Fertilization Eggssperms are destined to die within minutes

• Fertilization Eggs/sperms are destined to die within minutes, hours or days unless fertilization occurs. Activation of the egg Fusion of the haploid gametes. Invertebrates: sea urchins and external fertilization. Study internal fertilization in vitro. 1

Species-Specific Binding to the Zona Pellucida Induces the Sperm to Undergo an Acrosomal Reaction • 300, 000 human sperm ejaculated and yet only about 200 reach the egg. • Possibly chemical signals from the follicle cells that surround the egg attract the sperm. • But a sperm cell “ain’t no real sperm cell” until it has been “capacitated” or undergone capacitation • Requires about 5 -6 hours • Triggered by bicarbonate ions in the vagina • The bicarbonate ions enter the sperm and activates an enzyme (adenylyl cyclase) which will convert ATP to c. AMP. • All this serves to: • • • Alter lipid content of sperm plasma membrane. Alter glycoprotein composition of the plasma membrane. Increase sperm metabolism Increase sperm motility Sperm plasma membrane becomes more resistant to stimulation (hyperpolarized) 2

• Now the sperm can penetrate the egg’s follicle cells that surround the egg and bind to the zona pellucida. • ZP normally acts as a fertilization barrier between species. • ZP of mammalian eggs is composed of 3 main glycoproteins • ZP 2 is a long filament • ZP 3 is a long filament responsible for species-species binding of the sperm and therefore may have some receptors. • Possibility of infertility of ZP 3 glycoprotein is absent. • ZP 1 cross links the filaments. 3

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The Acrosomal Reaction • Contents of acrosome are released by exocytosis. • Contents are released after binding to the ZP and the influx of calcium ions into the sperm’s cytosol. • Hydrolytic enzymes help sperm get through ZP • More sperm surface receptors are exposed so that binding to the ZP 2 protein occurs • This ensures tight binding while the sperm wiggles its way through ZP. • Other proteins are exposed on the sperm’s plasma membrane that help with binding and fusion with egg’s membrane. • Infertility clinics will inject sperm directly into the egg and this will also cause fertilization. 5

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The Egg Cortical Reaction Helps to Ensure That Only One Sperm Fertilizes the Egg • Many sperm can “bind to the egg, ” but normally one fuses with the actual egg membrane. • Polyspermy results in faulty segregation of the chromosomes during cell division. • Mechanisms to prevent polyspermy • Fast Block: Depolarization of the egg’s plasma membrane • But the egg’s membrane quickly returns to its original charge separation • Slow or Secondary Block: Cortical Reaction of the egg • Calcium ion influx begins egg development • How could you be sure it was calcium ions causing the cortical reaction? • Cortical Granules and Cortical Reaction • Hydrolytic enzymes alter the ZP and “harden” it to prevent polyspermy. 7

The Egg Cortical Reaction Helps to Ensure That Only One Sperm Fertilizes the Egg 8

The Mechanism of Sperm-Egg Fusion is Still Unknown • Fertilin • transmembrane protein present in mouse sperm becomes exposed on the sperm’s surface during the acrosomal reaction. • May have a role in fusion • NH 2 ends are thought to bind to the egg’s plasma membrane. • Fertilin deficient mice are unable to bind to ZP or migrate out of the uterus into the oviduct. • Current Molecular Biology of Contraception: • ZP 3 or Fertilin could be targets. • Basically, just block any small molecules or receptors involved in 9 fertilization.

The Sperm Provides a Centriole for the Zygote • We have a zygote after fertilization • Differentiation • But first the two haploid nuclei must fuse, combine chromosomes to make a true diploid. 10

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• Summary: Mammalian fertilization begins when the _____ of a sperm binds in a ______-specific manner to the _______________ surrounding the egg. This induces the _________ reaction in the sperm, which releases the contents of the acrosomal vesicle, exposing enzymes that help the sperm to digest its way through the _____ to the egg plasma membrane in order to fuse with it. The fusion of the sperm with the _______ induces a _________ ion signal in the _____. The _______ ion signal activates the egg to undergo the ___________ reaction, in which __________________________ release their contents, including enzymes that alter the zona pellucida and thereby prevent the fusion of additional _______. The _______ ion signal also triggers the development of zygote, which begins after sperm and egg _______ nuclei have come together, and their ______________ have aligned on a single mitotic spindle, which mediates the first division of the ________. 13