We already discussed about the process of fertilization and how the fertilization cone formed, the fusion of plasma membrane of both the gametes i.e sperm and ovum and cortical reaction with the help of enzymes involved in it so and so. But it is specific to discuss the mechanism in-detail that how the process called block to polyspermy occured to check the entry of multiple sperm into ovum.
Ovum membrane has fusigenic nature which may allow multiple sperm into its ooplasm therefore it is necessary to change its fusigenic property. After fertilization the sperm haploid nucleus and ovum haploid nucleus combined to form diploid pronucleus in the form of zygote and along with centriole and mitochondria helps in cleavage of the zygote in which centriole divide to form two poles of zygote which initiate development.
Polyspermy is nothing but fertilization of ovum with more than one sperm. This results in formation of non-viable zygote and further it leads to critical condition i.e abnormalities in developmental process. To prevent this anomalies in developmental process the process required is called ‘Block to polyspermy’.
Mechanism for block to polyspermy :
To deduce the mechanism of ‘ block to polyspermy ‘, it is convenient to take the common model as Sea urchin. The methods of polyspermy prevention in Sea urchin have been described by two main pathways ;
i ) A fast reaction carried out by an electric change in ovum plasma membrane and
ii ) a slow reaction caused by exocytosis of cortical granules. These two pathways are called as fast block and slow block respectively.
i) Fast block :
Fast block can described interms of electric potential of ovum plasma membrane. There is a ionic difference present in between the ooplasm and outer environment, where ovum plasma membrane acat as a selective barrier. Ionic differentiation i.e sea water contain more sodium ion than ooplasm and vice-versa for potassium ions. It is seen that there is 70mV electric potential can be achieved in ovum plasma membrane by inserting a electrode into ooplasm and outside environment. Electric potential of ovum plasma membrane is maintained in negative i.e -70mV. Due to this steady electric potential it inhibits the influx and efflux of ions between ooplasm and it’s environment. This membrane potential called resting potential.
When the sperm binds to ovum plasma membrane the membrane potential shift to +20mV and there is a change in ions concentration and caused small influx of sodium ions and thus positive resting potential inhibits the other sperm to binds with ovum plasma membrane because sperm contains positively charged fusigenic protein.
ii ) Slow block :
Fast block is generally transient in nature and not sufficient for prevention of polyspermy. The slow block or mechanical block is carried out by the process of cortical granule reaction. This process is take about 10-60 seconds after which penetration of sperm into ovum’s plasm membrane takes place. By the cortical reaction there is a signal send to vitelline layer that the confirmation done by entry of specific sperm and there is a molecular and physical changes takes place during the fusion of membranes. This fusion caused the release of inositol triphosphate ( IP3 ) which triggers the release of Ca2+ ions. The release of Ca2+ ions caused vesicle present in ooplasm fuse with membrane to release enzymes which inturn causes number of changes in ovum.
- Receptors are removed from both the gametes i.e ovum and sperm membranes.
- Absorption of water molecules causes the ovum membranes to small and push the vitelline membrane away from the plasma membrane.
- The vitelline membrane hardens and works as a barrier for entry of super- numerary spermatozoa.
Polyspermy in humans :
In human also above described two mechanism operate to processed the block of polyspermy. Though the fast block i.e ions concentration change/changes in charge is not very feasible, therefore the molecular changes mechanism is considered as main mechanism for block of polyspermy.
Ca2+ ion oscillation event is play a key role in cortical reaction and extrusion of cortical granules when sperm attached with oocyte. Increase Ca2+ concentration leads to exocytosis of cortical granules from just below the oocyte membrane into perivitelline space. Release enzymes of cortical granules such as hydrolase, proteinase, and peroxidase which prevents the entry of further sperm by modifying the structure of sperm receptor such as ZP2 and ZP3. It results in hardening of zona pellucida and takes about 5 to 8 minutes after oocyte activation. Theodore Boveri’s analysis of polyspermy is a classic of experimental and descriptive biology to explain this process in details.