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

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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