Lecture 9 - Fertilization

Fertilization

the process whereby two gametes fuse together to create a new individual with genetic potentials derived from both parents

Parthenogenesis

embryonic development without a sperm

Fertilization in 4 steps

1. contact and recognition between sperm and egg, same species

2. Regulation of sperm entry into the egg, one sperm gets into the egg

3. Fusion of the genetic material of the sperm and egg

4. Activation of egg metabolism to start development

Recognition of egg and sperm

chemoattraction of sperm to egg by soluble molecules secreted by the egg, exocytosis of the acrosomal vesicle to release the enzyme

Chemoattraction

sperm is attracted to the egg by soluble molecules secreted by the egg

Chemotaxis

releasing of chemotactic molecule from the egg (the egg jelly) which attracts sperm to migrate toward the egg

Sperm activation or acrosomal reaction

the fusion of acrosomal vesicle with the sperm plasma membrane results in the exocytosis of the acrosomal vesicle and release of the protein digesting enzymes that can digest a path through the jelly coat to the egg surface.

When is the acrosomal activation triggered

once sperm makes contact with egg jelly

Cumulus

layer of cells surrounding eggs in mammals, made up of ovarian follicular cells.

Capacitation

mammalian sperms are activated by the contents of the female reproductive tract

What layer does mammalian sperm penetrate

cumulus layer

When does the acrosomal reaction occur in mammals?

When sperms reach the zona pellucida

Contact of gametes

major species-specific recognition step, binds sperm to extracellular envelope of egg, passes through envelope, fuses egg and sperm cell membranes

What is the extracellular envelope?

vitelline layer or zona pellucida

What is the mechanism for the contact of gametes?

Specific proteins on sperm surface and specific receptors of the egg

What happens once two gametes contact?

Membranes of the sperm and egg fuse, resulting in the injection of the sperm nucleus into the egg

Polyspermy

The entrance of multiple sperm

Fast block of polyspermy

the egg membrane changes in electric potential once the egg membrane fuses with a sperm membrane, influx of sodium ions

Slow block of polyspermy

Forms a fertilization envelope, cortical granules fuse with the egg plasma and release contents in the space between the cell membrane and vitelline membrane, expans the vitelline membrane creating the fertilization membrane, permanently blocks any more sperm

What is in a cortical granule

1. proteases

2. mucopolysaccharides

3. Peroxidase enzymes

4. Hyalin

What do proteases do?

Dissolves the vitelline posts that connect the vitelline membrane to plasma membrane, clips off bindin receptors

What do mucopolysaccharides do?

produce an osmotic gradient that causes water to rush into the space between the cell membrane and vitelline membrane, causes expansion to fertilization envelope

What do peroxidase enzymes do?

hardens the fertilization membrane

What does hyalin do?

Forms a coating around the egg

Fusion of genetic material

Male and female haploid pronuclei fuse to form the diploid zygote nucleus

How do the two pronuclei migrate towards each other?

Microtubular processes radiated from the centrosomes

Activation of egg metabolism

enacted once sperm fuses with egg, influx of free ions and release of calcium change ion concentration and pH levels in egg, activates. metabolic reactions

Maternal effects

After fertilization diploid nucleus is not immediately expressed, proteins required come from mRNAs stored in the oocyte

Hypothesis of how dormant mRNA in cytoplasm of oocyte suddenly acquire their competence for translation

1. masked maternal message hypothesis

2. Poly A tail hypothesis

3. translational efficiency hypothesis

4. Other mechanisms

   1. Uncapped messages

   2. Sequestered messages

Masked maternal message hypothesis

oocyte messages are physically masked by proteins, prevents mRNA from attaching to ribosomes, upon maturation masking proteins fall away

The poly A tail hypothesis

Size of poly A tail differs, long=translated now, short = silenced and stored, contain poly A tail sequence and cytoplasmic polyadenylation element (CPE), upon oocyte maturation mRNAs become polyadenylated again and can translate

Translational efficiency hypothesis

Lower pH impedes protein synthesis

Uncapped messages

at fertilization there is a burst of methylation in oocytes and caps are able to be added so translation can take place

Sequestered messages

mRNAs are sequestered and prevented from getting close to the ribosomes