What is polyspermy and why is it bad? Polyspermy is bad because, in addition to the extra set of chromosomes, a sea urchin sperm donates a centriole. The presence of additional centrioles during the first cell division will result in additional cleavage furrows and incorrect partitioning of the chromosomes (Fig.
What is polyspermy and what causes it?
Polyspermy is a condition where an egg is fertilized by more than one sperm cell. This can happen due to abnormalities of the formation of natural barriers after an egg has been fertilized by one sperm. It could also be the result of too many sperm cells reaching the egg and engaging in a strong competition...
What are the possible complications of polyspermy?
Pregnancies that result from polyspermy often result in spontaneous miscarriages during the second or third trimesters of pregnancy and fetuses that survive until childbirth are likely to not survive beyond a few days. Polyspermy can also complicate in vitro fertilization (IVF) by producing non-viable embryos.
How is polyspermy prevented?
Polyspermy is prevented by: modest sperm number; Fast block; Slow block. Acrosomal Reaction. The acrosome is the tip of the sperm head. The acrosomal reaction is a change in the sperm that is common to many animals. Its function is best understood in the sea urchin. Click to see full answer. Then, how is Polyspermy prevented quizlet?
What happens during the'slow block'of polyspermy?
During the 'slow block' of polyspermy that happens 5-8 minutes after fertilization, the egg releases enzymes that alter the zona pellucida, the outer layer of the egg, and a wall is formed surrounding the egg that stops more sperm from penetrating the egg. Defects of these mechanisms,...
What is meant by polyspermy?
: the entrance of several spermatozoa into one egg — compare dispermy, monospermy.
How is polyspermy prevented and why is this important?
An egg must be fertilized by a single sperm only. To prevent polyspermy, the zona pellucida, a structure that surrounds mammalian eggs, becomes impermeable upon fertilization, preventing the entry of further sperm. The structural changes in the zona upon fertilization are driven by the exocytosis of cortical granules.
What causes polyspermy in humans?
Recently, it was found that an abnormal zona pellucida is one of main causes of polyspermy in human eggs. A high proportion of polyspermy has resulted from the use of a high concentration of capacitated spermatozoa at the site of fertilization, irrespective of in the in vivo or in vitro environment.
How is polyspermy avoid in humans?
When a sperm comes in contact with the zona pellucida layer of the ovum it induces changes in the membrane of the ovum to block the entry of additional sperms. This thus prevents the polyspermy and ensures that only one sperm can fertilise an ovum.
Can polyspermy result in twins?
A likely cause of the above scenario is polyspermy, in which one egg is fertilized by multiple sperm cells. In this case, the egg carrying the maternal DNA is shared between the twins, while the different sperm cells allow for segregation of paternal DNA such that the set given to each developing fetus is distinct.
How is polyspermy avoided during fertilization quizlet?
How is polyspermy prevented during fertilization? The fusion of the sperm with the oocyte releases an enzyme that inactivates sperm binding protein.
Why should polyspermy be avoided?
Prevention of Polyspermy At fertilization, eggs are exposed to multiple sperm but entry of more than one sperm causes polyploidy, polyspermy, which results in abnormal embryo development. To ensure fusion of a single sperm, the egg becomes rapidly intolerant to additional sperm.
What is polyspermy and where does it occur?
Polyspermy occurs when the oocyte is penetrated by more than one sperm and can be pathological because an excess of centrosomes compromises development. However, in some taxa, multiple sperm enter the egg with no apparent adverse effect on zygote viability.
What happens if two sperms enter the egg?
Fraternal twins are formed when two eggs meet two sperm in the womb. Each is fertilized independently, and each becomes an embryo. With identical twins, one egg is fertilized by one sperm, and the embryo splits at some later stage to become two.
How often does polyspermy occur?
Approximately 7 % of fertilized human eggs are polyspermic, which is defined by appearance of three pronuclei, so-called triploid [19].
Why can't multiple sperm fertilize an egg?
To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg. If this mechanism fails, multiple sperm can fuse with the egg, resulting in polyspermy.
