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.
Species-Specific Binding to the Zona Pellucida Induces the Sperm to Undergo an Acrosomal Reaction • 300,000,000 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 cAMP. • 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)
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 • ZP2 is a long filament • ZP3 is a long filament responsible for species-species binding of the sperm and therefore may have some receptors. • Possibility of infertility of ZP3 glycoprotein is absent. • ZP1 cross links the filaments.
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 ZP2 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.
The Egg Cortical Reaction Helps to Ensure That Only One Sperm Fertilizes the Egg • Many sperm can “bind to the egg,” but normally only 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.
The Egg Cortical Reaction Helps to Ensure That Only One Sperm Fertilizes the Egg
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 • NH2 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 Contra- ception: • ZP3 or Fertilin could be targets. • Basically, just block any small molecules or receptors involved in 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.
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 ________________.