06 Fertilization

Fertilization

How will I know which way to go? How can I get inside the egg? If I'll get close enough, willI it recognize me? How can prevent others from entering same egg I pick? How willthe I know which egg is the right one for me ?

Sperm and Egg Attraction: Action at a distance

Sperms exhibit chemotaxis ● only mature eggs secrete chemoattractants sperms avoid immature eggs – prevents unnecessary –

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sperms swim up a concentration gradient of chemoattractants – faster swimming with increasing concentrations – concentrations provide

Sperm chemotaxis to resact of Arbacia

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chemoattractants are species-specific – prevents unproductive interactions between sperm and egg of different species

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chemoattractants in mammals work when sperm are in the oviduct * Read: Sidelights and Speculations

Gamete recognition and contact

Sea urchin

Acrosomal reaction

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species-specificity in sea urchin fertilization – sperm attraction – sperm activation – sperm adhesion to egg surface

Mammalian

Diagram of the fibrillar structure of the mouse zona pellucida. The major strands of the zona are composed of repeating dimers of proteins ZP2 and ZP3 occasionally crosslinked together

ZP3-induced

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ZP3 induces acrosomal reaction after binding sperm plasm with zona binding proteins is shed during acrosomal reaction

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secondary binding with fertilin in inner acrosomal membrane and ZP2 moderately speciesspecific

Gamete fusion and sperm entry

lysis of vitelline envelope or zona pellucida ● fusion of the sperm and egg plasma membranes – fusogenic proteins ●

entrance of sperm nucleus w/ centriole, mitochondria, flagellum ● formation of fertilization cone ●

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polymerization of actin homology of sperm and

Fusion of gametes and entry of sperm in sea urchins

Fusion of gametes and entry of sperm in golden hamster

Prevention of polyspermy

Consequences of polyspermy ● > 1 sperm nuclei = abnormal ploidy ● > 1 sperm centriole = > 2 daughter cells with unequal chromosomes ● = abnormal

Prevention of polyspermy in sea urchins ● fast block change in electrical potential of egg membrane

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slow block

Fast block to polyspermy ● change in the membrane resting potential – proteins from sperm activate Na+ channels in egg membranes

Implications to artificial fertilization polyspermy can be induced by keeping a – membrane potential (  Na+ in water) ● fertilization can be prevented by keeping a + ●

Slow block to polyspermy ● cortical granules fuse with egg membrane upon sperm entry exocytosis of proteins into space between egg membrane and vitelline envelope

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Proteins from cortical granules ●

protease – cleave proteins that attach vitelline envelope to egg membrane – cleave bindin

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mucopolysaccharides – production of osmotic gradient -> influx of water -> expansion of vitelline envelope -> fertilization envelope

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peroxidases – cross-link tyrosine residues in vitelline envelope – hardens the fertilization envelope

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hyalin – forms coating around the egg which plays several roles in cleavage * initiated by

Mammalian cortical granule reaction ● no fertilization envelope formed ● no hyalin layer created ● modification of ZP3 and ZP2 (zona reaction)

Nacetylglucosaminidases cleave Nacetylglucosamine from ZP3 ● proteases clip ZP2 ●

Cortical Granule Reaction

Inositol triphosphate

For lipid synthesis

* ER

Late reaction

Fusion of genetic materials

Sea urchin sperm nucleus decondenses -> male pronucleus ● exchange of proteins between sperm chromatin and egg ●

rotation of male o pronucleus by 180 ● formation of aster ● male and female pronuclei are brought together ● = zygote nucleus ●

Mammals tangent approach than perpendicular ● uncoiling of sperm chromatins by glutathione ● male pronuclues ●

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aster formation (centrosome of male pronuclei)

migration of pronuclei ● breakdown of nuclear membranes ● chromatin condense ●