How does IVF affect polyspermy?
The polyspermy rate was affected by the number of spermatozoa used for in vitro fertilization. Insemination with 0.5-0.8, 1.0, or 1.5-2.0 X 10(6) spermatozoa/oocyte resulted in a polyspermy rate of 6%, 20%, and 32%, respectively.
What is polyspermy in pregnancy?
Polyspermy refers to the fertilization of one egg with multiple sperm cells that result in the formation of an embryo with abnormal sets of chromosomes. This typically happens with conventional insemination where the egg and the sperm are placed in the same culture environment for fertilization. Pregnancies that result from polyspermy often result ...
How long can a baby survive polyspermy?
Pregnancies that result from polyspermy often result in spontaneous miscarriages during the second or third trimesters of pregnancy and fetuses that survive until childbirth are likely to not survive beyond a few days.
Can polyspermy cause IVF?
Polyspermy can also complicate in vitro fertilization (IVF) by producing non-viable embryos.
Can polyspermy cause triploid pregnancy?
But with advances in IVF techniques and preimplantation genetic screening (PGS), the risk of polyspermy leading to a triploid pregnancy is minimized.
What is polyspermy in animals?
Abstract#N#Polyspermy is a lethal condition where more than one sperm nucleus interacts with the female nucleus. This article looks at how animals avoid this condition in nature and shows that sperm numbers are regulated in various ways to ensure low sperm–oocyte ratios at the site of fertilization. Laboratory experiments as far back as the 1950s with oocytes deprived of their extracellular coats and exposed to unnaturally sperm numbers are artifactual and have led to the idea that oocytes possess mechanisms to repel excess spermatozoa. Under natural conditions selective pressure would have favored achieving monospermy rather than preventing polyspermy.
Why do eggs have polysperm?
Prevention of Polyspermy. At fertilization, eggs are exposed to multiple sperm but entry of more than one sperm causes polyploidy, polyspermy, which results in abnormal embryo development. To ensure fusion of a single sperm, the egg becomes rapidly intolerant to additional sperm.
Why is monospermic fertilization important?
Monospermic fertilization is critically important for successful onset of development and potent blocks to polyspermy are rapidly invoked following gamete fusion. There is a rapid block at the plasma membrane. Unlike other species, this block does not involve postfertilization changes in membrane voltage in mammals.
How does the electric potential of an egg affect polyspermy?
Changes in the electric potential of the egg plasma membrane are thought to play a central role in the fast block to polyspermy in various animals ( Gould and Stephano, 2003 ). In sea urchin eggs, a resting membrane potential of approximately −70 mV shifts to +20 mV within a few seconds after the contact of a fertilizing spermatozoon. When Na + concentration is lowered in the surrounding water, this sperm-induced jump-up in the membrane potential, i.e., depolarization, does not occur, resulting in polyspermy. In addition, holding the membrane potential of unfertilized eggs at either >+5 or <−20 mV prevents sperm entry, whereas holding them between −20 and +5 mV allows repeated sperm entries. Thus, it is thought that the fertilizing spermatozoa can enter the egg within the narrow window of time during which Na + -dependent depolarization takes place (or the egg becomes depolarized), ensuring monospermy at the initial phase of fertilization. However, it is a matter of debate whether or not the fast electrical polyspermy block is necessary under physiological conditions where not so many spermatozoa are assumed to reach the egg.
Why is 3PN abnormal?
Abnormal fertilization by conventional in vitro fertilization is mainly associated with abnormal polyspermy, due to the dispermic oocyte penetration resulting in tripronuclear (3PN) oocytes. Reasons for the appearance of 3PN include; uncontrollable multiple sperm penetrations, fertilization by a diploid sperm or oocyte, inhibition of second polar body extrusion and formation of two female pronuclei (or possibily one diploid) together with a single sperm pronucleus (monospermic digny). The identification and elimination of polysypermic oocytes are critical as it’s known that significantly higher spontaneous abortions are expected from the transfer of embryos derived from triploid zygotes. In addition to the dispermy and digyny the origin of tripronuclear zygotes may also be connected with their chromosomal constitution. New micromanipulation techniques such as microsurgical removal of a single pronucleus may provide an opportunity to repair the abnormalities of tripronuclear zygotes, especially for patients with high rate of hyperploidy (Ivakhnenko et al., 2000 ). Recent study by Escriba et al. (2006) demonstrating heteroparental blastocyst production from microsurgically corrected tripronucleated human embryos from conventional IVF had also been a hope for future studies of correction micromanipulations for hyperploidic embryos.
