Polyspermy

Key takeaways

Polyspermy belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Polyspermy to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Polyspermy from memory before moving on to harder problems.

Reference excerpt

In biology, polyspermy describes the fertilization of an egg by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome—one from the egg and one each from multiple sperm. Usually, the result is an unviable zygote. This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. Such a situation is often deleterious to the female: in other words, the male–male competition among sperm spills over to create sexual conflict.

Pathological polyspermy and the blocks In the classical case (at least from the historical human perspective), monospermic fertilization is the norm. For monospermic organisms, polyspermy is a detrimental process where eggs are incorrectly fertilized. It causes the formation of multiple microtubule-organizing centers from multiple, leading to disruption of mitosis, disarrayment in cleavage furrow formation, and ultimately, cell death. As a result, a number of "blocks" have evolved to prevent polyspermy in such organisms. Polyspermy is very rare in human reproduction. The decline in the numbers of sperm that swim to the oviduct is one of two ways that prevents polyspermy in humans. The other mechanism is the blocking of sperm in the fertilized egg. Only two cases of human polyspermy leading to birth of children have been reported.

Fast block of polyspermy The eggs of sexually-reproducing organisms are adapted to avoid this situation. The defenses are particularly well characterized in the sea urchin, which respond to the acceptance of one sperm by inhibiting the successful penetration of the egg by subsequent sperm. Similar defenses exist in other eukaryotes. The prevention of polyspermy in sea urchins depends on a change in the electrical charge across the surface of the egg, which is caused by the fusion of the first sperm with the egg. Unfertilized sea urchin eggs have a negative charge inside, but the charge becomes positive upon fertilization. When sea urchin sperm encounter an egg with a positive charge, sperm-egg fusion is blocked. Thus, after the first sperm contacts the egg and causes the change, subsequent sperms are prevented from fusing. This "electrical polyspermy block" is thought to result because a positively charged molecule in the sperm surface membrane is repelled by the positive charge at the egg surface. Electrical polyspermy blocks operate in many animal species, including frogs, clams, and marine worms, but not in the several mammals that have been studied (hamster, rabbit, mouse). In species without an electrical block, polyspermy is usually prevented by secretion of materials that establish a mechanical barrier to polyspermy. Animals such as sea urchins have a two-step polyspermy prevention strategy, with the fast, but transient, electrical block superseded after the first minute or so by a more slowly developing permanent mechanical block. Electrical blocks are helpful in species where a very fast block to polyspermy is needed, due to the presence of many sperm arriving simultaneously at the egg surface, as occurs in animals such as sea urchins. In sea urchins, fertilization occurs externally in the ocean, such that hundreds of sperm can encounter the egg within several seconds. In mammals, it occurs 2-3 seconds after the first sperm enters the egg. It is a chemical process which involves changing the potential of egg from a resting potential of -70 mv to 10 mv. It involves an influx of sodium ion into the egg. The membrane of the egg changes from negatively to positively charged. Sperm cannot enter a positively charged egg. The positive charge only lasts for 60 seconds.

Slow block of polyspermy In mammals, in which fertilization occurs internally, fewer sperm reach the fertilization site in the oviduct. This may be the result of the female genital tract being adapted to minimize the number of sperm reaching the egg. Nevertheless, polyspermy preventing mechanisms are essential in mammals; a secretion reaction, the "cortical reaction" modifies the extracellular coat of the egg (the zona pellucida), and additional mechanisms that are not well understood modify the egg's plasma membrane. The zona pellucida is modified by serine proteases that are released from the cortical granules. The proteases destroy the protein link between the cell membrane and the vitelline envelope, remove any receptors that other sperm have bound to, and help to form the fertilization envelope from the cortical granules. The cortical reaction occurs due to calcium oscillations inside the oocyte. What triggers such oscillations is PLC-zeta, a phospholipase unique to sperm that is very sensitive to calcium concentrations. When the first spermatozoa get inside the oocyte, it brings in PLC-zeta, that is activated by oocyte's basal calcium concentrations, initiates the formation of IP3 and causes calcium release from endoplasmic reticulum stores, generating the oscillations in calcium concentration that will activate the oocyte and block polyspermy.

Physiological polyspermy Physiological polyspermy happens when the egg normally accepts more than one sperm but only one of the multiple sperm will fuse its nucleus with the nucleus of the egg. Physiological polyspermy is present in some species of vertebrates and invertebrates. Some species utilize physiological polyspermy as the proper mechanism for developing their offspring. Some of these animals include birds, ctenophora, reptiles and amphibians. Some vertebrates that are both amniote or anamniote, including urodele amphibians, cartilaginous fish, birds and reptiles, undergo physiological polyspermy because of the internal fertilization of their yolky eggs. Sperm triggers egg activation by the induction of free calcium ion concentration in the cytoplasm of the egg. This induction plays a very critical role in both physiological polyspermy and monomeric polyspermy species. The rise in calcium causes activation of the egg. The egg will then be altered on both a biochemical and morphological level. In mammals as well as sea urchins, the sudden rise in calcium concentration occurs because of the influx of calcium ions within the egg. These calcium ions are responsible for the cortical granule reaction, and are also stored in the egg's endoplasmic reticulum.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Polyspermy

Start with the simplest possible case. Write down what Polyspermy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Polyspermy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Polyspermy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Polyspermy

In research: Polyspermy appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Polyspermy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Polyspermy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Reproduction, so understanding it makes those chapters shorter.
In everyday life: Look for Polyspermy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Polyspermy in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Polyspermy means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Polyspermy out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Polyspermy in simple terms?

In biology, polyspermy describes the fertilization of an egg by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent.

Why does Polyspermy matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Polyspermy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Polyspermy.