
   Prevents multiple fertilizations
   
10. 
    Secondary Oocyte divides (completion of second meiotic division)
    
11. 
    Polar body formed (release of the second polar body)
    
12. 
    Formation of the male and female pronucleus.
    
13. 
    Fusion of the male and female gamete pronucleus (Egg and sperm nucleus fuse)
    

The acrosome reaction provides the sperm with an enzymatic drill to get through the zona pellucida.

The same zona pellucida protein that serves as a sperm receptor also stimulates a series of events that lead to many areas of fusion between the plasma membrane and outer acrosomal membrane.

Membrane fusion (actually an exocytosis) and vesiculation, expose the acrosomal contents, leading to leakage of acrosomal enzymes from the sperm's head.

Sperm that lose their acrosomes before encountering the oocyte are unable to bind to the zona pellucida and thereby unable to fertilize.

Following fusion of the fertilizing sperm with the oocyte, the sperm head is incorporated into the egg cytoplasm. The nuclear envelope of the sperm disperses, and the chromatin rapidly loosens from its tightly packed state in a process called decondensation. In vertebrates, other sperm components, including mitochondria, are degraded rather than incorporated into the embryo.

Chromatin from both the sperm and egg are soon encapsulated in a nuclear membrane, forming pronuclei.