Week 2 Part 2

What happens when the sperm comes into contact with the egg?

The egg and sperm membranes fuse initiated by an increase in calcium in the egg.

What is the purpose of calcium increasing in the egg during egg activation and fertilization?

• It initiates the completion of meiosis II

• Prohibits polyspermy

What is the oogonia?

The immature oocyte that grows and differentiates to produce a primary oocyte

What is ovulation?

The release of the oocyte from the ovary and completion of metaphase 1

What does ovulation produce?

Secondary (mature) oocyte and first polar body

What happens after the secondary oocyte is produced?

It enters meiosis II and becomes blocked at metaphase

When the secondary oocyte is suspened in metaphase, what releases it?

Fertilization. Produces haploid pronucleus and 2nd polar body

How does the secondary oocyte stay in metaphase until fertilization?

High levels of maturation promoting factor (MPF)

What is MPF made of

cyclin B and cyclin dependent kinase

What is MPF?

Maturation promoting factor; holds the secondary oocyte in metaphase

How is the MPF level lowered so the secondary oocyte can exit metaphase II?

An increase in cytosolic calcium results in the degradation of cyclin B which decreases MPF activity and it exits metaphase

What does entry into the mitotic phase of the cell cycle require?

Activation of MP. Exit requires deactivation of MPF.

What do sea urchins do when the sperm reaches the egg?

When it reaches the jelly layer, acrosome reaction happens where the acrosome vesicle is opened to digest it. Actin behind the vesicle also forms the acrosome process which binds to the vitelline envelope. Acrosome process membrane and egg membrane fuse.

How do the sperm of sea urchins differ from mouses?

Sea urchin sperm does not require capacitation and contains actin behind the acrosome vesicle that forms the acrosome process

When the egg and sperm membranes fuse, what happens?

There is a wave of increasing calcium across the egg starting at the point of sperm entry which initiates the completion of meiosis II for the egg and prohibits polyspermy

What are the two “blocks” animals can use to prevent polyspermy during fertilization?

• Rapid (transient) block

• Slower (permanent) block

What is pathological polyspermy?

When more than one sperm in the egg causes defects during cleavage division

What is physiological polyspermy

When more than one sperm in the egg is the norm and still results in regular ploidy

Before that fast block to polyspermy, what are the charges around the egg membrane?

Positive on the outside, negative on the inside

What does the fast block to polyspermy depend on?

Changes to the voltage potential across the egg plasma membrane from negative to positive

After fertilization, what is the egg memrbanes charges after fast block?

Negative on the outside, positive on the inside

How does the fast block to polyspermy work?

Increase in cystolic calcium in the egg activates anion channels that allow chloride to be released from within the egg. This causes the charges outside to be changed to negative and the inside positive

What is the slow block to polyspermy in simple terms?

Involves the modification of the vitelline envelope to form the fertilization membrane which prevents extra sperm from adhering to or passing through 

Where the formation of the fertilization membrane begin in the slow block to polyspermy?

At the point of sperm entry

What is cortical granule exocytosis or the cortical reaction?

In the slow block to polyspermy, when the cortical granules (beginning at the sperm entry site) are released, they release chemicals and enzymes that modify/harden the vitelline membrane to form the fertilization membrane. Proteases cleave protein anchors linking the vitelline membrane to plasma membrane. Remaining cortical granule material forms the hyaline layer

What is the hyaline layer

Remaining cortical granule material around the egg that was released from slow block to polyspermy and helps provide structure, lubrication, and nutrients

How do echinoderms block polyspermy?

Slow and fast block

How do mammals block polyspermy?

Only slow block and cortical granule exocytosis alters the zona pellucida instead.

What is depolarization?

When a cells charge distribution changes to become more positive or less negative inside the cell

How do fish block polyspermy?

Fish eggs have a tiny opening called the micropyle and this is plugged up after a sperm enters by swelling cortical granule material and the shell gets thicker

How do amphibians block polyspermy?

Slow and fast block

What is genomic imprinting?

When only one copy of a gene (the maternal or paternal allele) is expressed due to parental-specific gene silencing is established in sperm and egg cells

What is a gynogenetic embryo?

An embryo with two maternal genomes 

What is an androgenetic embryo?

An embryo with two paternal genomes

What is maternal imprinting?

When the maternal allele is silenced (in the egg) and the paternal allele (in the sperm) is expressed

When does genomic imprinting happen?

Before fertilization. The effects of both genes being expressed happen after fertilization (as the egg and sperm fuse)

When does a gynogenetic embryo fail during development?

When it tries to grow the placenta as paternal genes are needed for this

When does an androgenetic embryo fail during development?

Embryonic growth doesn’t even start

What is an epigenetic modification?

Stable and heritable in gene expressions without changes to the original DNA sequence

• E.g., gene silencing

What is the mechanism behind genomic imprinting?

the epigenetic modification called gene silencing

How is gene silencing achieved in genomic imprinting?

DNA methylation and histone modification which influence chromatin structure 

How are imprinted genes often arranged?

In clusters that contain multiple protein coding mRNAs and at least one non-coding RNA

What is the expression of genes in the imprinted cluster controlled by?

Imprint control element (ICE)

What is the main example cluster of imprinted genes

In mice, includes insulin growth factor 2 (Igf2) and H19 (non-coding RNA)

• Igf2 necessary for normal growth

What is the purpose of the enhancer in imprinted gene cluster

Tells the genes around it to transcribe more

What is the purpose of the insulators in imprinted gene clusters?

When it is not methylated, will combine with protein to block the signal from the enhancer