Fertilization - Part 1

Gilbert - Chapter 7

pp. 181-186

Fertilization

“There is perhaps no phenomenon in the field of biology that touches so many fundamental questions as the union of the germ cells in the act of fertilization; in this supreme event all the strands of the webs of two lives are gathered in one know from which they diverge again and are rewoven into a new individual life-history. . . ” F.R. Lillie

Fertilization

• Fusion of two gametes to create a new individual, with a genome different from both parents

• HAS 2 MAJOR GOALS:– Joining of genetic material to create new

variations (sex)– Creation of a new organism (reproduction)

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

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

Recognition of Sperm and Egg

1. Chemoattraction of sperm to egg by soluble molecules

2. Exocytosis of acrosomal vesicle of sperm to release its enzymes

3. Binding of sperm to extracellular envelope (vitelline membrane or zona pellucida)

4. Passage of sperm through extracellular envelope

5. Fusion of egg and sperm membranesNOTE: In mammals, step 2 and 3 are reversed

SEA URCHIN

MOUSE

Step 1 - Chemoattraction - Sea urchin model

• Some species (marine animals) release eggs & sperm into vast environment

• Often other species are present• How do correct egg & sperm find each

other at dilute concentrations? • How are sperm and egg modified so

that only same species can successfully fertilize?

• Two mechanisms have evolved to address these issues:– Species-specific attraction of sperm– Species-specific sperm activation

• Example: – Sea urchins have resact peptide in the egg

jelly of the egg• 14 amino-acid peptide• Can diffuse in sea water• Attracts sperm to egg• Acts as a sperm-activating peptide

– Increases sperm motility– Increases mitochondrial production of ATP

Step 2: Acrosome Reaction(Sea Urchin Model)

• Initiated by species-specific compounds in the egg jelly

• Compounds bind to receptors on sperm cell membrane

• Calcium channels open; calcium flows into sperm head

• This induces fusion of the acrosomal membrane with sperm cell membrane

Extension of Acrosomal Process (Sea urchin model)

• After acrosomal vesicle fuses, an acrosomal process is extended.

• Occurs by polymerization of globular actin molecules into actin filaments

• Acrosomal process is important for species-specific recognition– In S. purpuratus the acrosomal process contains a

protein called bindin – This bindin can bind to the surface of S.

purpuratus egg but not A. punctulata

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Experimental evidence of the species-specific role of binding in

fertilization• Biochemical studies show other species

of sea urchin sperm contain bindin molecules that are closely related, but different

• Immunohistochemistry shows– bindin on the tip of the acrosomal process

Mammalian Gamete Binding and Recognition

• Zona pellucida plays the role of vitelline membrane

• Sperm binds at ZP; ZP initiates acrosomal reaction

• There is some species-specificity, but this is not as necessary if fertilization occurs internally

The Mouse Zona Pellucida• Made up of 3 proteins: ZP1, ZP2, ZP3• ZP3 is the protein that initially binds the sperm,

according to several lines of evidence• Carbohydrate moieties on ZP3 molecule bind to

proteins on sperm cell membrane• ZP3 also initiates the acrosome reaction, allowing

sperm to traverse the thick zona pellucida• This is mediated by an influx of calcium ions, as in

Sea Urchins• Sperm must then bind ZP2 in order to get across

the zona

Immunofluorescent staining for mouse ZP3 binding proteins

Radioactively labeled ZP3 binds to capacitated mouse sperm

Fusion of Egg and Sperm Membranes

• Mechanisms vary between species and are not well understood– Mammalian sperm fusion occurs on the side of the

sperm rather than the tip

• Cell adhesion molecules, such as integrin seem to be important– Female mice with gene knockout of integrin

associated CD9 molecule are infertile– Sperm can’t fuse with their eggs– Can be reverse by microinjecting the sperm into the

egg

QuickTime™ and aTIFF (Uncompressed) decompressor

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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