How does a fast block to polyspermy work?
The fast block to polyspermy, which has been best studied in sea urchins, consists of a rapid electrical depolarization of the plasma membrane of the egg. The resting membrane potential of the egg changes from about − 70 to + 10 mV within 2–3 s after fusion of the spermatozoon with the egg. This change in membrane potential prevents other spermatozoa from adhering to the egg's plasma membrane. The fast block in mammals is short-lived, lasting only several minutes, and may not be as heavily based on membrane depolarization as that in sea urchins. This time is sufficient for the egg to mount a permanent slow block. The exact nature of the fast block in the human egg is still not well defined.
Which molluscs have polyspermy block?
This permanent polyspermy block at the level of the plasma membrane also occurs in Spisula (a marine bivalve mollusc), in other bivalve molluscs such as Mytilus, Bankia, and Chama, and in the polychaete annelids Hydroides and Nereis.
Why is polyspermy blocked?
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.
Why is polyspermy considered an unviable zygote?
This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition.
What animals have polyspermy blocks?
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.
How does sperm affect the egg?
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.
What is the role of polysperm in the development of an egg?
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, just to name a few. Some vertebrates that are both amniote or anamniote. For example, 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.
How does polyspermy in sea urchins work?
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.
What is monospermic fertilization?
Unlike physiological polyspermy, monospermic fertilization deals with the analysis of the egg calcium waves, as this is the typical reproduction process in all species. Species that undergo physiological polyspermy have polyploidy -preventing mechanisms that act inside the egg.
Why is polysperm bad?
Also, what is Polyspermy and why is it bad? Polyspermy is bad because, in addition to the extra set of chromosomes, a sea urchin sperm donates a centriole. The presence of additional centrioles during the first cell division will result in additional cleavage furrows and incorrect partitioning of the chromosomes (Fig. 7.21).
How is polyspermy prevented?
Polyspermy is prevented by: modest sperm number; Fast block; Slow block. Acrosomal Reaction. The acrosome is the tip of the sperm head. The acrosomal reaction is a change in the sperm that is common to many animals. Its function is best understood in the sea urchin.
What is polyspermy in biology?
Polyspermy is the double fertilization of an ovum by two sperm. It is prevented by 3 methods during fertilization: 1) The intracellular wave of Ca2+ that travels across the oocyte membrane and causes hyper polarization after fusion of the sperm and oocyte membranes.
Overview
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 …
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 i…
Blocking polyspermy
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. According to Developmental Biology Interactive, if an egg becomes fertilized by multiple sperm, the embryo will then gain various paternal centrioles. When this happens, there is a struggle for extra chromosomes. This competition causes disarrayment in cleavage …
Evolutionary advantage
Female defenses select for ever more aggressive male sperm, however, leading to an evolutionary arms race. On the one hand, polyspermy creates inviable zygotes and lowers female fitness, but on the other, defenses may prevent fertilization altogether. This leads to a delicate compromise between the two, and has been suggested as one possible cause for the relatively high infertility rates seen in mammalian species. In some species, polyspermy is allowed to happen to result i…
See also
• Cortical reaction
Further reading
• Ginzberg, A. S. 1972. Fertilization in Fishes and the Problem of Polyspermy, Israel Program for Scientific Translations, Jerusalem.
• Jaffe, L. A. & M. Gould. 1985. Polyspermy-preventing mechanisms. In C. B. Metz & A. Monroy (editors) Biology of Fertilization. Academic, New York.Brendon magero; 223–250.
External links
• Animation of polyspermy