Contents
             Fertilization: Definition and types 
        Events of Fertilization in Sea Urchin
        Fig. 1 Recognition of Egg by Sperm in Sea Urchin and  Mouse
        Sperm contact with egg and Chemotaxix
        Fig. 2 Acrosomal interaction with egg membranes 
        Digestion of jelly layer in Sea Urchin
        Species-specific recognition in Sea Urchins
        Gamete binding and recognition in mammals
        Formation of Fertilization cone
        Fig. 3 Acrosomal reactions
        Prevention of polyspermy
        Formation of the zygote
        Biochemical Changes during Fertilization
            Summary
             Acknowledgements
       Fertilization: Definition and types 
    Fertilization is the process by which male and female gametes are fused together, 
     initiating the development of a new organism.
    The fertilization  process  in  animals  can  occur  either  internally  or  externally,  a 
     difference which is largely determined by the method of birth. 
    Animals which use viviparous  and ovoviviparous reproduction (embryos develop 
     within  the  animal’s  body),  and oviparous animals  which  lay  hard  shelled  eggs, 
     use internal fertilization.
    Animals which are oviparous, though produce eggs that are lacking, or have thin 
     egg membranes,  reproduce  by external  fertilization.  External  fertilization  is  a 
     reproductive strategy involving the joining of gametes outside of the body, either 
     in a spawning event, where gametes from both sexes are rapidly released into an 
     aquatic environment, or may occur when eggs are laid by a female on a substrate, 
     and are subsequently fertilized by a male.
    During  the  process  of  fertilization  the  sperm  and  ovum  of  the  same  species 
     approach and come in contact with each other. The entry of sperm initiates further 
     changes in the egg. The haploid nuclei of the sperm and ovum fuse, resulting in 
     the formation of a diploid zygote nucleus. This process of nuclear fusion is known 
     as syn-gamy or amphimixis.
     To understand the process one must know the structure of gametes, sperm and 
     egg.
       Events in Fertilization of 
       Sea Urchin
      The chemoattraction of the sperm to the egg by soluble 
      molecules secreted by the egg.
      2.The  contact  of  sperm  with  jelly  coat  of  the  egg.  The 
      exocytosis  of  the  acrosomal  vesicle  to  release  its 
      hydrolytic/proteolitic enzymes.
      3.The binding of the sperm to the extracellular envelope 
      (vitelline  layer  or  zona  pellucida  rupture)  the  hydrolytic 
      enzymes of acrosome degrade or make a hole in plasma 
      membrane.
      5. The sperm and egg plasma membranes fuse to form 
      cytoplasmic  bridge  through  which    sperm  pronucleus 
      (n)enters  the  egg  cytoplasm  and  fuses  with  egg 
      pronucleus (n), thus diploid cell (2n) zygote is produced.
   Fig. 1: Recognition of Egg and Sperm in (A)Sea 
   Urchin and  (B) Mouse
   The interaction of sperm and egg generally proceeds according 
   to five basic steps (Figure 7.8; Vacquier 1998):
         Sperm contact with egg and Chemotaxix
         How can sperm and eggs meet in such a dilute concentration, and 
           how can sperm be prevented from trying to fertilize  eggs  of 
           another species? 
         Two major mechanisms have evolved to solve these problems: 
           species-specific  attraction  of  sperm  and  species-specific  sperm 
           activation.
         One  chemotactic       molecule,   a   14-amino    acid   peptide 
           called resact, has  been  isolated  from  the  egg  jelly  of  the  sea 
           urchin Arbacia punctulata (Ward et al. 1985). 
         Resact diffuses readily in seawater and has a profound effect at 
           very  low  concentrations  when  added  to  a  suspension 
           of Arbacia sperm.
         A. punctulata sperm have receptors in their plasma membranes 
           that bind resact (Ramarao and Garbers 1985; Bentley et al. 1986) 
           and can swim up a concentration gradient of this compound until 
           they reach the egg.
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