Fertilization

Major Questions

• How do the egg and sperm find each other?

• How do they know that they are from the same species?

• How do we assure that only one sperm fertilizes the egg?

• How do the two nuclei combine to form one cell?

Sperm Components

• Sperm head

• Haploid nucleus

• Flagellum with mitochondria

• Acrosome

Acrosome

• Derived from the Golgi apparatus

• Contains proteases and enzymes to digest sugars

Meet the Egg or Ovum

• Each meiosis forms one haploid egg with accumulated cytoplasm

• 2-3 haploid polar bodies

• May become fertilized prior to final division (in mammals)

Egg Components

• Nutritive proteins

• Ribosomes and tRNAs

• mRNAs

  • Maternal RNAs may influence fetal development

• Morphogenetic factors

  • Transcription factors

  • Paracrine factors

• Protective chemicals

  • UV filters, DNA repair enzymes

When Egg Meets Sperm– External Fertilization

• Sperm released 

• Chemotaxis toward sperm-activating peptides (SAPs) secreted by egg– i.e. resact

• SAPs bind to receptors on sperm

• GTP converted to cGMP

• Opens calcium channels

• Sperm moves to egg

External Sperm/Egg recognition

• Attraction of sperm

• Exocytosis of acrosomal vesicle

• Extension of acrosomal process 

• Sperm binding to extracellular envelope of egg

• Sperm passage through extracellular envelope

• Fusion of gamete membranes

Species-specific Recognition (Sea Urchins)

• Bindin on sperm binds to jelly on egg

  • Bindin exposed once sperm binds to egg membrane 

  • Specific for each sea urchin species

  • Quick evolving protein

Egg Metabolism Activation

• Calcium wave

• Activates DNA replication and cell proliferation

• May be triggered by soluble proteins from sperm

Mammalian Fertilization and Early Development

• Oocyte released with cloud of cumulus cells

• Fertilized in ampulla

• Cleavage stages 

• Blastocyst forms

• Zona pellucida dissolves

• Implantation

Capacitation (Mammals)

• Sperm need oviduct to gain capacitation

  • Sperm motility

    • Flagella (especially once in the oviduct)

    • Uterine muscle contractions 

    • Follow thermal gradient (rather than only chemical)

• Oviduct may slow sperm and release them in batches to increase chances of fertilization

• Near egg, sperm gain motility and ability to do acrosome reaction

When Egg Meets Sperm– Internal Fertilization

• Sperm capacitation 

• Sperm binds to zona carbohydrates

• Acrosome reaction

• Enzymes released from acrosome make hole in zona pellucida

• Sperm and egg membranes fuse

Zona Pellucida

• Sperm must penetrate the cumulus cells and zona pellucida

• Carbohydrate layer

• Sperm have receptors for ZP carbohydrates

Fusion of Egg and Sperm mammals

• Sperm bind to egg membrane on its side

• Equatorial region

• Fusion of sperm and egg membranes

• Entry of sperm into egg cell

Activation of the Egg in Mammals

• Ca++ overload

• PLC pathway

• Will trigger meiosis II of egg

  • Cyclin B and separase degradation

  • 2^nd polar body formed

• Sperm mitochondria destroyed

  • Marked with ubiquitin to enable destruction in egg

• Sperm centriole survive

• Pronuclei fuse

Mammalian Fertilization

• Two female or two male nuclei cannot form viable fetus

• Imprinting of paternal and maternal genes

• Nonequivalence of nuclei

  • Hydatidiform mole

  • Parthogenesis

Molar Pregnancy

• One or two sperm fertilize egg that has lost DNA

• Hydatidiform mole

• Sperm duplicates DNA

• Usually 46 XX

• Can be triploid or tetraploid

• Uterus contains mass of cumulus cells

Parthenogenesis

• Non-fertilized egg or sperm starts to develop

• Natural process in some plants, algae, worms, and insects

• Not possible in human, though partial parthenogenesis has occurred