Fertilization: Part 2: Prevention of Polyspermy Gilbert - Chapter 7 pp. 187-192.
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Transcript of Fertilization: Part 2: Prevention of Polyspermy Gilbert - Chapter 7 pp. 187-192.
Fertilization: Part 2: Prevention of Polyspermy
Gilbert - Chapter 7
pp. 187-192
Today’s Objectives
• Identify the following important components of the process of fertilization: gametes, spermatogonia, acrosome, flagellum, tubulin, oocyte, pronuclei, vitelline membrane, zona pellucida, resting membrane potential, capacitation, cortical granule reaction
• Identify the structure of an oocyte• Recognize the harm of polyspermy
– Describe various ways that polyspermy is inhibited
• Recognize that fertilization is species-specific
Fertilization: 4 major events
• Sperm and egg make contact and must recognize each other as the same species
• ONE (and only one) sperm enters egg
• Fusion of the genetic material
• Activation of egg to begin development
Polyspermy
• Monospermy is the norm– Restores the diploid chromosome number– Sperm centriole becomes the mitotic
spindle
• Polyspermy is disastrous– Results in triploid nucleus– Multiple mitotic spindles form
Dispermic Sea Urchin egg
Dispermic Human Egg -Note 4 centrioles
Prevention of Polyspermy
• Usually done by preventing multiple sperm from entering the egg
• Sea Urchins have 2 mechanisms:– Fast Block
• Involves a change in egg cell membrane potential
– Slow Block• Involves exocytosis of the cortical granules in
the egg
Fast Block to Polyspermy(Sea Urchin model)
• The egg has a different ionic concentration from the seawater in which it exists
• Egg has lower sodium ion concentration; higher potassium concentration
• This is maintained by sodium/potassium pumps in the egg cell membrane
• The difference in charge across the egg membrane can be measured as -70mV and is called the resting membrane potential
• Sodium-Potassium Pump– Pumps Sodium Out of cell– Pumps Potassium Into Cell
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Fast Block to Polyspermy(Sea Urchin model)
• 1-3 seconds after first sperm binds, the membrane potential shifts to +20 mV
• Sperm can no longer fuse to the egg
• Experimental evidence - Polyspermy can occur if*:– Eggs are supplied with an electrical current
that keeps charge at -70mV– Fertilization occurs in water with a low
sodium ion concentration*Jaffe 1976
Slow Block to Polyspermy -Sea urchin model
• Slower, mechanical, permanent block
• Occurs about a minute after sperm-egg fusion
• Upon sperm entry cortical granules fuse with the cell membrane and release several molecules
Cortical Granule Molecues• Cortical granule serine protease
– Releases vitelline membrane from its anchors to the cell membrane
– Clips off bindin molecules
• Mucopolysaccharides – Cause osmotic gradient– Water rushes into space between vitelline envelope– Vitelline envelope expands (lifts) and becomes the
fertilization envelope
• Peroxidase – Hardens the fertilization envelope
• Hyaline– Forms a coating around the egg, protects during early
embryonic development
Formation of fertilization envelope in sea urchin
Mammalian Cortical Granule Reaction
• Does not form a fertilization envelope
• Does modify Zona Pellucida so sperm cannot bind
• In mice, cortical granules cleave an essential portion of the ZP3 molecule
Calcium and the Cortical Granule Reaction
• Upon fertilization, intracellular concentration of Calcium ion in the egg increases
• This is necessary for the fusion of cortical granules with the cell membrane
• Calcium comes not from outside the egg, but from inside the egg itself
• The fusion begins near the site of sperm entry and continues in a wave across the egg
• A similar wave of calcium ion release can be observed
Calcium experiments - Ca2+ is directly responsible for cortical
reaction
• A23187 is a calcium ionophore– Transports Ca2+ across lipid membranes
• Placing sea urchin embryos in sea water containing A23187 results in cortical granule reaction & fertilization envelope to rise (without presence of sperm)
• If Ca2+ chelator is injected into egg, no cortical reaction occurs
Fertilization: 4 major events
• Sperm and egg make contact and must recognize each other as the same species
• ONE (and only one) sperm enters egg
• Fusion of the genetic material
• Activation of egg to begin development