Embryology   Biology 441   Spring 2014   Albert Harris


Feb. 24, 2014: Blocks to Polyspermy

For an oocyte to be fertilized by two or more sperm destroys its ability to develop into a fertile adult. (Two sperm are just as bad as none.)

This results in a very strong Darwinian selection pressure favoring mechanisms that prevent any more sperm from fertilizing an oocyte (after the first sperm has fertilized it).

Question for class discussion:

Suppose that a mutation caused sperm carrying the "super-sperm" mutant gene to be able to overcome blocks to polyspermy: Would there be Darwinian selection in favor of that mutation?

Why or why not?

Suppose such a mutation also caused sperm nuclei to become able to destroy or inactivate other sperm that already had fertilized an oocyte? Would Darwinian selection favor this gene? Would the frequency of that mutation increase over time.

Suppose that a mutation increased the speed of sperm, or otherwise increased their competitive success rate in fertilizing oocytes?

Suppose that a certain "super-sperm" mutation had these two very different effects:

1) In the sperm (haploid, of course), the mutation greatly increases the frequency of success in fertilizing oocytes? (for example, if 90% of offspring of a heterozygous father resulted from fertilization by one of the 50% of his sperm that carried this mutation.)

2) Suppose that it is harmful or even lethal for animals to be homozygous for this "super-sperm" mutation, so that homozygotes leave fewer offspring.

Would the frequency of super-sperm mutant heterozygotes increase in the population? Or decrease?

What percentage increase in sperm success would balance what % decrease in survival or breeding success by individuals homozygous for the super-sperm mutation?
Or is there a predictable percentage trade-off?

*) What if this mutation is sex-linked (= located on the X chromosome)

*) What if this mutation is located on the Y chromosome?


The Fast Block to Polyspermy

Has a set of mechanisms very similar to nerve conduction

1) Resting potential (potassium concentration gradient across the plasma membrane

2) Action potential (voltage gated calcium channels)

(But differs in having a very long or pemenent depolarization )

3) Depolarization stimulates secretion of cytoplasmic vacuoles

(in nerves, these vacuoles contain acetyl choline or other neurotransmitters)

Positive outside, because positive potassium ions are more concentrated inside, and therefore diffuse outward through the membrane, carrying positive charges, and creating the positive-outside (=negative inside) resting potential.

One way to depolarize a nerve, muscle or oocyte is to put a concentrated solution of potassium ions outside.

When potassium ions are very concentrated outside, as well as inside, then there is no longer a diffusion gradient of potassium, And no tendency to diffuse either outward, or inward.

High concentrations of potassium ions outside cells stimulate all muscles to contract (constantly), including heart muscle, and stimulate all nerves nerves to depolarize, repeatedly, and briefly.

Injection of large amounts of potassium chloride, to stop the heart, has been the method of capital punishment in North Carolina! (preceded by anesthesia, so the person is not supposed to feel anything)

There has been discussion on the news whether the person would feel much pain if the general anesthetic failed to work, or its effect decreased.

Oocytes normally depolarize in response to sperm entry. Can you figure out at least two ways to induce this artificially?


Acrosome in sperm: enzymes to digest path through "jelly" around oocyte.

Cortical granules in oocyte: (20,000 or more per oocyte)
Increased concentration of calcium ions stimulates their secretion.
(analogous to secretion of synaptic vesicles, in nerves)


1) Enzymes to digest egg-sperm adhesion proteins. ("The Zona Reaction)
2) Other enzymes, that cut attachments between vitelline membrane + oocyte membrane
3) Components that attach to the inner surface of vitelline membrane
(to build the fertilization "membrane")
4) Dissolved materials that create an osmotic pressure (that lift the vitelline membrane, & inflate volume between it and cell surface).

Video: fertilization membrane in sea urchin

Sea urchins have all 4 of these

Vertebrates have only #1. (and the voltage depolarization)

"The fast block"

"The slow block" enzymes and fertilization membrane.

In mammals, the slow block is called the Zona Reaction.

And the vitelline membrane of mammals is called the Zona Pellucida.

(Not because it's different, but because different researchers worked on mammal embryos)



Video: galvanotaxis

{PLEASE NOTE: Voltage gradients of the sizes used in research on this subject can kill you.}

Experiments have discovered that most (maybe all) differentiated cell types respond to electrical voltage gradients

*by changing their shapes,

* by aligning either parallel or perpendicular to the voltage gradient,

* and sometimes by crawling toward the cathode, (negative electrode: where O2 [oxygen gas is released)

* and sometimes by crawling toward the anode.

(positive electrode: where H2 hydrogen gas is released

Mesenchymal cells and skeletal muscle cells line up perpendicular to voltage gradient. & crawl very slowly toward the negative electrode.

Epidermis cells crawl rapidly toward to negative electrodes

(so do some nerve axons)

Macrophages and osteoclasts crawl toward positive electrode.


When Paramecia are swimming along, and collide with something, they back up by temporarily reversing the direction of the power stroke of their cilia; Then they resume going forward.

This reversal is caused by depolarization of their plasma membrane, and by a brief increase in the calcium ion concentration of their cytoplasm.

Which probably evolved first, second, third and fourth:

The use of action potentials to control directional swimming in protozoa?

Or the use of action potentials to block polyspermy of oocytes.

Or the use of action potentials to control contraction of muscle cells?

Or the use of action potentials to propagate nerve impulses along nerve fibers?



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