Lecture 11: Prevention of Polyspermy PDF

Lecture 10 Prevention of polyspermy The prevention of polyspermy In normal monospermy, in which only one sperm enters the egg, a haploid sperm nucleus and a haploid egg nucleus combine to form the diploid nucleus of the fertilized egg (zygote), thus restoring the chromosome number appro...

Lecture 10 Prevention of polyspermy The prevention of polyspermy In normal monospermy, in which only one sperm enters the egg, a haploid sperm nucleus and a haploid egg nucleus combine to form the diploid nucleus of the fertilized egg (zygote), thus restoring the chromosome number appropriate for the species. The centriole of the sperm, will divide to form the two poles of the mitotic spindle during cleavage. The entrance of multiple sperm -------polyspermy  Consequences of polyspermy In the sea urchin Fertilization by two sperms results in a triploid nucleus (3 copies of each chromosome) Centriole of each sperm divides to form the two poles of a mitotic apparatus (4 poles total) So, triploid chromosomes divide into four cells  Consequences of polyspermy As there is no mechanism to control proper distribution of chromosome number and type So, chromosomes are distributed unequally. Some cells receive extra copies of certain chromosomes and other cells lack them. Such cells either die or develop abnormally. Block to polyspermy Species have evolved different ways to prevent union of more than two haploid nuclei. The most common way is to prevent the entry of more than one sperm into the egg.  Block to polyspermy The sea urchin egg has two mechanisms to avoid polyspermy Fast reaction ---- an electric change in the egg plasma membrane Slower reaction ------ exocytosis of the cortical granules. Fast Block to polyspermy The fast block to polyspermy Electric potential of the egg plasma membrane changes. Membrane potential = Voltage/ Charge/ ionic difference across two sides of membrane i.e., outside and inside. This membrane is a selective barrier between Egg cytoplasm Outside environment  The fast block to polyspermy The ionic concentration of Na+ and K+ is different Inside outside the plasma membrane This concentration difference is important for sodium and potassium ions. Seawater has high sodium ion concentration  The fast block to polyspermy Plasma membrane regulates differences by Inhibiting the entry of sodium ions into the oocyte Preventing potassium ions from leaking out into the environment The resting membrane potential is generally about -70 mV Because the inside of the cell is negatively charged with respect to  The fast block to polyspermy Occurs 1-3 seconds after the binding of the first sperm Membrane potential changes to +20 mV Small influx of sodium ions into the egg.  Significance Sperm can fuse if resting potential is -70 mV Cannot fuse if membrane potential is positive So, no more sperm can fuse with egg.  Confirmatory Experiments Polyspermy can be induced if sea urchin eggs are artificially supplied with an electric current that keeps their membrane potential negative. Conversely, fertilization can be prevented entirely by artificially keeping the membrane potential of the eggs positive. The fast block to polyspermy can also be prevented by lowering the concentration of sodium ions in the water. If the supply of sodium ions is not sufficient to cause the positive shift in membrane potential, polyspermy  The fast block to polyspermy How the changes in membrane potential of egg block secondary fertilization??? Not known Perhaps, sperm contains a voltage-sensitive component (possibly a positively charged fusogenic protein) Insertion of this component into the egg plasma membrane is regulated by the electric charge across the membrane. Electric block to polyspermy occurs in frogs Fast Block of polyspermy  Slow Block to polyspermy in Sea Urchin  The slow block to polyspermy The eggs of sea urchins (and many other animals) have a second mechanism to ensure that multiple sperms do not enter the egg cytoplasm. The fast block to polyspermy is transient, as the membrane potential of the sea urchin egg remains positive for only about a minute. The slow block is accomplished by the cortical granule reaction A slower, mechanical block to polyspermy  The slow block to polyspermy Directly beneath the sea urchin egg plasma membrane are about 15,000 cortical granules, each about 1 μm in diameter. Upon sperm entry, these cortical granules fuse with the egg plasma membrane They release their contents into the space between plasma membrane and vitelline envelope. The slow block to polyspermy  The slow block to polyspermy Several proteins are released during cortical granule exocytosis. 1. Proteases 2. Mucopolysaccharides 3. Peroxidase 4. Hyaline  The slow block to polyspermy 1. Proteases dissolve the protein that connect the vitelline envelope proteins to the cell membrane remove the bindin receptors and any sperm attached to it  The slow block to polyspermy 2. Mucopolysaccharides produce an osmotic gradient that causes water to rush into the space between the plasma membrane and the vitelline envelope This causes the envelope to expand and become the fertilization envelope The slow block to polyspermy --- sea urchin 3. Peroxidase hardens the fertilization envelope ---- by cross linking tyrosine residues on adjacent proteins. The fertilization envelope begins to form at the site of sperm entry and continues its expansion around the egg. As it forms, bound sperms are released from the envelope. This process starts about 20 seconds after sperm attachment and is completed by the end of the first minute of fertilization.  The slow block to polyspermy --- sea urchin 4. Hyaline forms a coating around the egg. Elongated microvilli extend from plasma membrane and their tips get attach with hyaline layer. This layer provides support for the blastomeres during cleavage.  Slow Block to polyspermy in Mammals The slow block to polyspermy --- mammals In mammals, the cortical granule reaction does not create a fertilization envelope ----- but its ultimate effect is the same. Released enzymes modify the zona pellucida sperm receptors in a way that they can no longer bind with the sperm. During this process ----- zona reaction, both ZP3 and ZP2 are modified. The slow block to polyspermy --- mammals The cortical granules of mouse eggs contain an enzyme that removes the terminal sugar residues of ZP3 This results in the release of bound sperm from the zona prevention of the attachment of other sperms The slow block to polyspermy --- mammals Cortical granules of mouse eggs contain N-acetylglucosaminidase enzymes Cleaves N-acetylglucosamine from ZP3 carbohydrate chains N-acetylglucosamine binds with sperm The slow block to polyspermy --- mammals Proteases Clip ZP2 ZP2 loses its ability to bind with sperm. Thus, once a sperm has entered the egg, other sperms can no longer initiate or maintain their binding to the zona pellucida and hence rapidly shed. Calcium as the initiator of the cortical granule reaction. The mechanism of the cortical granule reaction is similar to that of the acrosomal reaction. After fertilization, intracellular calcium ions are released from their intracellular stores (endoplasmic reticulum). In this high-calcium environment, cortical granule membranes fuse with the egg plasma membrane release their contents. Calcium as the initiator of the cortical granule reaction. Once the fusion of the cortical granules begins near the point of sperm entry, a wave of cortical granule exocytosis propagates around the cortex to the opposite side of the egg. In sea urchins and frogs, Endoplasmic reticulum is present in the cortex and surrounds the cortical granules